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  • 301.
    Freire, Francis
    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.
    Initial Results from Seafloor Characterization of Arctic and Antarctic Margins using Multibeam Backscatter2012In: Arctic Palaeoclimate and its Extremes: APEX VI / [ed] Ninna Immonen, Martin Jakobsson, Juha Pekka Lunkka, Kari Strand, Oulu: University of Oulu , 2012, p. 51-51Conference paper (Other academic)
    Abstract [en]

    The seafloor of high-latitude Polar margins is characterized by various submarine glacigenic landforms whose shape and texture were created, and subsequently modified, by ice. These glaciogenic landforms together with deposited seafloor sediments serve as a record of the past glacial history. The Multibeam Echo Sounder (MBES) technology provides a tool to map and study submarine glaciogenic landforms and seafloor texture. MBES bathymetric images have afforded scientists a way to understand many glacial processes such as iceberg movements, advance and retreat patterns of ice sheets, and polar underwater currents, among others. Aside from measuring the bathymetry, MBES systems also record the returned intensity, or backscatter, of the acoustic pulse. Recent developments have shown that the backscatter information can be used to distinguish/classify differences in the surface sediment types. Here we present the preliminary results of an analysis of backscatter data aimed to characterize sediment types at locations of the Arctic and Antarctic margins mapped with Swedish icebreaker Oden and the installed Kongsberg EM122, 12 kHz, deep water MBES. We apply the Angular Range Analysis (ARA) method that is included in the Geocoder Backscatter processing algorithm [Fonseca and Mayer, 2007]. The results are correlated with other geophysical data and core samples to ground-truth the resulting seafloor maps.

  • 302.
    Freire, Francis
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Zingerlersen, Karl
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Paleo-ice stream behavior inferred from cross-shelf troughs and submarine glaciogenic debris flows along the west Greenland continental marginManuscript (preprint) (Other academic)
  • 303.
    Freud, Eyal
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Leaitch, Richard
    Nguyen, Quynh T.
    Massling, Andreas
    Skov, Henrik
    Barrie, Leonard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pan-Arctic aerosol number size distributions: seasonality and transport patterns2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 13, p. 8101-8128Article in journal (Refereed)
    Abstract [en]

    The Arctic environment has an amplified response to global climatic change. It is sensitive to human activities that mostly take place elsewhere. For this study, a multi-year set of observed aerosol number size distributions in the diameter range of 10 to 500 nm from five sites around the Arctic Ocean (Alert, Villum Research Station - Station Nord, Zeppelin, Tiksi and Barrow) was assembled and analysed. A cluster analysis of the aerosol number size distributions revealed four distinct distributions. Together with Lagrangian air parcel back-trajectories, they were used to link the observed aerosol number size distributions with a variety of transport regimes. This analysis yields insight into aerosol dynamics, transport and removal processes, on both an intra- and an inter-monthly scale. For instance, the relative occurrence of aerosol number size distributions that indicate new particle formation (NPF) event is near zero during the dark months, increases gradually to similar to 40% from spring to summer, and then collapses in autumn. Also, the likelihood of Arctic haze aerosols is minimal in summer and peaks in April at all sites. The residence time of accumulation-mode particles in the Arctic troposphere is typically long enough to allow tracking them back to their source regions. Air flow that passes at low altitude over central Siberia and western Russia is associated with relatively high concentrations of accumulation-mode particles (N-acc) at all five sites - often above 150 cm(-3). There are also indications of air descending into the Arctic boundary layer after transport from lower latitudes. The analysis of the back-trajectories together with the meteorological fields along them indicates that the main driver of the Arctic annual cycle of N-acc, on the larger scale, is when atmospheric transport covers the source regions for these particles in the 10-day period preceding the observations in the Arctic. The scavenging of these particles by precipitation is shown to be important on a regional scale and it is most active in summer. Cloud processing is an additional factor that enhances the N-acc annual cycle. There are some consistent differences between the sites that are beyond the year-to-year variability. They are the result of differences in the proximity to the aerosol source regions and to the Arctic Ocean sea-ice edge, as well as in the exposure to free-tropospheric air and in precipitation patterns - to mention a few. Hence, for most purposes, aerosol observations from a single Arctic site cannot represent the entire Arctic region. Therefore, the results presented here are a powerful observational benchmark for evaluation of detailed climate and air chemistry modelling studies of aerosols throughout the vast Arctic region.

