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  • 301.
    Gebeyehu, Mekbib
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Geochemistry, Pb, C-O isotope data and genesis of some lower proterozoic massive sulfide deposits in the Bergslagen ore province, south central Sweden.1992Doctoral thesis, comprehensive summary (Other academic)
  • 302. Geiger, Harri
    et al.
    Mattsson, Tobias
    Deegan, Frances M.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Uppsala University, Sweden.
    Troll, Valentin R.
    Burchardt, Steffi
    Gudmundsson, Olafur
    Tryggvason, Ari
    Krumbholz, Michael
    Harris, Chris
    Magma plumbing for the 2014-2015 Holuhraun eruption, Iceland2016In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 17, no 8, p. 2953-2968Article in journal (Refereed)
    Abstract [en]

    The 2014-2015 Holuhraun eruption on Iceland was located within the Askja fissure swarm but was accompanied by caldera subsidence in the Baroarbunga central volcano 45 km to the southwest. Geophysical monitoring of the eruption identified a seismic swarm that migrated from Baroarbunga to the Holuhraun eruption site over the course of two weeks. In order to better understand this lateral connection between Baroarbunga and Holuhraun, we present mineral textures and compositions, mineral-melt-equilibrium calculations, whole rock and trace element data, and oxygen isotope ratios for selected Holuhraun samples. The Holuhraun lavas are compositionally similar to recorded historical eruptions from the Baroarbunga volcanic system but are distinct from the historical eruption products of the nearby Askja system. Thermobarometry calculations indicate a polybaric magma plumbing system for the Holuhraun eruption, wherein clinopyroxene and plagioclase crystallized at average depths of approximate to 17 km and approximate to 5 km, respectively. Crystal resorption textures and oxygen isotope variations imply that this multilevel plumbing system facilitated magma mixing and assimilation of low-O-18 Icelandic crust prior to eruption. In conjunction with the existing geophysical evidence for lateral migration, our results support a model of initial vertical magma ascent within the Baroarbunga plumbing system followed by lateral transport of aggregated magma batches within the upper crust to the Holuhraun eruption site.

  • 303. Gemery, Laura
    et al.
    Cronin, Thomas M.
    Poirier, Robert K.
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences. Aarhus University, Denmark.
    Barrientos, Natalia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Johansson, Carina
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Koshurnikov, Andrey
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Central Arctic Ocean paleoceanography from similar to 50 ka to present, on the basis of ostracode faunal assemblages from the SWERUS 2014 expedition2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 11, p. 1473-1489Article in journal (Refereed)
    Abstract [en]

    Late Quaternary paleoceanographic changes at the Lomonosov Ridge, central Arctic Ocean, were reconstructed from a multicore and gravity core recovered during the 2014 SWERUS-C3 Expedition. Ostracode assemblages dated by accelerator mass spectrometry (AMS) indicate changing sea-ice conditions and warm Atlantic Water (AW) inflow to the Arctic Ocean from similar to 50 ka to present. Key taxa used as environmental indicators include Acetabulastoma arcticum (perennial sea ice), Polycope spp. (variable sea-ice margins, high surface productivity), Krithe hunti (Arctic Ocean deep water), and Rabilimis mirabilis (water mass change/AWinflow). Results indicate periodic seasonally sea-ice-free conditions during Marine Isotope Stage (MIS) 3 (similar to 57-29 ka), rapid deglacial changes in water mass conditions (15-11 ka), seasonally sea-ice-free conditions during the early Holocene (similar to 10-7 ka) and perennial sea ice during the late Holocene. Comparisons with faunal records from other cores from the Mendeleev and Lomonosov ridges suggest generally similar patterns, although sea-ice cover during the Last Glacial Maximum may have been less extensive at the new Lomonosov Ridge core site (similar to 85.15 degrees N, 152 degrees E) than farther north and towards Greenland. The new data provide evidence for abrupt, large-scale shifts in ostracode species depth and geographical distributions during rapid climatic transitions.

  • 304. Gerok, Dmitrij
    et al.
    Gelumbauskaite, Leonora Zivile
    Flodén, Tom
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Grigelis, Algimantas
    Bitinas, Albertas
    New data on the palaeo-incisions network of the south-eastern Baltic Sea2014In: Baltica: an International Yearbook for Quaternary Geology and Palaeogeography, Coastal Morphology and Shore Processes, Marine Geology and Recent Tectonics of the Baltic Sea Area, ISSN 0067-3064, E-ISSN 1648-858X, Vol. 27, no 1, p. 1-14Article in journal (Refereed)
    Abstract [en]

    The present study area is located within the south-eastern segment of the Baltic Sea framed by 55 degrees 30'-56 degrees 30' N and 19 degrees 00'-21 degrees 15'E. The area is re-visited with the aim to describe in more detail the geologic prerequisite for development of the palaeo-incisions as well as the timing of their subsequent infillings. The channels form distinctive features in the sedimentary bedrock along the outer limits of pre-Weichselian ice sheets, on average reaching depths into the bedrock of 50 m in the nearshore zone of Lithuania to 100 m along the slope to the Gotland depression in the west. The development of palaeo-incisions system is governed by the easily eroded late Palaeozoic to Mesozoic bedrock of the present area. Only rare occurrences of channels have been reported from the middle and lower parts of the Palaeozoic further west in the Baltic. The present investigation supports a mechanism that the channels formed below the ice near the ice sheet margin by melt water erosion under high pressure. The channels start at random where a fracture in the ice develops forming outlet of water contained below the central part of the ice sheet. The channels often merge together in the direction of the ice margin, possibly gradually adapting to previous fracture systems in the bedrock. The investigated incisions were infilled prior to the advance of the Weichselian ice sheet and some have been reopened and repeatedly infilled.

  • 305. Gessner, Klaus
    et al.
    Gallardo, Luis A.
    Markwitz, Vanessa
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thomson, Stuart N.
    What caused the denudation of the Menderes Massif: Review of crustal evolution, lithosphere structure, and dynamic topography in southwest Turkey2013In: Gondwana Research, ISSN 1342-937X, E-ISSN 1878-0571, Vol. 24, no 1, p. 243-274Article, review/survey (Refereed)
    Abstract [en]

    The deformation of Earth's lithosphere in orogenic belts is largely forced externally by the sinking slab, but can also be driven by internal delamination processes caused by mechanical instabilities. Here we present an integrated analysis of geophysical and geological data to show how these processes can act contemporaneously and in close proximity to each other, along a lithosphere scale discontinuity that defines the lateral boundary between the Hellenide and Anatolide segments of the Tethyan orogen in western Turkey. The Hellenides and Anatolides have experienced similar rates of convergence, but display remarkable differences in the structure of Earth's crust and lithospheric mantle across the Aegean coast of the Anatolian peninsula. We review the tectonics of southwest Turkey in the light of new and published data on crustal structure, cooling history, topography evolution, gravity, Moho topography, earthquake distribution and seismic tomography. Geological data constrain that one of Earth's largest metamorphic core complexes, the Menderes Massif, experienced early Miocene tectonic denudation and surface uplift in the footwall of a north-directed extensional detachment system, followed by late Miocene to recent fragmentation by E-W and NW-SE trending graben systems. Gravity data, earthquake locations and seismic velocity anomalies highlight a north-south oriented boundary in the upper mantle between a fast slab below the Aegean and a slow asthenospheric region below western Turkey. Based on the interpretation of geological and geophysical data we propose that the tectonic denudation of the Menderes Massif and the delamination of its subcontinental lithospheric mantle reflect the late Oligocene/early Miocene onset of transtension along a lithosphere scale shear zone, the West Anatolia Transfer Zone (WATZ). We argue that the WATZ localised along the boundary of the Adriatic and Anatolian lithospheric domains in the Miocene, when southward rollback of the Aegean slab started to affect the central Aegean-Menderes portion of the Tethyan orogen. Transtension across the West Anatolia Transfer Zone affected the entire Menderes Massif in the Early Miocene. The current crustal expression of this boundary is a NNE-trending, distributed brittle deformation zone that localised at the western margin of the denuded massif. Here, sinistral transtension accommodates the continuing velocity difference between relatively slow removal of lithospheric mantle below western Anatolia and trench retreat in the rapidly extending Aegean Sea region. Our review highlights the significance of lateral variations of the lower plate in subduction-collision systems for evolving structure and surface processes in orogenic belts, particularly in relation to the formation of continental plateaux and metamorphic core complexes.

  • 306. Giasson, M-A
    et al.
    Ellison, A. M.
    Bowden, R. D.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Davidson, E. A.
    Drake, J. E.
    Frey, S. D.
    Hadley, J. L.
    Lavine, M.
    Melillo, J. M.
    Munger, J. W.
    Nadelhoffer, K. J.
    Nicoll, L.
    Ollinger, S. V.
    Savage, K. E.
    Steudler, P. A.
    Tang, J.
    Varner, R. K.
    Wofsy, S. C.
    Foster, D. R.
    Finzi, A. C.
    Soil respiration in a northeastern US temperate forest: a 22-year synthesis2013In: Ecosphere, ISSN 2150-8925, E-ISSN 2150-8925, Vol. 4, no 11, p. UNSP 140-Article in journal (Refereed)
    Abstract [en]

    To better understand how forest management, phenology, vegetation type, and actual and simulated climatic change affect seasonal and inter-annual variations in soil respiration (R-s), we analyzed more than 100,000 individual measurements of soil respiration from 23 studies conducted over 22 years at the Harvard Forest in Petersham, Massachusetts, USA. We also used 24 site-years of eddy-covariance measurements from two Harvard Forest sites to examine the relationship between soil and ecosystem respiration (R-e). R-s was highly variable at all spatial (respiration collar to forest stand) and temporal (minutes to years) scales of measurement. The response of R-s to experimental manipulations mimicking aspects of global change or aimed at partitioning R-s into component fluxes ranged from similar to 70% to +52%. The response appears to arise from variations in substrate availability induced by changes in the size of soil C pools and of belowground C fluxes or in environmental conditions. In some cases (e.g., logging, warming), the effect of experimental manipulations on R-s was transient, but in other cases the time series were not long enough to rule out long-term changes in respiration rates. Inter-annual variations in weather and phenology induced variation among annual R-s estimates of a magnitude similar to that of other drivers of global change (i.e., invasive insects, forest management practices, N deposition). At both eddy-covariance sites, aboveground respiration dominated R-e early in the growing season, whereas belowground respiration dominated later. Unusual aboveground respiration patterns-high apparent rates of respiration during winter and very low rates in mid-to-late summer-at the Environmental Measurement Site suggest either bias in R-s and R-e estimates caused by differences in the spatial scale of processes influencing fluxes, or that additional research on the hard-to-measure fluxes (e.g., wintertime R-s, unaccounted losses of CO2 from eddy covariance sites), daytime and nighttime canopy respiration and its impacts on estimates of R-e, and independent measurements of flux partitioning (e.g., aboveground plant respiration, isotopic partitioning) may yield insight into the unusually high and low fluxes. Overall, however, this data-rich analysis identifies important seasonal and experimental variations in R-s and R-e and in the partitioning of R-e above-vs. belowground.

