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  • 1. Bellucci, J. J.
    et al.
    Nemchin, A. A.
    Whitehouse, Martin J.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Snape, J. F.
    Kielman, Ross B.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Bland, P. A.
    Benedix, G. K.
    A Pb isotopic resolution to the Martian meteorite age paradox2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 433, p. 241-248Article in journal (Refereed)
    Abstract [en]

    Determining the chronology and quantifying various geochemical reservoirs on planetary bodies is fundamental to understanding planetary accretion, differentiation, and global mass transfer. The Pb isotope compositions of individual minerals in the Martian meteorite Chassigny have been measured by Secondary Ion Mass Spectrometry (SIMS). These measurements indicate that Chassigny has mixed with a Martian reservoir that evolved with a long-term U-238/Pb-204 (mu) value similar to two times higher than those inferred from studies of all other Martian meteorites except 4.428 Ga clasts in NWA7533. Any significant mixing between this and an unradiogenic reservoir produces ambiguous trends in Pb isotope variation diagrams. The trend defined by our new Chassigny data can be used to calculate a crystallization age for Chassigny of 4.526 +/- 0.027 Ga (2 sigma) that is clearly in error as it conflicts with all other isotope systems, which yield a widely accepted age of 1.39 Ga. Similar, trends have also been observed in the Shergottites and have been used to calculate a >4 Ga age or, alternatively, attributed to terrestrial contamination. Our new Chassigny data, however, argue that the radiogenic component is Martian, mixing occurred on the surface of Mars, and is therefore likely present in virtually every Martian meteorite. The presence of this radiogenic reservoir on Mars resolves the paradox between Pb isotope data and all other radiogenic isotope systems in Martian meteorites. Importantly, Chassigny and the Shergottites are likely derived from the northern hemisphere of Mars, while NWA 7533 originated from the Southern hemisphere, implying that the U-rich reservoir, which most likely represents some form of crust, must be widespread. The significant age difference between SNC meteorites and NWA 7533 is also consistent with an absence of tectonic recycling throughout Martian history.

  • 2.
    Chi Fru, Ernest
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Hemmingsson, Christoffer
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Holm, Mikaela
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chiu, Beverly
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rutgers University, USA.
    Iniguez, Enrique
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arsenic-induced phosphate limitation under experimental Early Proterozoic oceanic conditions2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 434, p. 52-63Article in journal (Refereed)
    Abstract [en]

    Comparison of phosphorus concentrations associated with modern hydrothermal Fe(III)(oxyhydr)oxides and ancient Fe(III) oxide-rich iron formations, is used to estimate bioavailable Precambrian marine phosphorus (P) concentrations. This led to the proposition of a low dissolved P budget of similar to 10-25% of present-day levels, before similar to 1.9 billion years ago. Estimates incorporating ancient marine Si levels >= 0.67 mM instead suggested global dissolved P levels greater than today. Here we unite current experimental models that have considered NaCl solutions containing elevated dissolved Fe(II), Si, Ca2+ and Mg2+ ions in the incorporation of P in Precambrian marine Fe(III)(oxyhydr)oxides, in addition to arsenic as a hydrothermal proxy. We show that the coprecipitation of dissolved P and Fe(III)(oxyhydr)oxides from arsenic-rich marine waters produces an average P distribution coefficient of similar to 0.072 (+/- 0.01) mu M-1. This is comparable to the similar to 0.07 mu M-1 predicted for Fe(III)(oxyhydr)oxides in modern arsenic-rich, submarine hydrothermal settings, from which the lower Early Proterozoic dissolved marine P concentrations were predicted. As/P molar ratios below modern seawater ratios removed the negative feedback effect high Si impose on P scavenging by Fe(III)(oxyhydr)oxides. The binding of As(III) to Fe(III)(oxyhydr)oxides exhibits a lower competitive influence on P fixation. As(V) that likely became prominent in the surficially oxidized Early Proterozoic oceans induced dissolved P limitation because of preferential P sequestration at the expense of dissolved As(V) enrichment. The control of As on P scavenging by the precipitating Fe(III)(oxyhydr)oxides is strong regardless of common seawater cations (Mg2+ and Ca2+). The data suggest that the application of Si and Fe(III)(oxyhydr)oxides as an ancient seawater P proxy should consider chemical variability between depositional basins, taking into account the rather strong role hydrothermal arsenic has on the distribution of P in Fe(Ill)(oxyhydr)oxides. We propose that the generalized lower dissolved P budgets estimated from Early Proterozoic iron formations are consistent with oceans thought to be at least 3-4 times more hydrothermally active than at present.

  • 3. Cuzzone, Joshua K.
    et al.
    Clark, Peter U.
    Carlson, Anders E.
    Ullman, David J.
    Rinterknecht, Vincent R.
    Milne, Glenn A.
    Lunkka, Juha-Pekka
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Marcott, Shaun A.
    Caffee, Marc
    Final deglaciation of the Scandinavian Ice Sheet and implications for the Holocene global sea-level budget2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 448, p. 34-41Article in journal (Refereed)
    Abstract [en]

    The last deglaciation of the Scandinavian Ice Sheet (SIS) from similar to 21,000 to 13,000 yr ago is well constrained by several hundred Be-10 and C-14 ages. The subsequent retreat history, however, is established primarily from minimum-limiting C-14 ages and incomplete Baltic-Sea varve records, leaving a substantial fraction of final SIS retreat history poorly constrained. Here we develop a high-resolution chronology for the final deglaciation of the SIS based on 79 Be-10 cosmogenic exposure dates sampled along three transects spanning southern to northern Sweden and Finland. Combining this new chronology with existing Be-10 ages on deglaciation since the Last Glacial Maximum shows that rates of SIS margin retreat were strongly influenced by deglacial millennial-scale climate variability and its effect on surface mass balance, with regional modulation of retreat associated with dynamical controls. Ice-volume estimates constrained by our new chronology suggest that the SIS contributed 8 m sea-level equivalent to global sea-level rise between similar to 14.5 ka and 10 ka. Final deglaciation was largely complete by similar to 10.5 ka, with highest rates of sea-level rise occurring during the Bolling-Allerod, a 50% decrease during the Younger Dryas, and a rapid increase during the early Holocene. Combining our SIS volume estimates with estimated contributions from other remaining Northern Hemisphere ice sheets suggests that the Antarctic Ice Sheet (AIS) contributed 14.4 +/- 5.9 m to global sea-level rise since 13 ka. This new constraint supports those studies that indicate that an ice volume of 15 m or more of equivalent sea-level rise was lost from the AIS during the last deglaciation.