  • 304. Fripiat, F.
    et al.
    Declercq, M.
    Sapart, C. J.
    Anderson, L. G.
    Brüchert, Volker
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Deman, F.
    Fonseca-Batista, D.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Roukaerts, A.
    Semiletov, I. P.
    Dehairs, F.
    Influence of the bordering shelves on nutrient distribution in the Arctic halocline inferred from water column nitrate isotopes2018In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 63, no 5, p. 2154-2170Article in journal (Refereed)
    Abstract [en]

    The East Siberian Sea and contiguous western Arctic Ocean basin are characterized by a subsurface nutrient maximum in the halocline, generally attributed to both Pacific inflow and intensive remineralization in shelf bottom waters that are advected into the central basin. We report nitrogen and oxygen isotopic measurement of nitrate from the East Siberian Sea and western Eurasian Basin, in order to gain insight into how nitrate is processed by the microbial community and redistributed in the Arctic Ocean. A large decoupling between nitrate delta N-15 and delta O-18 is reported, increasing and decreasing upward from the Atlantic temperature maximum layer toward the surface, respectively. A correlation between water and nitrate delta O-18 indicates that most of the nitrate (> 60%) at the halocline has been regenerated within the Arctic Ocean. The increase in nitrate delta N-15 correlates with the fixed N deficit, indicating a causal link between the loss of fixed N and the delta N-15 enrichment. This suggests that a significant share of benthic denitrification is driven by nitrate supplied by remineralization and partial nitrification, allowing residual delta N-15-enriched ammonium to diffuse out of the sediments. By increasing nutrient concentrations and fixed N deficit in shelf bottom waters, this imprint is attenuated offshore following advection into the halocline by nitrate regeneration and mixing. Estimation of the sedimentary isotope effect related to benthic denitrification yields values in the range of 2.4-3.8 parts per thousand, with its magnitude driven by both the degree of coupling between remineralization and nitrification, and fixed N concentrations in shelf bottom waters.

  • 305.
    Fritzon, Ruben
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Goodfellow, Bradley
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stroeven, Arjen
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Estholm, Madelene
    Caffee, Marc
    Evaluating geochemical evidence of earthquake periodicity, Sparta Fault, Southern GreeceManuscript (preprint) (Other academic)
    Abstract [en]

    Determining prehistoric earthquake periodicity and magnitudes is important for risk assessments in seismically active areas. We evaluate a geochemical method, which has previously been used to identify prehistoric slips on normal fault scarps through an analysis of variations in the concentration of rare earth elements and Y (REE-Y) along vertical transects. Our study object is the Sparta Fault, a normal fault in southern Greece, developed in limestone and previously documented, and dated using 36Cl, to have been last active 464 BC. From geochemical analyses of 39 fault rock samples, we conclude that REE-Y concentrations correlate strongly with the abundance of quartz and possibly other heterogeneities in the fault scarp. Because the sampled fault rock is a protocataclasite, formed at depth, variations in the abundance of quartz are not associated with prehistoric movements along the fault. We therefore conclude that geochemical evidence does not provide a reliable paleoseismic proxy for fault movement. We also present data indicating a co-variation between quartz and 36Cl concentrations, which we suggest requires a re-examination of this widely used application of the cosmogenic nuclide surface exposure dating method.

  • 306. Fu, Pingqing
    et al.
    Kawamura, Kimitaka
    Chen, Jing
    Qin, Mingyue
    Ren, Lujie
    Sun, Yele
    Wang, Zifa
    Barrie, Leonard A.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tachibana, Eri
    Ding, Aijun
    Yamashita, Youhei
    Fluorescent water-soluble organic aerosols in the High Arctic atmosphere2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 9845Article in journal (Refereed)
    Abstract [en]

    Organic aerosols are ubiquitous in the earth's atmosphere. They have been extensively studied in urban, rural and marine environments. However, little is known about the fluorescence properties of water-soluble organic carbon (WSOC) or their transport to and distribution in the polar regions. Here, we present evidence that fluorescent WSOC is a substantial component of High Arctic aerosols. The ratios of fluorescence intensity of protein-like peak to humic-like peak generally increased from dark winter to early summer, indicating an enhanced contribution of protein-like organics from the ocean to Arctic aerosols after the polar sunrise. Such a seasonal pattern is in agreement with an increase of stable carbon isotope ratios of total carbon (delta C-13(TC)) from -26.8 parts per thousand to -22.5 parts per thousand. Our results suggest that Arctic aerosols are derived from a combination of the long-range transport of terrestrial organics and local sea-to-air emission of marine organics, with an estimated contribution from the latter of 8.7-77% (mean 45%).