  • 307. Gierga, Merle
    et al.
    Hajdas, Irka
    van Raden, Ulrike J.
    Gilli, Adrian
    Wacker, Lukas
    Sturm, Michael
    Bernasconi, Stefano M.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Long-stored soil carbon released by prehistoric land use: Evidence from compound-specific radiocarbon analysis on Soppensee lake sediments2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 144, p. 123-131Article in journal (Refereed)
    Abstract [en]

    Compound-specific radiocarbon (C-14) analyses allow studying the fate of individual biomarkers in ecosystems. In lakes with small catchments, terrestrial biomarkers have the potential to be used for the dating of sediments that lack the traditionally targeted terrestrial macrofossils, if the specific organic compounds are deposited soon after production. On the other hand, if the biomarkers have been stored for a significant amount of time in the soils of the catchment before transported to the lake, their age can be used to reconstruct changes in average residence time of organic material on land through time. Here we present a study based on compound-specific C-14 analysis of the sedimentary record of Lake Soppensee, Switzerland, targeting long-chain n-alkanes of exclusive terrigenous origin, and comparing them with sediment ages obtained by high-resolution macrofossil dating. Additionally, we measured C-14 ages of bulk organic matter and carbonate samples to assess the hard water effect. Prior to 3100 cal BP n-alkanes had about the same age as the sediment or they were slightly older, indicating that the vast majority of the terrestrial organic carbon transported to the lake had a short residence time on land. In the samples younger than 3100 cal BP an increasing offset is observed, indicating liberation of old buried soil organic matter that must have accumulated over the previous millennia. Our results indicate that as long as stable ecosystem conditions have prevailed, the distribution and isotopic composition of the n-alkanes can be used as environmental proxies in small catchments with limited surface runoff, confirming a few earlier studies.

  • 308. Gierga, Merle
    et al.
    Schneider, Maximilian P. W.
    Wiedemeier, Daniel B.
    Lang, Susan Q.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hajdas, Irka
    Bernasconi, Stefano M.
    Schmidt, Michael W. I.
    Purification of fire derived markers for mu g scale isotope analysis (delta C-13, Delta C-14) using high performance liquid chromatography (HPLC)2014In: Organic Geochemistry, ISSN 0146-6380, E-ISSN 1873-5290, Vol. 70, p. 1-9Article in journal (Refereed)
    Abstract [en]

    Black carbon (BC) is the residue of incomplete biomass combustion. It is ubiquitous in nature and, due to its relative persistence, is an important factor in Earth's slow-cycling carbon pool. This resistant nature makes pure BC one of the most used materials for C-14 dating to elucidate its formation date or residence time in the environment. However, most BC samples cannot be physically separated from their matrices, precluding accurate C-14 values. Here we present a method for radiocarbon dating of the oxidation products of BC, benzene polycarboxylic acids, thereby circumventing interference from extraneous carbon. Individual compounds were isolated using high performance liquid chromatography (HPLC) and converted to CO2 via wet chemical oxidation for C-13 and C-14 isotope analysis. A detailed assessment was performed to identify and quantify sources of extraneous carbon contamination using two process standards with distinct isotopic signatures. The average blank was 1.6 +/- 0.7 mu g C and had an average radiocarbon content of 0.90 +/- 0.50 (FC)-C-14. We successfully analyzed the C-14 content of individual benzene polycarboxylic acids with a sample size as small as 20-30 mu g C after correcting for the presence of the average blank. The combination of delta C-13 and (FC)-C-14 analysis helps interpret the results and enables monitoring of extraneous carbon contribution in a fast and cost efficient way. Such a molecular approach to radiocarbon dating of BC residues enables the expansion of isotopic BC studies to samples that have either been too small or strongly affected by non-fire derived carbon.

  • 309. Giesler, Reiner
    et al.
    Lyon, Steve W.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Karlsson, Jan
    Karlsson, E. M.
    Jantze, Elin J.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Destouni, Georgia
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Catchment-scale dissolved carbon concentrations and exportestimates across six subarctic streams in northern Sweden2014In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 11, p. 525-537Article in journal (Refereed)
    Abstract [en]

    Climatic change is currently enhancing permafrostthawing and the flow of water through the landscape in subarcticand arctic catchments, with major consequences forthe carbon export to aquatic ecosystems. We studied streamwater carbon export in several tundra-dominated catchmentsin northern Sweden. There were clear seasonal differencesin both dissolved organic carbon (DOC) and dissolved inorganiccarbon (DIC) concentrations. The highest DOC concentrationsoccurred during the spring freshet while the highestDIC concentrations were always observed during winterbaseflow conditions for the six catchments considered in thisstudy. Long-term trends for the period 1982 to 2010 for oneof the streams showed that DIC concentrations has increasedby 9% during the 28 yr of measurement while no clear trendwas found for DOC. Similar increasing trends were alsofound for conductivity, Ca and Mg. When trends were discretizedinto individual months, we found a significant linearincrease in DIC concentrations with time for September,November and December. In these subarctic catchments, theannual mass of C exported as DIC was in the same orderof magnitude as DOC; the average proportion of DIC to thetotal dissolved C exported was 61% for the six streams. Furthermore,there was a direct relationship between total runoffand annual dissolved carbon fluxes for these six catchments.These relationships were more prevalent for annual DIC exportsthan annual DOC exports in this region. Our results alsohighlight that both DOC and DIC can be important in highlatitudeecosystems. This is particularly relevant in environmentswhere thawing permafrost and changes to subsurfaceice due to global warming can influence stream water fluxesof C. The large proportion of stream water DIC flux also hasimplications on regional C budgets and needs to be consideredin order to understand climate-induced feedback mechanismsacross the landscape.

  • 310. Gislason, Sigurdur R.
    et al.
    Oelkers, Eric H.
    Eiriksdottir, Eydis S.
    Kardjilov, Marin I.
    Gisladottir, Gudrun
    Sigfusson, Bergur
    Snorrason, Arni
    Elefsen, Sverrir
    Hardardottir, Jorunn
    Torssander, Peter
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Oskarsson, Niels
    Direct evidence of the feedback between climate and weathering2009In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 277, no 02-jan, p. 213-222Article in journal (Refereed)
    Abstract [en]

    Long-term climate moderation is commonly attributed to chemical weathering; the higher the temperature and precipitation the faster the weathering rate. Weathering releases divalent cations to the ocean via riverine transport where they promote the drawdown of CO2 from the atmosphere by the precipitation and subsequent burial of carbonate minerals. To test this widely-held hypothesis, we performed a field study determining the weathering rates of 8 nearly pristine north-eastern Iceland river catchments with varying glacial cover over 44 years. The mean annual temperature and annual precipitation of these catchments varied by 3.2 to 4.5 degrees C and 80 to 530%, respectively during the study period. Statistically significant linear positive correlations were found between mean annual temperature and chemical weathering in all 8 catchments and between mean annual temperature and both mechanical weathering and runoff in 7 of the 8 catchments. For each degree of temperature increase, the runoff, mechanical weathering flux, and chemical weathering fluxes in these catchments are found to increase from 6 to 16%, 8 to 30%, and 4 to 14% respectively, depending on the catchment. In contrast, annual precipitation is less related to the measured fluxes; statistically significant correlations between annual precipitation and runoff, mechanical weathering, and chemical weathering were found for 3 of the least glaciated catchments. Mechanical and chemical weathering increased with time in all catchments over the 44 year period. These correlations were statistically significant for only 2 of the 8 catchments due to scatter in corresponding annual runoff and average annual temperature versus time plots. Taken together, these results 1) demonstrate a significant feedback between climate and Earth surface weathering, and 2) suggest that weathering rates are currently increasing with time due to global warming. 

  • 311.
    Giustiniani, Michela
    et al.
    National Institute of Oceanography and Experimental Geophysics (OGS).
    Tinivella, Umberta
    National Institute of Oceanography and Experimental Geophysics (OGS).
    Jakobsson, M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rebesco, Michele
    National Institute of Oceanography and Experimental Geophysics (OGS).
    Arctic Ocean Gas Hydrate Stability in a Changing Climate2013In: Journal of Geological Research, ISSN 1687-8833, E-ISSN 1687-8841, no 783969, p. 1-10Article in journal (Refereed)
  • 312. Gleason, J. D.
    et al.
    Blum, J. D.
    Moore, T. C.
    Polyak, L.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Meyers, P. A.
    Biswas, A.
    Sources and cycling of mercury in the paleo Arctic Ocean from Hg stable isotope variations in Eocene and Quaternary sediments2017In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 197, p. 245-262Article in journal (Refereed)
    Abstract [en]

    Mercury stable isotopic compositions were determined for marine sediments from eight locations in the Arctic Ocean Basin. Mass dependent fractionation (MDF) and mass independent fractionation (MIF) of Hg stable isotopes were recorded across a variety of depositional environments, water depths, and stratigraphic ages. delta(202) Hg (MDF) ranges from -2.34% to -0.78%; Delta(199) Hg (MIF) from -0.18% to +0.12%; and Delta(201) Hg (MIF) from -0.29% to + 0.05% for the complete data set (n = 33). Holocene sediments from the Chukchi Sea and Morris Jesup Rise record the most negative Delta(199) Hg values, while Pleistocene sediments from the Central Arctic Ocean record the most positive Delta(199) Hg values. The most negative delta(202) Hg values are recorded in Pleistocene sediments. Eocene sediments (Lomonosov Ridge) show some overlap in their Hg isotopic compositions with Quaternary sediments, with a sample of the Arctic Ocean PETM (56 Ma) most closely matching the average Hg isotopic composition of Holocene Arctic marine sediments. Collectively, these data support a terrestrially-dominated Hg source input for Arctic Ocean sediment through time, although other sources, as well as influences of sea ice, atmospheric mercury depletion events (AMDEs), and anthropogenic Hg (in core top samples) on Hg isotopic signatures must also be considered.