  • 4. Davies, Siwan M.
    et al.
    Wastegård, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Abbott, P.M.
    Barbante, C.
    Bigler, M.
    Johnsen, S.J.
    Rasmussen, Tine L..
    Steffensen, J.P.
    Svensson, A.
    Tracing volcanic events in the NGRIP ice-core and synchronising North Atlantic marine records during the last glacial period2010In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 294, p. 69-79Article in journal (Refereed)
    Abstract [en]

    Abstract

    Five basaltic tephra horizons have been identified in the NGRIP ice-core during the last glacial period. Geochemical analyses indicate that these tephras are of Icelandic origin with one horizon identified as the Faroe Marine Ash Zone III (FMAZ III) — an important marker horizon in several marine records within the Faroe Islands margin. The FMAZ III is a few millimetres thick and is constrained to 38,122 ± 723 yr b2k (before year AD 2000) according to the Greenland Ice Core Chronology (GICC05). This event falls directly at the thermal peak of Greenland Interstadial 8 (GI-8) and a marine-ice correlation, based on the position of this tephra, suggests a close coupling of the marine and atmospheric systems at this time. The remaining tephras originate from the Eastern Volcanic Zone with the Katla volcanic system pinpointed as the dominant source. These are thought to represent previously unknown volcanic events which add considerably to our knowledge of Icelandic volcanism during this period. Independent ages are assigned to these events and together with major element characterisation, form the backbone of the tephrochronology framework presented here. Despite the identification of these tephras, this study also demonstrates that the sulphate record may not always act as a suitable proxy for the identification of tephra horizons in ice-core records.

  • 5. Dickens, Gerald R.
    et al.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    A comment on "Pliocene climate change of the Southwest Pacific and the impact of ocean gateways" by C. Karas, D. Nurnberg, R. Tiedemann, D. Garbe Schonberg, EPSL 301, 117-124 (2011)2012In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 331, p. 364-365Article in journal (Other academic)
  • 6. Drake, Henrik
    et al.
    Heim, Christine
    Roberts, Nick M. W.
    Zack, Thomas
    Tillberg, Mikael
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ivarsson, Magnus
    Whitehouse, Martin J.
    Åström, Mats E.
    Isotopic evidence for microbial production and consumption of methane in the upper continental crust throughout the Phanerozoic eon2017In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 470, p. 108-118Article in journal (Refereed)
    Abstract [en]

    Microorganisms produce and consume methane in terrestrial surface environments, sea sediments and, as indicated by recent discoveries, in fractured crystalline bedrock. These processes in the crystalline bedrock remain, however, unexplored both in terms of mechanisms and spatiotemporal distribution. Here we have studied these processes via a multi-method approach including microscale analysis of the stable isotope compositions of calcite and pyrite precipitated in bedrock fractures in the upper crust (down to 1.7 km) at three sites on the Baltic Shield. Microbial processes have caused an intriguing variability of the carbon isotopes in the calcites at all sites, with delta C-13 spanning as much as -93.1 parts per thousand (related to anaerobic oxidation of methane) to +36.5 parts per thousand (related to methanogenesis). Spatiotemporal coupling between the stable isotope measurements and radiometric age determinations (micro-scale dating using new high spatial methods: LA-ICP-MS U-Pb for calcite and Rb-Sr for calcite and co-genetic adularia) enabled unprecedented direct timing constraints of the microbial processes to several periods throughout the Phanerozoic eon, dating back to Devonian times. These events have featured variable fluid salinities and temperatures as shown by fluid inclusions in the calcite; dominantly 70-85 degrees C brines in the Paleozoic and lower temperatures (<50-62 degrees C) and salinities in the Mesozoic. Preserved organic compounds, including plant signatures, within the calcite crystals mark the influence of organic matter in descending surficial fluids on the microbial processes in the fracture system, thus linking processes in the deep and surficial biosphere. These findings substantially extend the recognized temporal and spatial range for production and consumption of methane within the upper continental crust.

  • 7. 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.

  • 8. 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. 

  • 9.
    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.

  • 10.
    Hammarlund, Emma U.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dahl, Tais W.
    Harper, David A. T.
    Bond, David P. G.
    Nielsen, Arne T.
    Bjerrum, Christian J.
    Schovsbo, Niels H.
    Schönlaub, Hans P.
    Zalasiewicz, Jan A.
    Canfield, Donald E.
    A sulfidic driver for the end-Ordovician mass extinction2012In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 331, p. 128-139Article in journal (Refereed)
    Abstract [en]