  • 307.
    Fuentes Guerrero, César
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Grain size analysis of a short sediment core from the Lomonosov Ridge, central Arctic Ocean2015Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Trigger core 07, is a 53 cm long sediment core that was collected during the Danish-Swedish expedition “Lomonosov Ridge off Greenland 2012” on the slope of the Lomonosov Ridge in the Arctic Ocean at a depth of 2522 m. This part of the world has experienced critical environmental changes during the Quaternary. Ice-sheets have advanced and retreated, and deposited sediments through all the Arctic Ocean. Glacial sediments contain coarser material and are gray, whereas interglacial sediments are brown, because of high amounts of manganese, and consist of fine-grained material.  The aim of this project is to make grain size analysis on TC 07 with the purpose to make an interpretation of the grain size data in relation to glaciation history and paleo-oceanography. For that, a correlation with piston core 07 has been made, and also a correlation between piston core 07 and the Arctic Coring Expedition, ACEX. The results showed that fine-grained material is more abundant in the top brown unit down to 32 cm, suggesting an interglacial period. This is followed by a gray-beige unit that goes down to 49 cm, and consist of coarser material, indicating glacial deposits. This unit can be linked to the Marine Isotope Stage 2, MIS 2, which began approximately 29000 years ago and ended about 14000 years ago 

  • 308. Gaines, Robert R.
    et al.
    Hammarlund, Emma U.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hou, Xianguang
    Qie, Chi
    Gabbott, Sarah E.
    Zhao, Yuanlong
    Peng, Jin
    Canfield, Donald E.
    Mechanism for Burgess Shale-type preservation2012In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 109, no 14, p. 5180-5184Article in journal (Refereed)
    Abstract [en]

    Exceptionally preserved fossil biotas of the Burgess Shale and a handful of other similar Cambrian deposits provide rare but critical insights into the early diversification of animals. The extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft-bodied fossil assemblages has remained enigmatic since Walcott’s initial discovery in 1909. Here, we demonstrate the mechanism of Burgess Shale-type preservation using sedimentologic and geochemical data from the Chengjiang, Burgess Shale, and five other principal Burgess Shale-type deposits. Sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation. Low sulfate concentrations in the global ocean and low-oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability. Subsequently, rapid entombment of fossils in fine-grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments. A permeability barrier, provided by bed-capping cements that were emplaced at the seafloor, is a feature that is shared among Burgess Shale-type deposits, and resulted from the unusually high alkalinity of Cambrian oceans. Thus, Burgess Shale-type preservation of soft-bodied fossil assemblages worldwide was promoted by unique aspects of early Paleozoic seawater chemistry that strongly impacted sediment diagenesis, providing a fundamentally unique record of the immediate aftermath of the “Cambrian explosion.”

  • 309. Galeczka, Iwona
    et al.
    Eiriksdottir, Eydis Salome
    Hardardottir, Jorunn
    Oelkers, Eric H.
    Torssander, Peter
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gislason, Sigurdur R.
    The effect of the 2002 glacial flood on dissolved and suspended chemical fluxes in the Skafta river, Iceland2015In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 301, p. 253-276Article in journal (Refereed)
    Abstract [en]