  • 313. Gleason, James D.
    et al.
    Blum, J.D.
    Moore, T.C.
    Polyak, Leonid
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hg Isotopes as Indicators of Paleoceanographic Change in the Arctic Ocean (55 Ma to Present)2011In: EOS Transactions, American Geophysical Union, American Geophysical Union , 2011, p. PP33A-1901-Conference paper (Other academic)
  • 314.
    Godinho, Jose
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Direct observations of the structures developed on fluorite surfaces with different orientations during dissolutionArticle in journal (Other academic)
  • 315.
    Godinho, Jose
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dissolution of fluorite type surfaces as analogues of spent nuclear fuel: Production of suitable analogues and study the effect of surface orientation on dissolution2011Licentiate thesis, monograph (Other academic)
    Abstract [en]

    It is accepted worldwide that the best final solution for spent nuclear fuel is to bury it in deep geological repositories. Despite the physical and chemical barriers that are supposed to isolate the nuclear waste for at least 100.000 years, some uncertainty factors may cause underground water to get in contact with the nuclear waste. Due to radioactivity and oxidation under air, dissolution experiments using UO2 pellets are difficult and frequently lead to incoherent results. Therefore, to enable a detailed study of the influence of microstructure and surface properties on the stability of spent nuclear fuel over time, it is necessary to produce analogues that closely resemble nuclear fuel in terms of crystallography and microstructure. At the same time, in-depth understanding of dissolution phenomena is crucial to geological processes such as dissolution precipitation creep and solvent mediated phase transformations.

    My thesis is based in two manuscripts. Paper I reports the microstructures obtained after sintering CaF2 powders at temperatures up to 1240°C. Pellets with microstructure, density and pore structure similar to that of UO2 spent nuclear fuel pellets were obtained in the temperature range between 900°C and 1000°C. Paper II reports how differences of surface chemistry and crystal symmetry, characteristics of each surface orientation, affect the topography of CaF2 pellets described in paper I during dissolution.

    I propose that every orientation of the fluorite structure can be decomposed in the three reference surfaces {100}, {110} and {111}. The {111} is the most stable surface with a dissolution rate of the top surface of 1,13x10-9 mol.m-2.s-1, and {112} the less stable surface with a dissolution rate 34 times faster that {111}. Surfaces that expose both Ca and F atoms in the same plan dissolve faster, possibly because the calcium is more susceptible to be solvated.

    The faster dissolving surfaces are replaced by the more stable {111} and {100} surfaces which causes the development of roughness on the top surface and stabilizes the surface on high energy sites; i.e. pores or grain boundaries. The main consequences of these observations are i) the increase of the total surface area; ii) the decrease of the overall surface energy.

    I present a dissolution model for surfaces of crystal with different surface energies. The main conclusions are: a) dissolution rates calculated from surface area are over estimated to the real dissolution rate; b) dissolution rates are faster at the beginning of dissolution and tend to diminish with time until a minimum value is reached.

  • 316.
    Godinho, Jose R. A.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Oak Ridge National Laboratory, United States.
    Putnis, Christine V.
    Piazolo, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences. Macquarie University, Australia.
    Direct Observations of the Dissolution of Fluorite Surfaces with Different Orientations2014In: Crystal Growth & Design, ISSN 1528-7483, E-ISSN 1528-7505, Vol. 14, no 1, p. 69-77Article in journal (Refereed)
    Abstract [en]

    Atomic force microscopy has been used to observe the surface dynamics during dissolution of polished fluorite surfaces with different orientations. These surfaces, with an initially high density of atomic scale defects, showed fast changes during the first seconds in contact with a solution. Different types of structures developed on each surface, depending on its initial orientation and solution composition. These structures dissolved slower than the main surface persisting for at least 67.5 days of continuous dissolution. A new interpretation of traditional kinetic and thermodynamic models of dissolution applied to surfaces with a high density of steps is proposed to explain the observations. The new model includes the following: (a) fast initial dissolution at defect sites, (b) formation of a fluid boundary layer at the mineral solution interface enriched in the dissolving ions, and (c) precipitation of more stable fluorite structures nucleated at surface defects. This model highlights the importance of considering surface defects and crystal orientation for advancing our understanding of processes happening at the mineral solution interface and for developing more accurate kinetic dissolution and crystal growth models essential in Earth and material sciences.

  • 317.
    Godinho, José
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Effect of surface structure for the development of topography during dissolution of fluorite surfacesArticle in journal (Other academic)
  • 318.
    Godinho, José R. A.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Piazolo, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Evins, L. Z.
    Effect of surface orientation on dissolution rates and topography of caf22012In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 86, p. 392-403Article in journal (Refereed)
    Abstract [en]

    This paper reports how during dissolution differences in surface chemistry affect the evolution of topography of CaF2 pellets with a microstructure similar to UO2 spent nuclear fuel. 3D confocal profilometry and atomic force microscopy were used to quantify retreat rates and analyze topography changes on surfaces with different orientations as dissolution proceeds up to 468 h. A NaClO4 (0.05 M) solution with pH 3.6 which was far from equilibrium relative to CaF2 was used. Measured dissolution rates depend directly on the orientation of the exposed planes. The {111} is the most stable plane with a dissolution rate of (1.2 +/- 0.8) x 10(-9) mol m(-2) s(-1), and {112} the least stable plane with a dissolution rate 33 times faster that {111}. Surfaces that expose both Ca and F atoms in the same plane dissolve faster. Dissolution rates were found to be correlated to surface orientation which is characterized by a specific surface chemistry and therefore related to surface energy. It is proposed that every surface is characterized by the relative proportions of the three reference planes {111}, {100} and {110}, and by the high energy sites at their interceptions. Based on the different dissolution rates observed we propose a dissolution model to explain changes of topography during dissolution. Surfaces with slower dissolution rate, and inferred lower surface energy, tend to form while dissolution proceeds leading to an increase of roughness and surface area. This adjustment of the surface suggests that dissolution rates during early stages of dissolution are different from the later stages. The time-dependency of this dynamic system needs to be taken into consideration when predicting long-term dissolution rates.

  • 319.
    Godinho, José R. A.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Oak Ridge National Laboratory, USA.
    Piazolo, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences. Macquarie University, Australia.
    Balic-Zunic, T.
    Importance of surface structure on dissolution of fluorite: Implications for surface dynamics and dissolution rates2014In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 126, p. 398-410Article in journal (Refereed)
    Abstract [en]

    Dissolution rates are usually calculated as a function of surface area, which is assumed to remain constant ignoring the changes occurring on the surface during dissolution. Here we present a study of how topography of natural fluorite surfaces with different orientation changes during up to 3200 h of dissolution. Results are analyzed in terms of changes in surface area, surface reactivity and dissolution rates. All surfaces studied present fast changes in topography during the initial 200 h of dissolution. The controlling factors that cause the development of topography are the stability of the step edges forming the initial surface and its inclination to the closest stable planes, which are specific for each surface orientation. During an initial dissolution regime dissolution rates decrease significantly, even though the total surface area increases. During a second dissolution regime, some surfaces continue to present significant changes in topography, while for others the topography tends to remain approximately constant. The observed variation of dissolution rates are attributed to a decrease of the density of step edges on the surface and the continuous increase in exposure of more stable surfaces. Calculations of dissolution rates, which assume that dissolution rates are directly proportional to surface area, are not valid for the type of surfaces studied. Instead, to develop accurate kinetic dissolution models and more realistic stochastic dissolution simulations the surface reactivity, determined by the relative stability of the planes and type of edges that constitute a surface needs to be considered. Significant differences between dissolution rates calculated based on surface area alone, and based on surface reactivity are expected for materials with the fluorite structure.

  • 320.
    Godinho, José R. A.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Oak Ridge National Laboratory, USA.
    Piazolo, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences. Macquarie University, Australia.
    Evans, L.
    Simulation of surface dynamics during dissolution as a function of the surface orientation: Implications for non-constant dissolution rates2014In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 408, p. 163-170Article in journal (Refereed)
    Abstract [en]

    An important problem in geochemistry is the understanding of how changes occurring on a surface during dissolution affect the variability of measured dissolution rates. In this study a new approach to study the effect of surface dynamics on dissolution rates is tested by coupling experimental data with a numerical model that simulates the retreat of surface profiles during dissolution. We present specific results from the simulation of dissolution of fluorite surfaces. The equations that determine the retreat of a surface are based on experimentally obtained equations that relate the retreat rate of a surface to a single variable, the crystallographic orientation of the surface. Our results show that depending on the starting orientation, different types of topography are developed, similar to those observed experimentally. During the initial dissolution phase, changes of topography are rapid and associated with fast dissolution rates. The progressively slower dissolution rates are coupled with the development of surface segments with orientations that dissolve at a slower rate. Consequently, the overall retreat rate of a profile decreases during the simulation, and tends to a near-constant value. The results show a close relationship between dissolution rates, surface orientation and surface dynamics, which suggests that the dissolution rate of a specific mineral phase is not constant but varies with dissolution time and surface structure. This variability needs to be considered in the evaluation of experimentally derived dissolution rates, future dissolution experiments, and predictive kinetic models of dissolution.

  • 321.
    Godinho, José R. A.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Piazolo, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stennett, Martin C.
    Hyatt, Neil C.
    Sintering of CaF2 pellets as nuclear fuel analog for surface stability experiments2011In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 419, no 1-3, p. 46-51Article in journal (Refereed)
    Abstract [en]

    To enable a detailed study of the influence of microstructure and surface properties on the stability of spent nuclear fuel, it is necessary to produce analogs that closely resemble nuclear fuel in terms of crystallography and microstructure. One such analog can be obtained by sintering CaF2 powder.

    This paper reports the microstructures obtained after sintering CaF2 powders at temperatures up to 1240 °C. Pellets with microstructure, density and pore structure similar to that of UO2 spent nuclear fuel pellets were obtained in the temperature range between 900 °C and 1000 °C. When CaF2 was sintered above 1100 °C the formation of CaO at the grain boundaries caused the disintegration of the pellet due to hydration occurring after sintering.