    The end-Ordovician extinction consisted of two discrete pulses, both linked, in various ways, to glaciation at the South Pole. The first phase, starting just below the Normalograptus extraordinarius Zone, particularly affected nektonic and planktonic species, while the second pulse, associated with the Normalograptus persculptus Zone, was less selective. Glacially induced cooling and oxygenation are two of many suggested kill mechanisms for the end-Ordovician extinction, but a general consensus is lacking. We have used geochemical redox indicators, such as iron speciation, molybdenum concentrations, pyrite framboid size distribution and sulfur isotopes to analyze the geochemistry in three key Hirnantian sections. These indicators reveal that reducing conditions were occasionally present at all three sites before the first pulse of the end-Ordovician extinction, and that these conditions expanded towards the second pulse. Even though the N. extraordinarius Zone appears to have been a time of oxygenated deposition, pyrite is significantly enriched in 34S in our sections as well as in sections reported from South China. This suggests a widespread reduction in marine sulfate concentrations, which we attribute to an increase in pyrite burial during the early Hirnantian. The S-isotope excursion coincides with a major positive carbon isotope excursion indicating elevated rates of organic carbon burial as well. We argue that euxinic conditions prevailed and intensified in the early Hirnantian oceans, and that a concomitant global sea level lowering pushed the chemocline deeper than the depositional setting of our sites. In the N. persculptus Zone, an interval associated with a major sea level rise, our redox indicators suggests that euxinic conditions, and ferruginous in some places, encroached onto the continental shelves. In our model, the expansion of euxinic conditions during the N. extraordinarius Zone was generated by a reorganization of nutrient cycling during sea level fall, and we argue, overall, that these dynamics in ocean chemistry played an important role for the end-Ordovician mass extinction. During the first pulse of the extinction, euxinia and a steepened oxygen gradient in the water column caused habitat loss for deep-water benthic and nektonic organisms. During the second pulse, the transgression of anoxic water onto the continental shelves caused extinction in shallower habitats.

  • 11.
    Heyman, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Harbor, Jonathan M.
    Caffee, Marc W.
    Too young or too old: Evaluating cosmogenic exposure dating based on an analysis of compiled boulder exposure ages2011In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 302, no 1-2, p. 71-80Article in journal (Refereed)
    Abstract [en]

    Cosmogenic exposure dating has greatly enhanced our ability to define glacial chronologies spanning several global cold periods, and glacial boulder exposure ages are now routinely used to constrain deglaciation ages. However, exposure dating involves assumptions about the geological history of the sample that are difficult to test and yet may have a profound effect on the inferred age. Two principal geological factors yield erroneous inferred ages: exposure prior to glaciation (yielding exposure ages that are too old) and incomplete exposure due to post-depositional shielding (yielding exposure ages that are too young). Here we show that incomplete exposure is more important than prior exposure, using datasets of glacial boulder 10Be exposure ages from theTibetan Plateau (1420 boulders), Northern Hemisphere palaeo-ice sheets (631 boulders), and present-day glaciers (208 boulders). No boulders from present-day glaciers and few boulders from the palaeo-ice sheets have exposure ages significantly older than independently known deglaciation ages, indicating that prior exposure is of limited significance. Further, while a simple post-depositional landform degradation model can predict the exposure age distribution of boulders from the Tibetan Plateau, a prior exposure model fails, indicating that incomplete exposure is important. The large global dataset demonstrates that, in the absence of other evidence, glacial boulder exposure ages should be viewed as minimum limiting deglaciation ages.

  • 12. Hofmann, Axel
    et al.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wilson, Allan
    Gold mobility during Palaeoarchaean submarine alteration2017In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 462, p. 47-54Article in journal (Refereed)
    Abstract [en]

    Seafloor alteration provides large amounts of solutes to the hydrosphere. In order to investigate gold mobility during water-rock interaction prior to 3-billion-years ago, low detection limit analysis of Au concentrations was carried out on rocks from marine alteration zones. Stratiform zones recording low temperature (<= 150 degrees C) seafloor alteration are a characteristic feature of greenstone belts older than 3.0 Ga. Hydrothermal processes were operating on, and immediately below, the seafloor, giving rise to extensive silicification of sub-seafloor volcanic rocks and silicification of seafloor sediments. In order to investigate gold mobility during silicification, unaltered and variably silicified volcanic rocks and associated cherts from Palaeoarchaean greenstone successions (c. 3.4 Ga) of South Africa were analyzed. Results show mobility of gold during silicification of mafic/ultramafic rocks and transfer to the Archaean ocean. Some gold was incorporated into carbonaceous marine sediments overlying the alteration zones. A combination of pervasive silicification, rarity of black shales, and low gold content in komatiites can explain the low mineralization potential of Palaeoarchaean greenstone belts for orogenic gold deposits.

  • 13.
    Holzkämper, Steffen
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Lee-Thorp, Julia
    Geographical and Environmental Sciences, University of Bradford, UK.
    Talma, Siep
    Natural Resources and the Environment, CSIR, Pretoria, South Africa.
    Mangini, Augusto
    Heidelberger Akademie der Wissenschaften, Heidelberg, Germany.
    Partridge, Tim
    School of Geography, University of Witwatersrand, Johannesburg, South Africa.
    Late Pleistocene stalagmite growth in Wolkberg Cave, South Africa2009In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 282, p. 212-221Article in journal (Refereed)
    Abstract [en]

    Little is known about the sequence of climate and environmental change in southern Africa during the last glacial period, in spite of the intimations from records, such as Antarctic ice cores and archaeological sites, that very marked changes took place which would have had profound effects on vegetation and animal distributions across the sub-continent. High-resolution, (semi-) continuous climate and environmental records can be extracted from suitable cave speleothems. Speleothems are reasonably abundant in southern Africa, but their occurrence is patchy in time and space and the records can be difficult to interpret. Here we report our assessment of the stalagmite W5 from Wolkberg Cave in the northeastern part of South Africa, as an archive for glacial-period climatic and environmental shifts. The cave is located at 1450 m asl, in the dolomitic limestones of the Transvaal System in an area currently dominated by C4 grass vegetation. Nine U/Th dates show growth from 58 to 46 ka, and a second brief phase ca. 40 ka, indicating that the available moisture was sufficient to allow speleothems to form. The δ18O and δ13C values along the growth axis show variability in the order of 2‰ for the former, while variability in the latter is characterized by a shift from values near − 2‰ in the older section to + 2‰ or more in the younger part. These high δ13C values are probably the combined result of CO2 degassing of the percolating soil water prior to the carbonate precipitation in the cave chamber, the increasing dominance of C4 over C3 vegetation, and the high percentage of aragonite towards the stalagmite's top. The retrieved data point towards increasingly drier and colder conditions during the growth period of the stalagmite. Furthermore, the high-frequency variations of δ18O values indicate the presence of short term climate oscillations that are probably linked to shifts of the Intertropical Convergence Zone.