    This study describes the chemical composition of dissolved, degased and suspended fluxes of the 2002 Skafta glacial flood, which emerged from one of the Skafta subglacial lake due to geothermal activity beneath the Icelandic Vatnajokull glacier. The dissolved and suspended fluxes during the flood are compared with those normally observed in the Skafta river to determine the effect of such floods on the annual fluxes of material delivered to the coastal waters. Concentrations of most dissolved elements during the flood were significantly higher than those normally observed in the Skafta river. In addition, dissolved concentrations of nutrients such as SiO2, Fe, and V, increased more than an order of magnitude during the flood. These will affect biological processes on a local scale. The delta S-34 composition in the flood water suggests that the dissolved SO4 was derived from the oxidation of H2S and the geothermal fluid. The total suspended particulate load measured in the Skafta river during the 8-day 2002 flood was approximately half of the non-flood total annual Skafta suspended load. As particles carry the bulk of limiting nutrients to the oceans, this demonstrates the importance of glacial floods for primary production of coastal waters. The composition of the flood water and the Skafta subglacial lake, together with reaction path modelling suggest that substantial degasing of CO2 and H2S occurred at the glacial outlet during the flood. This degasing may have released as much as 262,000 and 7,980 tonnes of CO2 and H2S, respectively, to the atmosphere having a considerable impact on the local carbon and sulphur cycles during the flood event.

  • 310. Galfalk, Magnus
    et al.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bastviken, David
    Making methane visible2016In: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 6, no 4, p. 426-430Article in journal (Refereed)
    Abstract [en]

    Methane (CH4) is one of the most important greenhouse gases, and an important energy carrier in biogas and natural gas. Its large-scale emission patterns have been unpredictable and the source and sink distributions are poorly constrained. Remote assessment of CH4 with high sensitivity at a m(2) spatial resolution would allow detailed mapping of the near-ground distribution and anthropogenic sources in landscapes but has hitherto not been possible. Here we show that CH4 gradients can be imaged on the <m(2) scale at ambient levels (similar to 1.8 ppm) and filmed using optimized infrared (IR) hyperspectral imaging. Our approach allows both spectroscopic confirmation and quantification for all pixels in an imaged scene simultaneously. It also has the ability to map fluxes for dynamic scenes. This approach to mapping boundary layer CH4 offers a unique potential way to improve knowledge about greenhouse gases in landscapes and a step towards resolving source-sink attribution and scaling issues.

  • 311. Gallagher, K
    et al.
    Bodin, T
    Sambridge, M
    Weiss, D
    Kylander, Malin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Large, David
    Inference of abrupt changes in noisy geochemical records using transdimensional changepoint models2011In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 311, no 1-2, p. 182-194Article in journal (Refereed)
    Abstract [en]

    We present a method to quantify abrupt changes (or changepoints) in data series, represented as a function of depth or time. These changes are often the result of climatic or environmental variations and can be manifested in multiple datasets as different responses, but all datasets can have the same changepoint locations/timings. The method we present uses transdimensional Markov chain Monte Carlo to infer probability distributions on the number and locations (in depth or time) of changepoints, the mean values between changepoints and, if required, the noise variance associated with each dataset being considered. This latter point is important as we generally will have limited information on the noise, such as estimates only of measurement uncertainty, and in most cases it is not practical to make repeat sampling/measurement to assess other contributions to the variation in the data. We describe the main features of the approach (and describe the mathematical formulation in supplementary material), and demonstrate its validity using synthetic datasets, with known changepoint structure (number and locations of changepoints) and distribution of noise variance for each dataset. We show that when using multiple data, we expect to achieve better resolution of the changepoint structure than when we use each dataset individually. This is conditional on the validity of the assumption of common changepoints between different datasets. We then apply the method to two sets of real geochemical data, both from peat cores, taken from NE Australia and eastern Tibet. Under the assumption that changes occur at the same time for all datasets, we recover solutions consistent with those previously inferred qualitatively from independent data and interpretations. However, our approach provides a quantitative estimate of the relative probability of the inferred changepoints, allowing an objective assessment of the significance of each change.

  • 312. Garcia, Marga
    et al.
    Dowdeswell, Julian A.
    Ercilla, Gemma
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Recent glacially influenced sedimentary processes on the East Greenland continental slope and deep Greenland Basin2012In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 49, p. 64-81Article, review/survey (Refereed)
    Abstract [en]

    This paper presents the morpho-sedimentary characterization and interpretations of the assemblage of landforms of the East Greenland continental slope and Greenland Basin, based on swath bathymetry and sub-bottom TOPAS profiles. The interpretation of landforms reveals the glacial influence on recent sedimentary processes shaping the seafloor, including mass-wasting and turbidite flows. The timing of landform development points to a predominantly glacial origin of the sediment supplied to the continental margin, supporting the scenario of a Greenland Ice Sheet extending across the continental shelf, or even to the shelf-edge, during the Last Glacial Maximum (LGM). Major sedimentary processes along the central section of the eastern Greenland Continental Slope, the Norske margin, suggest a relatively high glacial sediment input during the LGM that, probably triggered by tectonic activity, led to the development of scarps and channels on the slope and debris flows on the continental rise. The more southerly Kejser Franz Josef margin has small-scale mass-wasting deposits and an extensive turbidite system that developed in relation to both channelised and unconfined turbidity flows which transferred sediments into the deep Greenland Basin.