    First results from a novel set-up of dissolution experiments show that changes in roughness, dissolution rate and etch pit shape of fluorite surfaces are strongly dependent on the crystallographic orientation of the expose surface. Consequently, the differences observed for each orientation will affect the overall dissolution rate and will lead to uncertainties in the estimation of dissolution rates of spent nuclear fuel.

  • 322.
    Godinho, José Ricardo Assunção
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    A surface approach to understanding the dissolution of fluorite type materials: Implications for mineral dissolution kinetic models2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Traditional dissolution models are based in the analyses of bulk solution compositions and ignore the fact that different sites of a surface dissolve at different rates. Consequently, the variation of surface area and surface reactivity during dissolution are not considered for the calculation of the overall dissolution rate, which is expected to remain constant with time. The results presented here show the limitations of this approach suggesting that dissolution rates should be calculated as a function of an overall surface reactivity term that accounts for the reactivity of each of the sites that constitute the surface. In contrast to previous studies, here the focus is put on studying the surface at different dissolution times. Significant changes in surface topography of CaF2 were observed during the initial seconds and up to 3200 hours of dissolution. The observed changes include the increase of surface area and progressive exposure of the most stable planes, with consequent decrease in overall reactivity of the surface. The novelty of a proposed dissolution model for fluorite surfaces, when compared with traditional dissolution models, is that it differentiates the reactivity of each characteristic site on a surface, e.g. plane or step edge, and considers the time dynamics. The time dependency of dissolution rates is a major factor of uncertainty when calculating long term dissolution rates using equations derived from dissolution experiments running for short periods of time and using materials with different surface properties. An additional factor of uncertainty is that the initial dissolution times are the most dynamic periods of dissolution, when significant variations of surface area and reactivity occur. The results are expected to have impact in the field of nuclear waste management and to the larger geological and material science community.

  • 323. Goldhammer, Tobias
    et al.
    Brüchert, Volker
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ferdelman, Timothy G.
    Zabel, Matthias
    Microbial sequestration of phosphorus in anoxic upwelling sediments2010In: Nature geoscience, ISSN 1752-0894, Vol. 3, no 8, p. 557-561Article in journal (Refereed)
    Abstract [en]

    Phosphorus is an essential nutrient for life. In the ocean, phosphorus burial regulates marine primary production(1,2). Phosphorus is removed from the ocean by sedimentation of organic matter, and the subsequent conversion of organic phosphorus to phosphate minerals such as apatite, and ultimately phosphorite deposits(3,4). Bacteria are thought to mediate these processes(5), but the mechanism of sequestration has remained unclear. Here, we present results from laboratory incubations in which we labelled organic-rich sediments from the Benguela upwelling system, Namibia, with a P-33-radiotracer, and tracked the fate of the phosphorus. We show that under both anoxic and oxic conditions, large sulphide-oxidizing bacteria accumulate P-33 in their cells, and catalyse the nearly instantaneous conversion of phosphate to apatite. Apatite formation was greatest under anoxic conditions. Nutrient analyses of Namibian upwelling waters and sediments suggest that the rate of phosphate-to-apatite conversion beneath anoxic bottom waters exceeds the rate of phosphorus release during organic matter mineralization in the upper sediment layers. We suggest that bacterial apatite formation is a significant phosphorus sink under anoxic bottom-water conditions. Expanding oxygen minimum zones are projected in simulations of future climate change(6), potentially increasing sequestration of marine phosphate, and restricting marine productivity.

  • 324.
    Goodfellow, Bradley W.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stanford University, USA; Lund University, Sweden.
    Hilley, George E.
    Webb, Samuel M.
    Sklar, Leonard S.
    Moon, Seulgi
    Olson, Christopher A.
    The chemical, mechanical, and hydrological evolution of weathering granitoid2016In: Journal of Geophysical Research - Earth Surface, ISSN 2169-9003, E-ISSN 2169-9011, Vol. 121, no 8, p. 1410-1435Article in journal (Refereed)
    Abstract [en]

    Surprisingly few studies connect the chemical, mechanical, and hydrological evolution of rock as it weathers to saprolite and soil. We assess this coevolution in granodiorite from Monterey Peninsula, California, by measuring changes in bulk chemistry, mineralogy, volumetric strain, the oxidation state of Fe in biotite crystals, tensile strength, abrasion rate, connected porosity, and hydraulic conductivity in samples covering a range of weathering grades. We identify the oxidative dissolution of biotite as the key chemical reaction because of the volumetric expansion that accompanies formation of altered biotite and precipitation of ferrihydrite. We show how the associated accumulation of elastic strain produces an energy density that is sufficient to support rock fracturing over length scales equivalent to constituent crystals. The resulting intragranular and intergranular cracking profoundly reduces tensile strength and increases the abrasion rate, connected porosity, and hydraulic conductivity of the rock matrix. These changes increase the rate of plagioclase weathering, and ultimately the rock disintegrates into grus and clay. Major changes in rock properties can occur with only minor element leaching, and the threshold behavior of weathering that arises from the coevolution of chemical, hydrological, and mechanical properties may be difficult to capture using simplified weathering models that fail to incorporate these properties. Our results, which combine the mechanical and hydrological evolution of weathering rock with more common measurements of chemical changes, should help to more accurately model the effects of, and mechanical and hydrological feedbacks upon, chemical weathering of rock.

  • 325.
    Goodfellow, Bradley W.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Stanford University, USA.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Martel, Stephen J.
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jansson, Krister N.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hättestrand, Clas
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Controls of tor formation, Cairngorm Mountains, Scotland2014In: Journal Of Geophysical Research: Earth Surface, ISSN 2169-9003, Vol. 119, no 2, p. 225-246Article in journal (Refereed)
    Abstract [en]

    Tors occur in many granitic landscapes and provide opportunities to better understand differential weathering. We assess tor formation in the Cairngorm Mountains, Scotland, by examining correlation of tor location and size with grain size and the spacing of steeply dipping joints. We infer a control on these relationships and explore its potential broader significance for differential weathering and tor formation. We also assess the relationship between the formation of subhorizontal joints in many tors and local topographic shape by evaluating principle surface curvatures from a digital elevation model of the Cairngorms. We then explore the implications of these joints for tor formation. We conclude that the Cairngorm tors have formed in kernels of relatively coarse grained granite. Tor volumes increase with grain size and the spacing of steeply dipping joints. We infer that the steeply dipping joints largely formed during pluton cooling and are more widely spaced in tor kernels because of slower cooling rates. Preferential tor formation in coarser granite with a wider joint spacing that is more easily grusified indicates that joint spacing is a dominant control on differential weathering. Sheet jointing is well developed in tors located on relatively high convex surfaces. This jointing formed after the gross topography of the Cairngorms was established and before tor emergence. The presence of closely spaced (tens of centimeters), subhorizontal sheeting joints in tors indicates that these tors, and similarly sheeted tors elsewhere, formed either after subaerial exposure of bedrock or have progressively emerged from a regolith only a few meters thick. Key Points <list list-type=bulleted id=jgrf20195-list-0001> <list-item id=jgrf20195-li-0001>Tors form in kernels of coarse-grained granite among finer-grained granite <list-item id=jgrf20195-li-0002>Wide joint spacing in tors attributable to a slow cooling rate of the granite <list-item id=jgrf20195-li-0003>Sheet jointing discounts tor formation within a thick regolith

  • 326.
    Goodfellow, Bradley W.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Lund University, Sweden.
    Viola, Giulio
    Bingen, Bernard
    Nuriel, Perach
    Kylander-Clark, Andrew R. C.
    Palaeocene faulting in SE Sweden from U-Pb dating of slickenfibre calcite2017In: Terra Nova, ISSN 0954-4879, E-ISSN 1365-3121, Vol. 29, no 5, p. 321-328Article in journal (Refereed)
    Abstract [en]

    Estimating the timing of faulting is crucial to modelling tectonics, palaeoseismicity, landscape evolution and fault mechanics. Four slickenfibre calcite samples from a conjugate strike-slip fault set in a platformal limestone, SE Sweden, were dated using U-Pb. Three of the samples yielded an average age of 64.8 +/- 6.5Ma, while the fourth yielded a marginally younger age of 54.7 +/- 5.5Ma. Precipitation of the fibres is interpreted as syn-deformational. Age uncertainty and dispersion reflect incorporation of common Pb and tiny host-rock components into the dated calcite and/or possible fault reactivation through ca. 55Ma. We infer from crystal characteristics, stable isotopes (O-18 and C-13) and rare-earth elements that fibres formed in an environment rich in deep-seated fluids, at temperatures of 40-200 degrees C, with shear stresses exceeding 10 MPa and at a maximum burial depth of c. 4km. This Palaeocene faulting may reflect far-field stresses from shortening in the Alps.

  • 327. Goodrich, Jordan P.
    et al.
    Varner, Ruth K.
    Frolking, Steve
    Duncan, Bryan N.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    High-frequency measurements of methane ebullition over a growing season at a temperate peatland site2011In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 38, p. L07404-Article in journal (Refereed)
    Abstract [en]

    Bubbles can contribute a significant fraction of methane emissions from wetlands; however the range of reported fractions is very large and accurate characterization of this pathway has proven difficult. Here we show that continuous automated flux chambers combined with an integrated cavity output spectroscopy (ICOS) instrument allow us to quantify both CH(4) ebullition rate and magnitude. For a temperate poor fen in 2009, ebullition rate varied on hourly to seasonal time scales. A diel pattern in ebullition was identified with peak release occurring between 20:00 and 06:00 local time, though steady fluxes (i.e., those with a linear increase in chamber headspace CH(4) concentration) did not exhibit diel variability. Seasonal mean ebullition rates peaked at 843.5 +/- 384.2 events m(-2) d(-1) during the summer, with a mean magnitude of 0.19 mg CH(4) released in each event.