  • 14. Insel, Nadja
    et al.
    Poulsen, Christopher J.
    Ehlers, Todd A.
    Sturm, Christophe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Response of meteoric delta O-18 to surface uplift - Implications for Cenozoic Andean Plateau growth2012In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 317, p. 262-272Article in journal (Refereed)
    Abstract [en]

    The timing and magnitude of surface uplift provide important constraints on geodynamic models of orogen formation. Oxygen isotope (delta O-18) and mass-47 isotopolog (Delta(47)) compositions from terrestrial carbonate sediments have been used with modern isotope and temperature lapse rates to infer past surface elevations of the Andes. However, these paleoaltimeny interpretations are contentious because variations in the oxygen isotope composition in meteoric water (delta O-18(p)) are caused by changes in elevation (orographic) and regional climate. Here, we use a limited-domain isotope-tracking general circulation model to simulate changes in delta O-18(p) and isotopic lapse rates in response to Andean surface uplift, and to re-evaluate delta O-18 and Delta(47) changes in late Miocene carbonates previously associated with rapid Andean growth. Results indicate that Andean surface uplift leads to changes in low-level atmospheric circulation and an increase in precipitation along the eastem Andean flank which influences isotopic source and amount effects. Simulated changes in Andean delta O-18(p) are not systematic with an increase in surface elevation, but are instead a function of orographic thresholds that abruptly change regional climate. A delta O-18(p) decrease of >5%. over the central Andes and an increase in isotopic lapse rates (up to 0.8%. km(-1)) coincide with Andean surface uplift from 75 to 100% of modem elevation. These changes in the isotopic signature could account for the entire 3-4%. delta O-18 depletion in late Miocene carbonate nodules, and suggest an Andean paleoelevation of similar to 3000 m (75% of modem elevations) before 10 Ma.

  • 15.
    Jansen, John D.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Nanson, G. C.
    Cohen, T. J.
    Fujioka, T.
    Fabel, D.
    Larsen, J. R.
    Codilean, A. T.
    Price, D. M.
    Bowman, H. H.
    May, J. -H
    Gliganic, L. A.
    Lowland river responses to intraplate tectonism and climate forcing quantified with luminescence and cosmogenic Be-102013In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 366, p. 49-58Article in journal (Refereed)
    Abstract [en]

    Intraplate tectonism has produced large-scale folding that steers regional drainage systems, such as the 1600 km-long Cooper Ck, en route to Australia's continental depocentre at Lake Eyre. We apply cosmogenic Be-10 exposure dating in bedrock, and luminescence dating in sediment, to quantify the erosional and depositional response of Cooper Ck where it incises the rising Innamincka Dome. The detachment of bedrock joint-blocks during extreme floods governs the minimum rate of incision (17.4 +/- 6.5 mm/ky) estimated using a numerical model of episodic erosion calibrated with our 10Be measurements. The last big-flood phase occurred no earlier than similar to 112-121 ka. Upstream of the Innamincka Dome long-term rates of alluvial deposition, partly reflecting synclinal-basin subsidence, are estimated from 47 luminescence dates in sediments accumulated since similar to 270 ka. Sequestration of sediment in subsiding basins such as these may account for the lack of Quaternary accumulation in Lake Eyre, and moreover suggests that notions of a single primary depocentre at base-level may poorly represent lowland, arid-zone rivers. Over the period similar to 75-55 ka Cooper Ck changed from a bedload-dominant, laterally-active meandering river to a muddy anabranching channel network up to 60 km wide. We propose that this shift in river pattern was a product of base-level rise linked with the slowly deforming syncline-anticline structure, coupled with a climate-forced reduction in discharge. The uniform valley slope along this subsiding alluvial and rising bedrock system represents an adjustment between the relative rates of deformation and the ability of greatly enhanced flows at times during the Quaternary to incise the rising anticline. Hence, tectonic and climate controls are balanced in the long term.

  • 16.
    Kylander, Malin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Klaminder, Jonatan
    Department of Ecology and Environmental Sciences, Umeå University .
    Bindler, Richard
    Department of Ecology and Environmental Sciences, Umeå University.
    Weiss, Dominik
    Department of Earth Science and Engineering, Imperial College London.
    Natural lead isotope variations in the atmosphere2010In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 290, no 1-2, p. 44-53Article in journal (Refereed)
    Abstract [en]

    Stable lead (Pb) isotope data from pre-contamination peat sections has the potential to contribute to our understanding of earth system processes (e.g., atmospheric circulation, erosion, volcanic activity) in the past. Key questions arise however. Can the Pb isotopes archived in peat records be used for assessing aerosol dynamics on a hemispheric scale or do they mainly reflect inputs from local soils? What natural Pb sources are important and do contributions vary over time? In order to answer these questions we have synthesized all available Pb isotope data from pre-contamination peat sections in Europe, Australia, North America and South America. We specifically examine the spatial and temporal variability of the Pb isotope records and identify regionally important trends and Pb sources.