  • 313.
    Garcia, Rina
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Löwemark, L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Terrestrial organic mater deposition on the Lomonosov Ridge, central Arctic Ocean2011In: APEX Fith International Conference and Workshop: Quaternary Glacial and Climate Extremes, The University Centre in Svalbard (UNIS) , 2011, p. 47-48Conference paper (Other academic)
  • 314.
    Gdaniec, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    231Pa and Th isotopes as tracers of deep water ventilation and scavenging in the Mediterranean Sea2017Licentiate thesis, monograph (Other academic)
    Abstract [en]

    The naturally occurring isotopes 231Pa and 230Th are used as tracers of marine biogeochemical processes. They are both produced from the radioactive decay of their uniformly distributed uranium parents (235U and 234U) in seawater. After production, 231Pa and 230Th are removed by adsorption onto settling particles (scavenging) and subsequently buried in marine sediments. 230Th is more particle reactive compared to 231Pa. Consequently, 230Th will be removed from the open ocean by adsorption onto settling particles, while 231Pa tend to be laterally transported by currents and removed by scavenging in areas of high particle flux (e.g. ocean margins). The primordial 232Th indicates lithogenic supply via rivers and resuspension of sediments, which provides additional information about processes involved in the cycling of particle reactive elements in the ocean. The preferential deposition of particle reactive elements at ocean margins (boundary scavenging) has important implications for our understanding of the distribution and dispersion of micronutrients (e.g. iron) and pollutants in the ocean. It is therefore valuable to understand the nature of boundary scavenging processes in order to evaluate the relative contribution of circulation and scavenging behaviors.The major characteristics of thermohaline circulation in the Mediterranean are well known and have been studied for decades. This sea is an almost land-locked area, where limited water-exchange with the Atlantic Ocean only occurs through the Strait of Gibraltar. Therefore, this marginal sea is often referred to as a “miniature ocean” suitable as a “laboratory” for marine environmental research. In this licentiate thesis, distributions of 231Pa, 230Th and 232Th in seawater and marine particles collected during the GEOTRACES MedSeA-GA04-S cruise in 2013 are presented. Observed nuclide distributions indicate the impact of deep water formation processes, where observed differences can be linked to the type of deep water formation process that occurs in respective basin. Essentially all in-situ produced 230Th is buried in Mediterranean Sea sediments. Despite lower affinity of 231Pa for marine particles, most 231Pa is also scavenged and deposited in Mediterranean Sea sediments. The efficient scavenging of 231Pa produces a relatively low fractionation between 231Pa and 230Th in terms of the fractionation factor FTh/Pa. This licentiate thesis presents a summary of the methods used for the analysis of 231Pa and Th-isotopes with details on the exchange chromatography method and the treatment of mass spectrometric data. The study of 231Pa, 230Th and 232Th in the Mediterranean Sea has important implications for our understanding of processes that control their water column distributions and how their behavior can be utilized to trace chemical flux in modern and past ocean environments.