  • 328. Gottlieb, Erik S.
    et al.
    Pease, Victoria
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Miller, Elizabeth L.
    Akinin, Vyacheslav V.
    Neoproterozoic basement history of Wrangel Island and Arctic Chukotka: integrated insights from zircon U–Pb, O and Hf isotopic studies2018In: Geological Society Special Publication, ISSN 0305-8719, E-ISSN 2041-4927, Vol. 460, no 1, article id 183Article in journal (Refereed)
    Abstract [en]

    The pre-Cenozoic kinematic and tectonic history of the Arctic Alaska Chukotka (AAC) terrane is not well known. The difficulties in assessing the history of the AAC terrane are predominantly due to a lack of comprehensive knowledge about the composition and age of its basement. During the Mesozoic, the AAC terrane was involved in crustal shortening, followed by magmatism and extension with localized high-grade metamorphism and partial melting, all of which obscured its pre-orogenic geological relationships. New zircon geochronology and isotope geochemistry results from Wrangel Island and western Chukotka basement rocks establish and strengthen intra- and inter-terrane lithological and tectonic correlations of the AAC terrane. Zircon U–Pb ages of five granitic and one volcanic sample from greenschist facies rocks on Wrangel Island range between 620±6 and 711±4 Ma, whereas two samples from the migmatitic basement of the Velitkenay massif near the Arctic coast of Chukotka yield 612±7 and 661±11 Ma ages. The age spectrum (0.95–2.0 Ga with a peak at 1.1 Ga and minor 2.5–2.7 Ga) and trace element geochemistry of inherited detrital zircons in a 703±5 Ma granodiorite on Wrangel Island suggests a Grenville–Sveconorwegian provenance for metasedimentary strata in the Wrangel Complex basement and correlates with the detrital zircon spectra of strata from Arctic Alaska and Pearya. Temporal patterns of zircon inheritance and O–Hf isotopes are consistent with Cryogenian–Ediacaran AAC magmatism in a peripheral/external orogenic setting (i.e. a fringing arc on rifted continental margin crust).

  • 329. Graham, Alastair G. C.
    et al.
    Dutrieux, Pierre
    Vaughan, David G.
    Nitsche, Frank O.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Greenwood, Sarah L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Larter, Robert D.
    Jenkins, Adrian
    Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier2013In: Journal of Geophysical Research: Earth Surface, ISSN 2169-9011, Vol. 118, no 3, p. 1356-1366Article in journal (Refereed)
    Abstract [en]

    [1] Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed “corrugations” (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high-resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long-term change in Pine Island Glacier. Corrugations and ploughmarks on a sub-ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed-shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a “stable” grounding-line configuration throughout the Holocene.

  • 330.
    Graham, Robert M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The Location and Variability of Southern ocean Fronts2013Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. Moreover, the Subtropical Front (STF) is believed play a key role in the global climate system. Model simulations have shown that a wind induced poleward shift of the STF may strengthen the Atlantic Meridional Overturning Circulation by allowing a stronger salt flux from the Indian to the Atlantic Ocean. This hypothesis has important implications for our future climate, as global warming scenarios predict an intensification and southward shift of the Southern Hemisphere Westerlies. Nonetheless, confirmation of the theory has been limited by a lack of data and also our poor dynamical understanding of fronts. In this thesis we produce a new working dynamical definition of the STF and study the relation of this and other Southern Ocean fronts to the winds and topography.

    We first explore the relative importance of bottom topography and winds for determining the location and structure of Southern Ocean fronts, using 100 years of a control and climate change simulation on the high resolution coupled climate model HiGEM. Topography has primary control on the number and intensity of fronts at each longitude. However, there is no strong relationship between the position or spacing of jets and underlying topographic gradients because of the effects of upstream and downstream topography. The Southern Hemisphere Westerlies intensify and shift south by 1.3° in the climate change simulation, but there is no comparable meridional displacement of the Antarctic Circumpolar Current’s (ACC) path or the fronts within its boundaries, even over flat topography. Instead, the current contracts meridionally and weakens. North of the ACC, the STF shifts south gradually, even over steep topographic ridges. We suggest the STF reacts more strongly to the wind shift because it is strongly surface intensified. In contrast, fronts within the ACC are more barotropic and are therefore more sensitive to the underlying topography.

    We then use satellite sea surface temperature (SST) data to show that the traditional STF, as defined by water mass properties, is comprised of two distinct dynamical regimes. On the western side of each basin the traditional STF coincides with a deep current that has strong SST gradients and no seasonal cycle. We define this as the Dynamical STF (DSTF). Further east, the DSTF diverges from the traditional STF and tracks south-eastwards into the centre of each basin to merge with the Sub-Antarctic Front. The traditional STF continues to the eastern side of the basins where it coincides with the so-called Subtropical Frontal Zone, a zone of shallow SST fronts that have little transport and large seasonal cycles.

    Finally, we compare the position of our DSTF and previous STF climatologies to the mean wind stress curl field, from satellite scatterometry winds. We find that contrary to previous suggestions, the position of the STF does not coincide with the zero or maximum wind stress curl. Using output from the HiGEM model we show that instead of being controlled purely by the wind field, transport south of the subtropical gyre, including the latitude of the zero wind stress curl, is forced strongly by the bottom pressure torque that is a product of the interaction of the ACC with the ocean floor topography.

    Here in these studies we have provided a new simple and reproducible method for identifying fronts. We have also given new insights into the seasonal and decadal variability of fronts, as well as how fronts may respond to future climate change. This has highlighted previous misconceptions regarding the relationship between the position of fronts and winds. Finally we have provided a new framework to study the behaviour of the STF and interpret observations, paving the way for better predictions on the likelihood and impact of future STF changes.

  • 331.
    Graham, Robert M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The role of Southern Ocean fronts in the global climate system2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. However, until recently fronts have been poorly resolved by available data and climate models. In this thesis we utilise a combination of high resolution satellite data, model output and ARGO data to improve our basic understanding of fronts.

    A method is derived whereby fronts are identified as local maxima in sea surface height gradients. In this way fronts are defined locally as jets, rather than continuous-circumpolar water mass boundaries. A new climatology of Southern Ocean fronts is presented. This climatology reveals a new interpretation of the Subtropical Front. The currents associated with the Subtropical Front correspond to the western boundary current extensions from each basin, and we name these the Dynamical Subtropical Front. Previous studies have instead suggested that the Subtropical Front is a continuous feature across the Southern Ocean associated with the super gyre boundary.

    A comprehensive assessment of the relationship between front locations and wind stress is conducted. Firstly, the response of fronts to a southward shift in the westerly winds is tested using output from a 100 year climate change simulation on a high resolution coupled model. It is shown that there was no change in the location of fronts within the Antarctic Circumpolar Current as a result of a 1.3° southward shift in the westerly winds. Secondly, it is shown that the climatological position of the Subtropical Front is 5-10° north of the zero wind stress curl line, despite many studies assuming that the location of the Subtropical Front is determined by the zero wind stress curl.

    Finally, we show that the nutrient supply at ocean fronts is primarily due to horizontal advection and not upwelling. Nutrients from coastal regions are entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front. 

  • 332.
    Graham, Robert M.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    De Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The Dynamical Subtropical Front2013In: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 118, no 10, p. 5676-5685Article in journal (Refereed)
    Abstract [en]

    The Southern Ocean Subtropical Front (STF) is thought to play a key role in the global climate system. Theory suggests that the latitude of the STF regulates the volume of saline Agulhas Leakage into the Atlantic Ocean from the Indian. Here we use satellite sea surface temperature (SST) and sea surface height (SSH) data to study the physical characteristics of the STF water mass boundary. We find that the strong currents in this region do not align with the surface water mass boundary. Therefore, we provide a new climatology for these currents which we define as the Dynamical STF (DSTF). The DSTF is the eastward extension of the western boundary current in each basin and is characterized by strong SST and SSH gradients and no seasonal cycle. At the center of each basin it merges with the Sub-Antarctic Front. On the eastern side of basins, the STF surface water mass boundary coincides with a separate region of multiple SST fronts. We call this the Subtropical Frontal Zone (STFZ). The fronts in the STFZ have a large seasonal cycle and no SSH signature. Despite lying close to the same water mass boundary, the DSTF and STFZ are completely unrelated. We therefore suggest the term STF only be used when referring to the surface water mass boundary. When studying the strong currents on the western side of basins the term DSTF is more relevant and, similarly, the term STFZ better describes the region of enhanced SST gradients towards the east.

  • 333.
    Graham, Robert M.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of East Anglia, UK.
    de Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Heywood, Karen J.
    Chapman, Mark R.
    Stevens, David P.
    Southern Ocean fronts: Controlled by wind or topography?2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, article id C08018Article in journal (Refereed)
    Abstract [en]

    The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. Here we explore the relative importance of bottom topography and winds for the location of Southern Ocean fronts, using 100 years of a control and climate change simulation from the high resolution coupled climate model HiGEM. Topography has primary control on the number and intensity of fronts at each longitude. However, there is no strong relationship between the position or spacing of jets and underlying topographic gradients because of the effects of upstream and downstream topography. The Southern Hemisphere Westerlies intensify and shift south by 1.3 degrees in the climate change simulation, but there is no comparable meridional displacement of the Antarctic Circumpolar Current's (ACC) path or the fronts within its boundaries, even over flat topography. Instead, the current contracts meridionally and weakens. North of the ACC, the Subtropical Front (STF) shifts south gradually, even over steep topographic ridges. We suggest the STF reacts more strongly to the wind shift because it is strongly surface intensified. In contrast, fronts within the ACC are more barotropic and are therefore more sensitive to the underlying topography. An assessment of different methods for identifying jets reveals that maxima of gradients in the sea surface height field are the most reliable. Approximating the position of fronts using sea surface temperature gradients is ineffective at high latitudes while using sea surface height contours can give misleading results when studying the temporal variability of front locations.

  • 334.
    Graham, Robert M.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    De Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kohfeld, Karen E.
    Schlosser, Christian
    Identifying sources and transport pathways of iron in the Southern OceanIn: Deep Sea Research Part I: Oceanographic Research Papers, ISSN 0967-0637, E-ISSN 1879-0119Article in journal (Refereed)
    Abstract [en]

    Over large regions of the global ocean primary productivity is limited by the availability of dissolved iron. Changes in the supply of iron to these regions could have major impacts on primary productivity and the carbon cycle. One of the largest sources of dissolved iron to the ocean is thought to be from shelf sediments, and this source is often parameterized in biogeochemical models as a depth dependent iron flux through the seafloor. Using the knowledge that Southern Ocean surface waters are iron limited, we infer source regions of iron to the Southern Ocean by identifying where the most intense chlorophyll blooms develop. We further derive surface current patterns from satellite sea surface height fields to assess the role of the ocean circulation in transporting iron away from these source regions. We find a tight relationship between satellite chlorophyll concentrations and sea surface height. Large chlorophyll blooms develop on the shelf and where the western boundary currents detach from the continental shelves and turn eastward into the Southern Ocean. This is likely due to shelf supplied iron becoming entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front. The most intense chlorophyll blooms are located along coastal margins of islands and continents. Blooms do not develop over submerged seamounts or plateaus in the open ocean. This suggests that shelf sediments in coastal regions act as large bioavailable iron sources to the Southern Ocean. We recommend that a more accurate method of parameterizing the shelf sediment iron flux could be to prescribe this flux only through grid cells neighboring coastlines. Finally, we hypothesize how changes in sea level during glacial-interglacial cycles may have altered the distribution of shelf sediment iron sources in the Southern Ocean and helped to drive export production anomalies in the Sub-Antarctic Zone.