    A pooling of all available pre-contamination peat data generated an average natural 206Pb/207Pb background ratio of 1.21±0.05 (2σ, n=300)(206Pb/204Pb= 18.90±0.86, 207Pb/204Pb= 15.66±0.10 and 208Pb/204Pb= 38.74±0.57, n=207). The majority of the records showed limited temporal and compositional agreement, suggesting that the peat record receives mainly inputs from local (<10 km) and/or regional (10-500 km) sources. Three-isotope plots also support local and regional control and evidence a wide natural spread at some sites, particularly those located in radiogenic geological settings. A temporally synchronous isotope excursion to values between 1.16-1.18 at sites across Europe ca 4000-3000 B.C. was detected, however. While usually associated with anthropogenic sources, there are indeed natural aerosols having 206Pb/207Pb signatures as low as 1.16 as evidenced in several peat and ice core records globally. Three-isotope plots suggest that this unlikely to be a signal of mineral dust contributions, which tend to have 206Pb/207Pb ratios ≥1.19, but rather sourced to volcanic emissions. These results stress caution when using estimates of the upper continental crust to constrain natural Pb sources in, e.g., mixing equations. Considering the strong influence from local and regional sources on Pb-containing aerosols in the peat record, the assessment of aerosol dynamics at lower latitudes is likely best achieved using a compliment of archives rather than just polar ice cores, for example. 

  • 17. Lang, K. A.
    et al.
    Ehlers, T. A.
    Kamp, P. J. J.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sediment storage in the Southern Alps of New Zealand: New observations from tracer thermochronology2018In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 493, p. 140-149Article in journal (Refereed)
    Abstract [en]

    Careful study of the processes transporting sediment across Earth's surface is critical for robust interpretation of the sedimentary record. Here we consider the specific influence of cyclic glaciation on the export of sediment from mountain landscapes to ocean basins. Using detrital apatite fission-track tracer thermochronology, we present new observations of sediment provenance from six large river systems draining the eastern flank of the Southern Alps, New Zealand. Detrital cooling ages in all six rivers reflect erosion of partially-reset and fully-unreset bedrock exposed in lower catchment areas and indicate that sediment is not currently contributed in proportion to long-term (>10(6) yr) erosion patterns. Instead, detrital cooling ages are better explained by either localized erosion along the eastern mountain front or intermontane sediment storage. Of these two alternatives, only intermontane sediment storage is further consistent with suspended sediment flux measurements in eastern rivers. Our observations are consistent with prior interpretations of Holocene sediment retention, and contrast with tracer thermochronology from continental margin deposits indicating sediment was rapidly exported to the continental shelf during the late Pleistocene. Collectively, this evidence argues for a reactive sediment routing system east of the main drainage divide that responds to cyclic glaciation by retaining sediment onshore following deglaciation and evacuating sediment reservoirs offshore during the subsequent glacial advance. Our research demonstrates the importance of intermontane sediment storage on the transmission of high-frequency (similar to 10(4-5) yr) climate signals to offshore sedimentary archives while highlighting a novel approach to detailing sediment provenance in tectonically active mountain ranges.

  • 18. Li, Yingkui
    et al.
    Li, Dewen
    Liu, Gengnian
    Harbor, Jon
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Purdue University, USA.
    Caffee, Marc
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Patterns of landscape evolution on the central and northern Tibetan Plateau investigated using in-situ produced Be-10 concentrations from river Sediments2014In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 398, p. 77-89Article in journal (Refereed)
    Abstract [en]

    Quantifying long-term erosion rates across the Tibetan Plateau and its bordering mountains is of critical importance to an understanding of the interaction between climate, tectonic movement, and landscape evolution. We present a new dataset of basin-wide erosion rates from the central and northern Tibetan Plateau derived using in-situ produced Be-10 concentrations of river sediments. Basin-wide erosion rates from the central plateau range from 10.1 +/- 0.9 to 36.8 +/- 3.2 mm/kyr, slightly higher than published local erosion rates measured from bedrock surfaces. These values indicate that long-term downwearing of plateau surfaces proceeds at low rates and that the landscape is demonstrably stable in the central plateau. In contrast, basin-wide erosion rates from the Kunlun Shan on the northern Tibetan Plateau range from 19.9 +/- 1.7 to 163.2 +/- 15.9 mm/kyr. Although the erosion rates of many of these basins are much higher than the rates from the central plateau, they are lower than published basin-wide erosion rates from other mountains fringing the Tibetan Plateau, probably because the basins in the Kunlun Shan include both areas of low-relief plateau surface and high-relief mountain catchments and may also result from retarded fluvial sediment transport in an arid climate. Significantly higher basin-wide erosion rates derived from the Tibetan Plateau margin, compared to the central plateau, reflect a relatively stable plateau surface that is being dissected at its margins by active fluvial erosion.

  • 19. Lupi, Matteo
    et al.
    Frehner, Marcel
    Weis, Philipp
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Saenger, Erik H.
    Tisato, Nicola
    Geiger, Sebastian
    Chiodini, Giovanni
    Driesner, Thomas
    Regional earthquakes followed by delayed ground uplifts at Campi Flegrei Caldera, Italy: Arguments for a causal link2017In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 474, p. 436-446Article in journal (Refereed)
    Abstract [en]

    Earthquake-triggered volcanic activity promoted by dynamic and static stresses are considered rare and difficult-to-capture geological processes. Calderas are ideal natural laboratories to investigate earthquake volcano interactions due to their sensitivity to incoming seismic energy. The Campi Flegrei caldera, Italy, is one of the most monitored volcanic systems worldwide. We compare ground elevation time series at Campi Flegrei with earthquake catalogues showing that uplift events at Campi Flegrei are associated with large regional earthquakes. Such association is supported by (yet non-definitive) binomial tests. Over a 70-year time window we identify 14 uplift events, 12 of them were preceded by an earthquake, and for 8 of them the earthquake-to-uplift timespan ranges from immediate responses to 1.2 yr. Such variability in the response delay may be due to the preparedness of the system with faster responses probably occurring in periods during which the Campi Flegrei system was already in a critical state. To investigate the process that may be responsible for the proposed association we simulate the propagation of elastic waves and show that passing body waves impose high dynamic strains at the roof of the magmatic reservoir of the Campi Flegrei at about 7 km depth. This may promote a short-lived embrittlement of the magma reservoir's carapace otherwise marked by a ductile behaviour. Such failure allows magma and exsolved volatiles to be released from the magmatic reservoir. The fluids, namely exsolved volatiles and/or melts, ascend through a nominally plastic zone above the magmatic reservoir. This mechanism and the associated inherent uncertainties require further investigations but the new concept already implies that geological processes triggered by passing seismic waves may become apparent several months after passage of the seismic waves. (C) 2017 Elsevier B.V. All rights reserved.