  • 315.
    Gdaniec, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    231Pa and Th isotopes as tracers of deep water ventilation and scavenging in the Mediterranean Sea2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The naturally occurring isotopes 231Pa and 230Th are used as tracers of marine biogeochemical processes. They are both produced from the radioactive decay of their uniformly distributed uranium parents (235U and 234U) in seawater. After production, 231Pa and 230Th are removed by adsorption onto settling particles (scavenging) and subsequently buried in marine sediments. 230Th is more particle reactive compared to 231Pa. Consequently, 230Th will be removed from the open ocean by adsorption onto settling particles, while 231Pa tend to be laterally transported by currents and removed by scavenging in areas of high particle flux (e.g. ocean margins). The primordial 232Th indicates lithogenic supply via rivers and resuspension of sediments, which provides additional information about processes involved in the cycling of particle reactive elements in the ocean. The preferential deposition of particle reactive elements at ocean margins (boundary scavenging) has important implications for our understanding of the distribution and dispersion of micronutrients (e.g. iron) and pollutants in the ocean. It is therefore valuable to understand the nature of boundary scavenging processes in order to evaluate the relative contribution of circulation and scavenging behaviors.The major characteristics of thermohaline circulation in the Mediterranean are well known and have been studied for decades. This sea is an almost land-locked area, where limited water-exchange with the Atlantic Ocean only occurs through the Strait of Gibraltar. Therefore, this marginal sea is often referred to as a “miniature ocean” suitable as a “laboratory” for marine environmental research. In this licentiate thesis, distributions of 231Pa, 230Th and 232Th in seawater and marine particles collected during the GEOTRACES MedSeA-GA04-S cruise in 2013 are presented. Observed nuclide distributions indicate the impact of deep water formation processes, where observed differences can be linked to the type of deep water formation process that occurs in respective basin. Essentially all in-situ produced 230Th is buried in Mediterranean Sea sediments. Despite lower affinity of 231Pa for marine particles, most 231Pa is also scavenged and deposited in Mediterranean Sea sediments. The efficient scavenging of 231Pa produces a relatively low fractionation between 231Pa and 230Th in terms of the fractionation factor FTh/Pa. This licentiate thesis presents a summary of the methods used for the analysis of 231Pa and Th-isotopes with details on the exchange chromatography method and the treatment of mass spectrometric data. The study of 231Pa, 230Th and 232Th in the Mediterranean Sea has important implications for our understanding of processes that control their water column distributions and how their behavior can be utilized to trace chemical flux in modern and past ocean environments.

  • 316.
    Gdaniec, Sandra
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden; Université Paris-Saclay, France.
    Roy-Barman, Matthieu
    Foliot, Lorna
    Thil, Francois
    Dapoigny, Arnaud
    Burckel, Pierre
    Garcia-Orellana, Jordi
    Masqué, Pere
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andersson, Per S.
    Thorium and protactinium isotopes as tracers of marine particle fluxes and deep water circulation in the Mediterranean Sea2018In: Marine Chemistry, ISSN 0304-4203, E-ISSN 1872-7581, Vol. 199, p. 12-23Article in journal (Refereed)
    Abstract [en]

    Pa-231, Th-230 and Th-232 were analyzed in unfiltered seawater samples (n = 66) and suspended particles (n = 19) collected in the Mediterranean Sea during the MedSeA-GA04-S cruise along the GEOTRACES section GA04S and used to investigate mechanisms controlling the distribution and fractionation of Pa and Th in an ocean margin environment. Pa-231 and Th-230 are particle reactive radionuclides and are often used as tracers of processes such as boundary scavenging, particle transport and ocean circulation. The depth profiles of total Pa-231 and Th-230 concentrations in the Mediterranean Sea displayed non-linear shapes. Higher total Th-232 concentrations were observed at the straits and in deep waters pointing at lithogenic sources. Fractionation factors F-Th/Pa ranged from 1.4 to 9. Application of a box-model illustrated that 94% of the Pa-231 and almost all of the Th-230 (99.9%) produced in the Mediterranean Sea is removed to the sediment by scavenging. The negligible export of Th-230 to the Atlantic Ocean, leads to a reevaluation of the mean settling speed of the filtered particles, which is now estimated to 500-1000 m/y. The low F-Th/Pa fractionation factors are attributed to the efficient scavenging and lack of transport of Pa-231 to the Atlantic Ocean.

  • 317.
    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)
  • 318. 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.

  • 319. 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.

  • 320. 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.

  • 321. 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.

  • 322. 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.

  • 323. 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.

  • 324. 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.

  • 325. 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.

  • 326. 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. 

  • 327.
    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)
  • 328. 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.

  • 329. 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)
  • 330.
    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)
  • 331.
    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.

  • 332.
    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.

  • 333.
    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)
  • 334.
    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.

  • 335.
    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.

  • 336.
    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.

  • 337.
    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.

  • 338.
    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.

  • 339. 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.

  • 340.
    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.

  • 341.
    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

  • 342.
    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.

  • 343. 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.

  • 344. 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).

  • 345. 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.

  • 346.
    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.

  • 347.
    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. 

  • 348.
    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.

  • 349.
    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.

  • 350.
    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.

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