  • 335.
    Graham, Robert M.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    De Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    van Sebille, Erik
    Kohfeld, Karen E.
    Schlosser, Christian
    Inferring source regions and supply mechanisms of iron in the Southern Ocean from satellite chlorophyll data2015In: Deep Sea Research Part I: Oceanographic Research Papers, ISSN 0967-0637, E-ISSN 1879-0119, Vol. 104, p. 9-25Article in journal (Refereed)
    Abstract [en]

    Primary productivity is limited by the availability of iron over large areas of the global ocean. Changes in the supply of iron to these regions could have major impacts on primary productivity and the carbon cycle. However, source regions and supply mechanisms of iron to the global oceans remain poorly constrained. Shelf sediments are considered one of the largest sources of dissolved iron to the global ocean, and a large shelf sediment iron flux is prescribed in many biogeochemical models over all areas of bathymetry shallower than 1000 m. Here, we infer the likely location of shelf sediment iron sources in the Southern Ocean, by identifying where satellite chlorophyll concentrations are enhanced over shallow bathymetry (< 1000 m). We further compare chlorophyll concentrations with the position of ocean fronts, to assess the relative role of horizontal advection and upwelling for supplying iron to the ocean surface. We show that mean annual chlorophyll concentrations are not visibly enhanced over areas of shallow bathymetry that are located more than 500 km from a coastline. Mean annual chlorophyll concentrations > 2 mg m(-3) are only found within 50 km of a continental or island coastline. These results suggest that sedimentary iron sources only exist on continental and island shelves. Large sedimentary iron fluxes do not seem present on seamounts and submerged plateaus. Large chlorophyll blooms develop where the western boundary currents detach from the continental shelves, and turn eastward into the Sub-Antarctic Zone. Chlorophyll concentrations are enhanced along contours of sea surface height extending off the continental shelves, as shown by the trajectories of virtual water parcels in satellite altimetry data. These analyses support the hypothesis that bioavailable iron from continental shelves is entrained into western boundary currents, and advected into the Sub-Antarctic Zone along the Dynamical Subtropical Front. Our results indicate that upwelling at fronts in the open ocean is unlikely to deliver iron to the ocean surface from deep sources. Finally, we hypothesise how a reduction in sea level may have altered the distribution of shelf sediment iron sources in the Southern Ocean and increased export production over the Sub-Antarctic Zone during glacial intervals.

  • 336.
    Granberg, Therese
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mineralogy and grain size of eolian sediments from the Shatsky Rise, NW Pacific Ocean: mineralogy as proxy for eolian record comparisons2015Independent thesis Advanced level (degree of Master (Two Years)), 40 credits / 60 HE creditsStudent thesis
  • 337.
    Greenwood, Sarah L.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Clason, Caroline C.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Helanow, Christian
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Margold, Martin
    Stockholm University, Faculty of Science, Department of Physical Geography. Durham University, UK.
    Theoretical, contemporary observational and palaeo-perspectives on ice sheet hydrology: Processes and products2016In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 155, p. 1-27Article in journal (Refereed)
    Abstract [en]

    Meltwater drainage through ice sheets has recently been a key focus of glaciological research due to its influence on the dynamics of ice sheets in a warming climate. However, the processes, topologies and products of ice sheet hydrology are some of the least understood components of both past and modem ice sheets. This is to some extent a result of a disconnect between the fields of theoretical, contemporary observational and palaeo-glaciology that each approach ice sheet hydrology from a different perspective and with different research objectives. With an increasing realisation of the potential of using the past to inform on the future of contemporary ice sheets, bridging the gaps in the understanding of ice sheet hydrology has become paramount. Here, we review the current state of knowledge about ice sheet hydrology from the perspectives of theoretical, observational and palaeo-glaciology. We then explore and discuss some of the key questions in understanding and interpretation between these research fields, including: 1) disagreement between the palaeo-record, glaciological theory and contemporary observations in the operational extent of channelised subglacial drainage and the topology of drainage systems; 2) uncertainty over the magnitude and frequency of drainage events associated with geomorphic activity; and 3) contrasts in scale between the three fields of research, both in a spatial and temporal context The main concluding points are that modem observations, modelling experiments and inferences from the palaeo-record indicate that drainage topologies may comprise a multiplicity of forms in an amalgam of drainage modes occurring in different contexts and at different scales. Drainage under high pressure appears to dominate at ice sheet scale and might in some cases be considered efficient; the sustainability of a particular drainage mode is governed primarily by the stability of discharge. To gain better understanding of meltwater drainage under thick ice, determining what drainage topologies are reached under high pressure conditions is of primary importance. Our review attests that the interconnectivity between research sub-disciplines in progressing the field is essential, both in interpreting the palaeo-record and in developing physical understanding of glacial hydrological processes and systems.

  • 338.
    Greenwood, Sarah L.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Clason, Caroline C.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Mikko, Henrik
    Nyberg, Johan
    Peterson, Gustaf
    Smith, Colby A.
    Integrated use of LiDAR and multibeam bathymetry reveals onset of ice streaming in the northern Bothnian Sea2015In: GFF, ISSN 1103-5897, E-ISSN 2000-0863, Vol. 137, no 4, p. 284-292Article in journal (Refereed)
    Abstract [en]

    Geomorphological mapping from the new LiDAR (Light Detection and Ranging)-derived digital elevation model for Sweden and a high-resolution multibeam bathymetry data-set for the Gulf of Bothnia reveals a continuous system of glacial landforms crossing the transition between the modern terrestrial and marine environments. A palaeo-ice stream in the northern Bothnian Sea is reconstructed, with an onset tributary over the present-day angstrom ngermanland-Vasterbotten coastline. Systematic contrasts in landform morphology and lineation length indicate that this ice stream comprised a relatively narrow (approximate to 40km) corridor of fast flow, flowing first SW then S, and likely fed by converging flow around the upper Bothnian Sea. The geometry and landform associations of this system imply that ice, at the time period represented here, did not flow across the Gulf of Bothnia: SSE-ward ice flow indicators on the northern Swedish coast do not correspond directly with landform assemblages of the large SE-oriented Finnish deglacial lobes. Instead, we suggest they may contribute to a late-stage fast-flow event to the S and SW. Multibeam bathymetry data offer entirely new access into the rich, landform-scale geomorphological record on the seafloor of the Gulf of Bothnia. The combination of offshore multibeam with the new terrestrial LiDAR data provides unprecedented insight into and renewed understanding of the glacial dynamics of the Bothnian Sea sector of the Fennoscandian Ice Sheet, hitherto interpreted over large areas of unmapped ice sheet bed.

  • 339.
    Greenwood, Sarah L.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Clason, Caroline C.
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Plymouth, UK.
    Nyberg, Johan
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Holmlund, Per
    Stockholm University, Faculty of Science, Department of Physical Geography.
    The Bothnian Sea ice stream: early Holocene retreat dynamics of the south-central Fennoscandian Ice Sheet2017In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 46, no 2, p. 346-362Article in journal (Refereed)
    Abstract [en]

    The Gulf of Bothnia hosted a variety of palaeo-glaciodynamic environments throughout the growth and decay of the last Fennoscandian Ice Sheet, from the main ice-sheet divide to a major corridor of marine-and lacus-trine-based deglaciation. Ice streaming through the Bothnian and Baltic basins has been widely assumed, and the damming and drainage of the huge proglacial Baltic Ice Lake has been implicated in major regional and hemispheric climate changes. However, the dynamics of palaeo-ice flow and retreat in this large marine sector have until now been inferred only indirectly, from terrestrial, peripheral evidence. Recent acquisition of high-resolution multibeam bathymetry opens these basins up, for the first time, to direct investigation of their glacial footprint and palaeo-ice sheet behaviour. Here we report on a rich glacial landform record: in particular, a palaeo-ice stream pathway, abundant traces of high subglacial meltwater volumes, and widespread basal crevasse squeeze ridges. The Bothnian Sea ice stream is a narrow flow corridor that was directed southward through the basin to a terminal zone in the south-central Bothnian Sea. It was activated after initial margin retreat across the Aland sill and into the Bothnian basin, and the exclusive association of the ice-stream pathway with crevasse squeeze ridges leads us to interpret a short-lived stream event, under high extension, followed by rapid crevasse-triggered break-up. We link this event with a c. 150-year ice-rafted debris signal in peripheral varved records, at c. 10.67 cal. ka BP. Furthermore, the extensive glacifluvial system throughout the Bothnian Sea calls for considerable input of surface meltwater. We interpret strongly atmospherically driven retreat of this marine-based ice-sheet sector.

  • 340.
    Greenwood, Sarah L.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Anderson, John B.
    Ice-flow switching and East/West Antarctic Ice Sheet roles in glaciation of the western Ross Sea2012In: Geological Society of America Bulletin, ISSN 0016-7606, E-ISSN 1943-2674, Vol. 124, no 11-12, p. 1736-1749Article in journal (Refereed)
    Abstract [en]

    The long-term behavior of the East and West Antarctic Ice Sheets, and their respective responses to forcing provide essential context for assessment of modern dynamic changes in ice-flow regimes and ice-sheet and shelf margins. The western Ross Sea discharges ice from both the East and West Antarctic Ice Sheets, and the paleoglacial record from this region is therefore valuable in unraveling their long-term behavior. New, high-resolution multibeam bathymetric data reveal snapshots of well-preserved glacial landforms on the seafloor around Ross Island and McMurdo Sound. Glacial lineations, grounding zone wedges, draped recessional moraines, and meltwater channels record a series of different ice-flow events in the region, contradictions between which require major phases of ice-flow reorganization. From the glacial geomorphology, we reconstruct a four-stage model of ice-flow evolution for the last glacial cycle, consisting of: (1) northeastward flow into the Ross Sea from McMurdo Sound; (2) westward flow from the Ross Sea, around Ross Island, and onto the Victoria Land coast and coastal seafloor trough; (3) a deglacial phase of ice-sheet thinning, minor shifts in flow, and grounding line retreat into McMurdo Sound; and (4) grounding line pinning on Ross Island during regional retreat, uncoupling of a remnant Ross Island ice cap, and local oscillation of Victoria Land outlet glaciers. We find that East Antarctic Ice Sheet ice discharge had a strong influence on ice-flow geometry in this part of the Ross Sea during the last glacial stage, but that it was not necessarily in phase with the behavior of the West Antarctic Ice Sheet. It is similarly evident that the ice streams that drained the Ross Sea over the continental shelf at the Last Glacial Maximum did not all operate synchronously, and exerted different drawdown power at different times. Finally, we conclude that Ross Island acts as an important pinning point in the Ross Sea ice-sheet-shelf system, stabilizing grounding line retreat and encouraging lasting ice-shelf development.