  • 20. Mamtani, Manish A.
    et al.
    Piazolo, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Greiling, Reinhard O.
    Kontny, Agnes
    Hrouda, Frantisek
    Process of magnetite fabric development during granite deformation2011In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 308, no 1-2, p. 77-89Article in journal (Refereed)
    Abstract [en]

    This study evaluates the fabric defined by magnetite grains in a syntectonically deformed granite and deciphers the processes that led to magnetite fabric development. Anisotropy of anhysteretic remanence magnetization (AARM) analysis is performed in samples taken from different parts of the granite to establish that the magnetite grains define a fabric. Along with microstructural studies, the AARM data help conclude that this fabric is on account of shape preferred orientation (SPO) of the magnetite grains. The intensity of magnetite fabric (degree of anisotropy of the AARM ellipsoid) is higher in the southern parts as compared to the north, which is inferred to indicate a strain gradient. Electron back scattered diffraction (EBSD) analyses of magnetite grains were performed to determine if there are intracrystalline deformation features that could have influenced magnetite shape and SPO, and thus AARM data. Detailed crystallographic orientation data coupled with orientation contrast imaging did not reveal any subgrains and/or significant variations in crystallographic orientations within magnetite grains. Instead, grains exhibit fractures and are in places associated with quartz pressure fringes. Hence, neither the SPO nor the variation in the magnetite fabric intensity in the granite can be attributed to intracrysta Hine deformation of magnetite by dislocation creep. It is concluded that the magnetite grains were rheologically rigid and there was relative movement between the magnetite and the matrix minerals (quartz, feldspar and biotite). These matrix minerals actually define the fabric attractor and the magnetite grains passively rotated to align with it. Thus it is demonstrated that the magnetite fabric in the granite stems from rigid body movement rather than dislocation creep.

  • 21. Marin-Carbonne, Johanna
    et al.
    Rollion-Bard, Claire
    Bekker, Andrey
    Rouxel, Olivier
    Agangi, Andrea
    Cavalazzi, Barbara
    Wohlgemuth-Ueberwasser, Cora C.
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Johannesburg, South Africa.
    Hofmann, Axel
    McKeegan, Kevin D.
    Coupled Fe and S isotope variations in pyrite nodules from Archean shale2014In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 392, p. 67-79Article in journal (Refereed)
    Abstract [en]

    Iron and sulfur isotope compositions recorded in ancient rocks and minerals such as pyrite (FeS2) have been widely used as a proxy for early microbial metabolisms and redox evolution of the oceans. However, most previous studies focused on only one of these isotopic systems. Herein, we illustrate the importance of in-situ and coupled study of Fe and S isotopes on two pyrite nodules in a c. 2.7 Ga shale from the Bubi Greenstone Belt (Zimbabwe). Fe and S isotope compositions were measured both by bulk-sample mass spectrometry techniques and by ion microprobe in-situ methods (Secondary Ion Mass Spectrometry, SIMS). Spatially-resolved analysis across the nodules shows a large range of variations at micrometer-scale for both Fe and S isotope compositions, with delta Fe-56 and delta S-34 values from -2.1 to +0.7 parts per thousand and from -0.5 to +8.2 parts per thousand, respectively, and Delta S-33 values from -1.6 to +2.9 parts per thousand. The Fe and S isotope variations in these nodules cannot be explained by tandem operation of Dissimilatory Iron Reduction (DIR) and Bacterial Sulfate Reduction (BSR) as was previously proposed, but rather they reflect the contributions of different Fe and S sources during a complex diagenetic history. Pyrite formed from two different mineral precursors: (1) mackinawite precipitated in the water column, and (2) greigite formed in the sediment during early diagenesis. The in-situ analytical approach reveals a complex history of the pyrite nodule growth and allows us to better constrain environmental conditions during the Archean.

  • 22. McNeill, Lisa C.
    et al.
    Dugan, Brandon
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pickering, Kevin T.
    Pouderoux, Hugo F. A.
    Henstock, Timothy J.
    Petronotis, Katerina E.
    Carter, Andrew
    Chemale, Farid
    Milliken, Kitty L.
    Kutterolf, Steffen
    Mukoyoshi, Hideki
    Chen, Wenhuang
    Kachovich, Sarah
    Mitchison, Freya L.
    Bourlange, Sylvain
    Colson, Tobias A.
    Frederik, Marina C. G.
    Guerin, Gilles
    Hamahashi, Mari
    House, Brian M.
    Hüpers, Andre
    Jeppson, Tamara N.
    Kenigsberg, Abby R.
    Kuranaga, Mebae
    Nair, Nisha
    Owari, Satoko
    Shan, Yehua
    Song, Insun
    Torres, Marta E.
    Vannucchi, Paola
    Vrolijk, Peter J.
    Yang, Tao
    Zhao, Xixi
    Thomas, Ellen
    Understanding Himalayan erosion and the significance of the Nicobar Fan2017In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 475, p. 134-142Article in journal (Refereed)
    Abstract [en]

    A holistic view of the Bengal-Nicobar Fan system requires sampling the full sedimentary section of the Nicobar Fan, which was achieved for the first time by International Ocean Discovery Program (IODP) Expedition 362 west of North Sumatra. We identified a distinct rise in sediment accumulation rate (SAR) beginning similar to 9.5 Ma and reaching 250-350 m/Myr in the 9.5-2 Ma interval, which equal or far exceed rates on the Bengal Fan at similar latitudes. This marked rise in SAR and a constant Himalayan-derived provenance necessitates a major restructuring of sediment routing in the Bengal-Nicobar submarine fan. This coincides with the inversion of the Eastern Himalayan Shillong Plateau and encroachment of the west-propagating Indo-Burmese wedge, which reduced continental accommodation space and increased sediment supply directly to the fan. Our results challenge a commonly held view that changes in sediment flux seen in the Bengal-Nicobar submarine fan were caused by discrete tectonic or climatic events acting on the Himalayan-Tibetan Plateau. Instead, an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation.

  • 23. Menzies, Catriona D.
    et al.
    Wright, Sarah L.
    Craw, Dave
    James, Rachael H.
    Alt, Jeffrey C.
    Cox, Simon C.
    Pitcairn, Lain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Teagle, Damon A. H.
    Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand2018In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 481, p. 305-315Article in journal (Refereed)
    Abstract [en]

    Collisional mountain building influences the global carbon cycle through release of CO2 liberated by metamorphic reactions and promoting mechanical erosion that in turn increases chemical weathering and drawdown of atmospheric CO2. The Southern Alps is a carbonate-poor, siliciclastic mountain belt associated with the active Australian Pacific plate boundary. On-going, rapid tectonic uplift, metamorphism and hydrothermal activity are mobilising carbon. Here we use carbon isotope measurements of hot spring fluids and gases, metamorphic host rocks, and carbonate veins to establish a metamorphic carbon budget. We identify three major sources for CO2 within the Southern Alps: (1) the oxidation of graphite; (2) consumption of calcite by metamorphic reactions at the greenschist-amphibolite fades boundary, and (3) the dissolution of groundmass and vein-hosted calcite. There is only a minor component of mantle CO2 arising on the Alpine Fault. Hot springs have molar HCO3-/Ca2+ similar to 9, which is substantially higher than produced by the dissolution of calcite indicating that deeper metamorphic processes must dominate. The total CO2 flux to the near surface environment in the high uplift region of the Southern Alps is estimated to be similar to 6.4 x 10(8) mol/yr. Approximately 87% of this CO2 is sourced from coupled graphite oxidation (25%) and disseminated calcite decarbonation (62%) reactions during prograde metamorphism. Dissolution of calcite and mantle-derived CO2 contribute 10% and 3% respectively. In carbonate-rich orogens CO2 production is dominated by metamorphic decarbonation of limestones. The CO2 flux to the atmosphere from degassing of hot springs in the Southern Alps is 1.9 to 3.2 x 10(8) mol/yr, which is 30-50% of the flux to the near surface environment. By contrast, the drawdown of CO2 through surficial chemical weathering ranges between 2.7 and 20 x 10(9) mol/yr, at least an order of magnitude greater than the CO2 flux to the atmosphere from this orogenic belt. Thus, siliciclastic mountain belts like the Southern Alps are net sinks for atmospheric CO2, in contrast to orogens involving abundant carbonate rocks, such as the Himalaya, that are net CO2 sources.

  • 24.
    Moerner, Nils-Axel
    et al.
    Stockholm University.
    SUBSun, Guangyu
    Paleoearthquake deformations recorded by magnetic variables2008In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 267, no 04-mar, p. 495-502Article in journal (Refereed)
    Abstract [en]

    Faults and related deformational structures give evidence of earthquakes in the past. The ground shaking at high-magnitude seismic events usually generates liquefaction; the higher seismic magnitude, the larger area of liquefaction. We can now demonstrate that it is possible to discriminate between plastic and liquefied deformations with magnetic methods. Besides, it is possible to record internal crypto-deformations not visibly observed. This offers new means of recording past earthquakes. Fine particles carrying the magnetic remanence are shown to move, rotate and re-align with respect to the prevailing geomagnetic field direction or in response to the applied shaking forces. We call this seismomagnetization. This implies that paleoseismics may generate excursion-like pateomagnetic signals despite the absence of visible deformation structures. 

  • 25. Montagnat, Maurine
    et al.
    Blackford, Jane R.
    Piazolo, Sandra
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arnaud, Laurent
    Lebensohn, Ricardo A.
    Measurements and full-field predictions of deformation heterogeneities in ice2011In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 305, no 1-2, p. 153-160Article in journal (Refereed)
    Abstract [en]

    We have made creep experiments on columnar grained ice and characterised the microstructure and intragranular misorientations over a range of length scales. A FFT full-field model was used to predict the deformation behaviour, using the experimentally characterised microstructure as the starting material. This is the first time this combination of techniques has been used to study the deformation of ice. The microstructure was characterised at the cm scale using an optical technique, the automatic ice texture analyser AITA and at the micron scale using electron backscattered diffraction EBSD. The crystallographic texture and intragranular misorientations were fully characterised by EBSD (3 angles). The deformed microstructure frequently showed straight subgrain boundaries often originating at triple points. These were identified as kink bands, and for the first time we have measured the precise misorientation of the kink bands and deduced the nature of the dislocations responsible for them. These dislocations have a basal edge nature and align in contiguous prismatic planes enabling deformation along the c-axis. In addition, non-uniform grain boundaries and regions of recrystallization were seen. We present coupling between fine scale characterization of intragranular misorientations, from experiments, and prediction of internal stresses that cause it. The model predicts the morphology of the observed local misorientations within the grains, however it over predicts the misorientation values. This is because the annealing and recrystallization mechanisms are not taken into account in the model. Ice is excellent as a model material for measuring, predicting and understanding deformation behaviour for polycrystalline materials. Specifically for ice this knowledge is needed to improve models of ice sheet dynamics that are important for climatic signal interpretation.