  • 341.
    Greenwood, Sarah L.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kleman, Johan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Glacial landforms of extreme size in the Keewatin sector of the Laurentide Ice Sheet2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 15-16, p. 1894-1910Article in journal (Refereed)
    Abstract [en]

    Assemblages of glacial landforms of a 'mega-scale' are here identified in the Keewatin sector of the Laurentide Ice Sheet. Large till 'belts' or 'ridges', apparent only on satellite imagery and lying beneath the drumlins, flutes and ribbed moraine which comprise the known regional glacial landform record, form extensive and coherent patterns throughout the Keewatin region. Planform and crestline mapping from remotely sensed imagery yields a mapped population of >2500 individual landforms, whose dimensions are on average similar to 10 km long and similar to 1.5 km wide. Based on analysis of their morphology and morphometry, their spatial arrangement and pattern, and comparison with analogues and reference populations of glacial landform types, we interpret three morphological groups of different genetic origin. Two of these are examples of currently known landform types: i) a set of heavily overprinted, i.e. non-pristine, mega-scale glacial lineations, feeding from the heart of the Keewatin region north into Queen Maud Gulf; and ii) a 350 km long moraine zone, overrun by later ice flow paths, and likely associated with the terminal position of an ice sheet prior to the final deglacial episode. A third group, comprising a significant number of the Keewatin population, does not fit any existing category of glacial landforms. Here we report a major new finding: subglacial bedforms, of a mega-scale, transverse to the palaeo-ice flow direction. Mega-scale transverse bedforms have not been previously reported from any palaeo-(or contemporary) ice sheet. Close spatial integration with the ribbed moraine population in Keewatin suggests a similar mode of genesis. The Keewatin landforms indicate there is a fundamental transverse organisation of till at a scale beyond that of conventional transverse bedforms (ribbed moraine), and with as yet unknown implications for our understanding of subglacial processes and ice-bed coupling.

  • 342.
    Greenwood, Sarah L.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Swärd, Henrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Flodén, Tom
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ananyev, Roman
    Chernykh, Denis
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Multiple re-advances of a Lake Vättern outlet glacier during Fennoscandian Ice Sheet retreat, south-central Sweden2015In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 44, no 4, p. 619-637Article in journal (Refereed)
    Abstract [en]

    Lake Vättern represents a critical region geographically and dynamically in the deglaciation of the Fennoscandian Ice Sheet. The outlet glacier that occupied the basin and its behaviour during ice-sheet retreat were key to the development and drainage of the Baltic Ice Lake, dammed just west of the basin, yet its geometry, extent, thickness, margin dynamics, timing and sensitivity to regional retreat forcing are rather poorly known. The submerged sediment archives of Lake Vättern represent a missing component of the regional Swedish deglaciation history. Newly collected geophysical data, including high-resolution multibeam bathymetry of the lake floor and seismic reflection profiles of southern Lake Vättern, are used here together with a unique 74-m sediment record recently acquired by drill coring, and with onshore LiDAR-based geomorphological analysis, to investigate the deglacial environments and dynamics in the basin and its terrestrial environs. Five stratigraphical units comprise a thick subglacial package attributed to the last glacial period (and probably earlier), and an overlying > 120-m deglacial sequence. Three distinct retreat-re-advance episodes occurred in southern Lake Vättern between the initial deglaciation and the Younger Dryas. In the most recent of these, ice overrode proglacial lake sediments and re-advanced from north of Visingsö to the southern reaches of the lake, where ice up to 400 m thick encroached on land in a lobate fashion, moulding crag-and-tail lineations and depositing till above earlier glacifluvial sediments. This event precedes the Younger Dryas, which our data reveal was probably restricted to north-central sectors of the basin. These dynamics, and their position within the regional retreat chronology, indicate a highly active ice margin during deglaciation, with retreat rates on average 175 m a(-1). The pronounced topography of the Vättern basin and its deep proglacial-dammed lake are likely to have encouraged the dynamic behaviour of this major Fennoscandian outlet glacier.

  • 343.
    Greenwood, Sarah L.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Simkins, Lauren M.
    Halberstadt, Anna Ruth W.
    Prothro, Lindsay O.
    Anderson, John B.
    Holocene reconfiguration and readvance of the East Antarctic Ice Sheet2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 3176Article in journal (Refereed)
    Abstract [en]

    How ice sheets respond to changes in their grounding line is important in understanding ice sheet vulnerability to climate and ocean changes. The interplay between regional grounding line change and potentially diverse ice flow behaviour of contributing catchments is relevant to an ice sheet's stability and resilience to change. At the last glacial maximum, marine-based ice streams in the western Ross Sea were fed by numerous catchments draining the East Antarctic Ice Sheet. Here we present geomorphological and acoustic stratigraphic evidence of ice sheet reorganisation in the South Victoria Land (SVL) sector of the western Ross Sea. The opening of a grounding line embayment unzipped ice sheet sub-sectors, enabled an ice flow direction change and triggered enhanced flow from SVL outlet glaciers. These relatively small catchments behaved independently of regional grounding line retreat, instead driving an ice sheet readvance that delivered a significant volume of ice to the ocean and was sustained for centuries.

  • 344. Greiling, Reinhard O.
    et al.
    Kathol, Benno
    Kumpulainen, Risto A.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nappe units along the Caledonian margin in central Scandinavia (Grong-Olden to Nasafjallet): definition, distinction criteria and tectonic evolution2018In: GFF, ISSN 1103-5897, E-ISSN 2000-0863, Vol. 140, no 1, p. 66-89Article in journal (Refereed)
    Abstract [en]

    This study reviews lithological and stratigraphic data, new geological mapping results and microscopic work from the lower and middle Caledonian nappes in central Scandinavia, and suggests a model of their tectonic evolution. The Jamtlandian and Blaik Nappes represent the lower nappes, and are overlain, successively, by the Stalon, Danabergen and Slussfors Nappes. The latter occupies the level immediately beneath the Seve Nappe Complex. Farther north, around Lake Gautojaure, the Akerberget Nappe represents the level between the Stalon Nappe and the Ammarnas Complex. Their hangingwall is the Sadenaive Nappe and the overlying Seve Nappe Complex. The Danabergen, Akerberget, Sadenaive and Slussfors Nappes are formally defined here. In the Blaik Nappe, brittle deformation gave rise to composite thrust systems. The overlying nappes show a more ductile deformation. The arkose sequence of the Stalon Nappe shows fine-grained recrystallization of muscovite. In contrast, finely recrystallized mica minerals form a phyllitic foliation in the Akerberget Nappe. The Danabergen Nappe contains small garnet porphyroblasts. The Ammarnas Complex and the Sadenaive Nappes show garnet with a straight internal foliation (S-i), while the external foliation is refolded. This fabric suggests a time interval after early foliation development, in which garnet grew but no deformation occurred, and subsequent deformation. This may represent an episode of arrest of deformational activity and thrust movement. Spiral-shaped S-i in garnets of the Slussfors Nappe indicate a contrasting and more complex metamorphic and structural evolution, perhaps similar to that of the Seve Nappe Complex, with a well-known, polyphase tectonic evolution.

  • 345.
    Griffiths, Jennifer R.
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Kadin, Martina
    Stockholm University, Faculty of Science, Stockholm Resilience Centre.
    Nascimento, Francisco J. A.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Tamelander, Tobias
    Törnroos, Anna
    Bonaglia, Stefano
    Stockholm University, Faculty of Science, Department of Geological Sciences. Lund University, Sweden.
    Bonsdorff, Erik
    Brüchert, Volker
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gårdmark, Anna
    Järnström, Marie
    Kotta, Jonne
    Lindegren, Martin
    Nordström, Marie C.
    Norkko, Alf
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre. University of Helsinki, Finland.
    Olsson, Jens
    Weigel, Benjamin
    Zydelis, Ramunas
    Blenckner, Thorsten
    Stockholm University, Faculty of Science, Stockholm Resilience Centre.
    Niiranen, Susa
    Stockholm University, Faculty of Science, Stockholm Resilience Centre.
    Winder, Monika
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    The importance of benthic-pelagic coupling for marine ecosystem functioning in a changing world2017In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 23, no 6, p. 2179-2196Article, review/survey (Refereed)
    Abstract [en]

    Benthic-pelagic coupling is manifested as the exchange of energy, mass, or nutrients between benthic and pelagic habitats. It plays a prominent role in aquatic ecosystems, and it is crucial to functions from nutrient cycling to energy transfer in food webs. Coastal and estuarine ecosystem structure and function are strongly affected by anthropogenic pressures; however, there are large gaps in our understanding of the responses of inorganic nutrient and organic matter fluxes between benthic habitats and the water column. We illustrate the varied nature of physical and biological benthic-pelagic coupling processes and their potential sensitivity to three anthropogenic pressures - climate change, nutrient loading, and fishing - using the Baltic Sea as a case study and summarize current knowledge on the exchange of inorganic nutrients and organic material between habitats. Traditionally measured benthic-pelagic coupling processes (e.g., nutrient exchange and sedimentation of organic material) are to some extent quantifiable, but the magnitude and variability of biological processes are rarely assessed, preventing quantitative comparisons. Changing oxygen conditions will continue to have widespread effects on the processes that govern inorganic and organic matter exchange among habitats while climate change and nutrient load reductions may have large effects on organic matter sedimentation. Many biological processes (predation, bioturbation) are expected to be sensitive to anthropogenic drivers, but the outcomes for ecosystem function are largely unknown. We emphasize how improved empirical and experimental understanding of benthic-pelagic coupling processes and their variability are necessary to inform models that can quantify the feedbacks among processes and ecosystem responses to a changing world.

  • 346. Gu, Guansheng
    et al.
    Dickens, Gerald R.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rice University, USA.
    Bhatnagar, Gaurav
    Colwell, Frederick S.
    Hirasaki, George J.
    Chapman, Walter G.
    Abundant Early Palaeogene marine gas hydrates despite warm deep-ocean temperatures2011In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 4, p. 848-851Article in journal (Refereed)
    Abstract [en]

    Abrupt periods of global warming between 57 and 50 million years ago—known as the Early Palaeogene hyperthermal events—were associated with the repeated injection of mas- sive amounts of carbon into the atmosphere1–4. The release of methane from the sea floor following the dissociation of gas hydrates is often invoked as a source5. However, seafloor temperatures before the events were at least 4–7 ◦ C higher than today1, which would have limited the area of sea floor suitable for hosting gas hydrates6,7. Palaeogene gas hydrate reservoirs may therefore not have been sufficient to provide a significant fraction of the carbon released. Here we use numer- ical simulations of gas hydrate accumulation8 at Palaeogene seafloor temperatures to show that near-present-day values of gas hydrates could have been hosted in the Palaeogene. Our simulations show that warmer temperatures during the Palaeogene would have enhanced the amount of organic carbon reaching the sea floor as well as the rate of methanogenesis. We find that under plausible temperature and pressure condi- tions, the abundance of gas hydrates would be similar or higher in the Palaeogene than at present. We conclude that methane hydrates could have been an important source of carbon during the Palaeogene hyperthermal events. 

  • 347.
    Guelland, Kathi
    et al.
    Institute of biogeochemistry and pollutant dynamics, ETH Zurich.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Göransson, H.
    Bernasconi, S. M.
    Hajdas, I.
    Kretzschmar, R.
    Evolution of carbon fluxes during initial soil formation along the forefield of Damma glacier, Switzerland2013In: Biogeochemistry, ISSN 0168-2563, E-ISSN 1573-515X, Vol. 113, no 1-3, p. 545-561Article in journal (Refereed)
    Abstract [en]

    Soil carbon (C) fluxes, soil respiration and dissolved organic carbon (DOC) leaching were explored along the young Damma glacier forefield chronosequence (7-128 years) over a three-year period. To gain insight into the sources of soil CO2 effluxes, radiocarbon signatures of respired CO2 were measured and a vegetation-clipping experiment was performed. Our results showed a clear increase in soil CO2 effluxes with increasing site age from 9 +/- A 1 to 160 +/- A 67 g CO2-C m(-2) year(-1), which was linked to soil C accumulation and development of vegetation cover. Seasonal variations of soil respiration were mainly driven by temperature; between 62 and 70 % of annual CO2 effluxes were respired during the 4-month long summer season. Sources of soil CO2 effluxes changed along the glacier forefield. For most recently deglaciated sites, radiocarbon-based age estimates indicated ancient C to be the dominant source of soil-respired CO2. At intermediate site age (58-78 years), the contribution of new plant-fixed C via rhizosphere respiration amounted up to 90 %, while with further soil formation, heterotrophically respired C probably from accumulated 'older' soil organic carbon (SOC) became increasingly important. In comparison with soil respiration, DOC leaching at 10 cm depth was small, but increased similarly from 0.4 +/- A 0.02 to 7.4 +/- A 1.6 g DOC m(-2) year(-1) over the chronosequence. A strong rise of the ratio of SOC to secondary iron and aluminium oxides strongly suggests that increasing DOC leaching with site age results from a faster increase of the DOC source, SOC, than of the DOC sink, reactive mineral surfaces. Overall, C losses from soil by soil respiration and DOC leaching increased from 9 +/- A 1 to 70 +/- A 17 and further to 168 +/- A 68 g C m(-2) year(-1) at the < 10, 58-78, and 110-128 year old sites. By comparison, total ecosystem C stocks increased from 0.2 to 1.1 and to 3.1 kg C m(-2) from the young to intermediate and old sites. Therefore, the ecosystem evolved from a dominance of C accumulation in the initial phase to a high throughput system. We suggest that the relatively strong increase in soil C stocks compared to C fluxes is a characteristic feature of initial soil formation on freshly exposed rocks.

  • 348. Gunnarsson-Robin, Jóhann
    et al.
    Stefánsson, Andri
    Ono, Shuhei
    Torssander, Peter
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sulfur isotopes in Icelandic thermal fluids2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 346, p. 161-179Article in journal (Refereed)
    Abstract [en]

    Multiple sulfur isotope compositions of thermal fluids from Iceland were measured in order to evaluate the sources and reactions of sulfur and sulfur isotope fractionation in geothermal systems at Icelandic divergent plate boundaries, characterized by MORB-like basalts. The geothermal systems studied had a wide range of reservoir temperatures of 56-296 degrees C and Cl concentrations of 18-21,000 ppm. Dissolved sulfide (Sigma S-II) and SO4 concentrations in liquid water measured <0.01-165 ppm and 1.3-300 ppm, respectively, and H2S(g) concentrations in the vapor 4.9-2000 ppm. The delta S-34 and Delta S-33 values for different phases and oxidation states were highly variable: delta S-34 Sigma s(-II) = -11.6 to 10.5 parts per thousand (n = 99), Delta S-33 Sigma s(-II) = -0.12 to 0.00 parts per thousand (n = 45), delta(34)Sso(4) = -1.0 to 24.9 parts per thousand (n = 125), Delta(33)Sso(4) = -0.04 to 0.02 parts per thousand (n = 50), delta(34)SH(2)s(g) = -2.6 to 5.9 parts per thousand (n = 112) and Delta(33)SH(2)s(g) = -0.03 to 0.00 parts per thousand (n = 56). The multiple sulfur isotope values of the thermal fluids are interpreted to reflect various sources of sulfur in the fluids, as well as isotope fractionation occurring within the geothermal systems associated with fluid-rock interaction, boiling and oxidation and reduction reactions. The results of isotope geochemical modeling demonstrate that the sources of S-II in the thermal fluid are leaching of basalt (MORB) and seawater SO4 reduction for saline systems with insignificant magma gas input, and that the observed ranges of delta S-34 and Delta S-33 for Sigma S-II and H2S(g) reflect isotope fractionation between minerals and aqueous and gaseous species upon fluid -rock interaction and boiling. The sources of SO4 are taken to be multiple, including oxidation of S-II originating from basalt, leaching of S-VI from the basalts and the seawater itself in the case of saline systems. In low-temperature fluids, the delta S-34 and Delta S-33 values reflect the various sources of sulfur. For high-temperature fluids, fluid -rock interaction, Sigma S-II oxidation and SO4 reduction and sulfide and sulfate mineral formation result in a large range of delta S-34 and Delta S-33 values for Sigma S-II and SO4 in the fluids, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements like sulfur. Such effects needed to be quantified in order to reveal the various sources of an element.

  • 349.
    Gustafsson, Erik
    et al.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Deutsch, Barbara
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Gustafsson, Bo G.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Carbon cycling in the Baltic Sea - The fate of allochthonous organic carbon and its impact on air-sea CO2 exchange2014In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 129, p. 289-302Article in journal (Refereed)
    Abstract [en]

    A coupled physical-biogeochemical model (BALTSEM) is used to estimate carbon fluxes in the Baltic Sea over the 1980-2006 period. Budget calculations for organic carbon indicate that of the total allochthonous organic carbon (TOCT) supplied to the system, on average 56% is mineralized, 36% is exported out of the system, and the remainder is buried. River discharge is the main source of dissolved inorganic carbon (DIC) to the Baltic Sea. However, model results indicate that in the Gulf of Bothnia (northern Baltic Sea), the contribution to the DIC stock by TOCT mineralization is of the same order as direct river input of DIC In the Kattegat and Danish Straits (southwestern Baltic Sea) on the other hand, net uptake of atmospheric CO2 comprises the major DIC source. Despite large variations within the system, with net outgassing from some sub-basins and net absorption in others, the Baltic Sea as a whole was estimated to be a net sink for atmospheric CO2. Mineralization of allochthonous dissolved organic carbon (DOCT) influences air-sea CO2 exchange. A sensitivity study indicates that depending on the labile fraction of DOCT, the contribution from CO2 absorption to total external DIC sources can amount to 10-25%.

  • 350.
    Gustafsson, Erik
    et al.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre. Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Björk, Göran
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of New Hampshire, USA.
    Andersson, Leif G.
    Geibel, Marc C.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre. Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sundbom, Marcus
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Semiletov, Igor P.
    Thornton, Brett F.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gustafsson, Bo G.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Carbon cycling on the East Siberian Arctic Shelf – a change in air-sea CO2 flux induced by mineralization of terrestrial organic carbon2017In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189Article in journal (Refereed)
    Abstract [en]

    Measurements from the SWERUS-C3 and ISSS-08 Arctic expeditions were used to calibrate and validate a new physical-biogeochemical model developed to quantify key carbon cycling processes on the East Siberian Arctic Shelf (ESAS). The model was used in a series of experimental simulations with the specific aim to investigate the pathways of terrestrial dissolved and particulate organic carbon (DOCter and POCter) supplied to the shelf. Rivers supply on average 8.5 Tg C yr−1 dissolved inorganic carbon (DIC), and further 8.5 and 1.1 Tg C yr−1 DOCter and POCter respectively. Based on observed and simulated DOC concentrations and stable isotope values (δ13CDOC) in shelf waters, we estimate that only some 20 % of the riverine DOCter is labile. According to our model results, an additional supply of approximately 14 Tg C yr−1 eroded labile POCter is however required to describe the observed stable isotope values of DIC (δ13CDIC). Degradation of riverine DOCter and POCter results in a 1.8 Tg C yr−1 reduction in the uptake of atmospheric CO2, while degradation of eroded POCter results in an additional 10 Tg C yr−1 reduction. Our calculations indicate nevertheless that the ESAS is an overall small net sink for atmospheric CO2 (1.7 Tg C yr−1). The external carbon sources are largely compensated by a net export from the shelf to the Arctic Ocean (31 Tg C yr−1), and to a smaller degree by a permanent burial in the sediments (2.7 Tg C yr−1).

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