  • 26.
    Pausata, Francesco S. R.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Impacts of dust reduction on the northward expansion of the African monsoon during the Green Sahara period2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 434, p. 298-307Article in journal (Refereed)
    Abstract [en]

    The West African Monsoon (WAM) is crucial for the socio-economic stability of millions of people living in the Sahel. Severe droughts have ravaged the region in the last three decades of the 20th century, highlighting the need for a better understanding of the WAM dynamics. One of the most dramatic changes in the West African Monsoon (WAM) occurred between 15000-5000 yr BP, when increased summer rainfall led to the so-called Green Sahara and to a reduction in dust emissions from the region. However, model experiments are unable to fully reproduce the intensification and geographical expansion of the WAM during this period, even when vegetation over the Sahara is considered. Here, we use a fully coupled simulation for 6000 yr BP (Mid-Holocene) in which prescribed Saharan vegetation and dust concentrations are changed in turn. A closer agreement with proxy records is obtained only when both the Saharan vegetation changes and dust decrease are taken into account. The dust reduction strengthens the vegetation-albedo feedback, extending the monsoon's northern limit approximately 500 km further than the vegetation-change case only. We therefore conclude that accounting for changes in Saharan dust loadings is essential for improving model simulations of the WAM during the Mid-Holocene.

  • 27. Reed, Daniel C.
    et al.
    Gustafsson, Bo G.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Slomp, Caroline P.
    Shelf-to-basin iron shuttling enhances vivianite formation in deep Baltic Sea sediments2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 434, p. 241-251Article in journal (Refereed)
    Abstract [en]

    Coastal hypoxia is a growing and persistent problem largely attributable to enhanced terrestrial nutrient (i.e., nitrogen and phosphorus) loading. Recent studies suggest phosphorus removal through burial of iron (II) phosphates, putatively vivianite, plays an important role in nutrient cycling in the Baltic Sea the world's largest anthropogenic dead zone yet the dynamics of iron (II) phosphate formation are poorly constrained. To address this, a reactive-transport model was used to reconstruct the diagenetic and depositional history of sediments in the Faro basin, a deep anoxic and sulphidic region of the Baltic Sea where iron (II) phosphates have been observed. Simulations demonstrate that transport of iron from shelf sediments to deep basins enhances vivianite formation while sulphide concentrations are low, but that pyrite forms preferentially over vivianite when sulphate reduction intensifies due to elevated organic loading. Episodic reoxygenation events, associated with major inflows of oxic waters, encourage the retention of iron oxyhydroxides and iron-bound phosphorus in sediments, increasing vivianite precipitation as a result. Results suggest that artificial reoxygenation of the Baltic Sea bottom waters could sequester up to 3% of the annual external phosphorus loads as iron (II) phosphates, but this is negligible when compared to potential internal phosphorus loads due to dissolution of iron oxyhydroxides when low oxygen conditions prevail. Thus, enhancing vivianite formation through artificial reoxygenation of deep waters is not a viable engineering solution to eutrophication in the Baltic Sea. Finally, simulations suggest that regions with limited sulphate reduction and hypoxic intervals, such as eutrophic estuaries, could act as important phosphorus sinks by sequestering vivianite. This could potentially alleviate eutrophication in shelf and slope environments.

  • 28.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Uysal, I. Tonguc
    Yuce, Galip
    Unal-Imer, Ezgi
    Italiano, Francesco
    Imer, Ali
    Zhao, Jian-xin
    Recent mantle degassing recorded by carbonic spring deposits along sinistral strike-slip faults, south-central Australia2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 454, p. 304-318Article in journal (Refereed)
    Abstract [en]

    The interior of the Australian continent shows evidence for late Quaternary to Recent fault-controlled mantle He-3 and CO2 degassing. A series of interconnected NW-striking sinistral faults, the Norwest fault zone (NFZ), in south-central Australia are associated with travertine mounds, the latter show a regular spacing of 50-70 km. U-series ages on 26 samples range from 354 +/- 7 to 1.19 +/- 0.02 ka (2 sigma errors) and suggest a clustering every similar to 3-4 ka since similar to 26 ka. Geochemical data demonstrate a remarkable mantle to-groundwater connection. Isotopic data indicate that the groundwater is circulating to depths >3 km and interacting with Neoproterozoic/Cambrian basement and mantle volatiles. He-3/He-4 isotope ratios show that the He comes in part from the mantle. This demonstrates that the NFZ cuts through the entire crust and provides pathways for mantle degassing. Scaling relationships suggest that the series of sinistral faults that make up the NFZ are interconnected at depths and have a significant strike length of 60-70 km or more. The NFZ occurs where a major compositional boundary and a significant heat flow anomaly occurs, and a major step in lithospheric thickness has been mapped. We discuss a tectonic model in which recent stress field, heat flow and lithospheric structure in central Australia reactivated a set of steeply dipping Neoproterozoic faults, which may now be growing into a crustal/lithospheric-scale structure.

  • 29. Roberts, Andrew P.
    et al.
    Florindo, Fabio
    Larrasoana, Juan C.
    O'Regan, Matthew A.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Zhao, Xiang
    Complex polarity pattern at the former Plio-Pleistocene global stratotype section at Vrica (Italy): Remagnetization by magnetic iron sulphides2010In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 292, no 02-jan, p. 98-111Article in journal (Refereed)
    Abstract [en]

    The Vrica section in Calabria, southern Italy, was the global stratotype for the Pliocene-Pleistocene boundary until this boundary was redefined in 2009. Several paleomagnetic investigations have been carried out at Vrica to determine the age of the formerly defined Pliocene-Pleistocene boundary, which was a key calibration point for the astronomical polarity timescale (APTS). Each study has documented a complex polarity pattern at and above the top of the Olduvai subchron and in relation to the existence of the so-called Vrica subchron. When constructing the APTS, two alternative interpretations for the Vrica section were proposed, neither of which could be conclusively supported. Authigenic growth of magnetic iron sulphide minerals was proposed to explain the complex magnetic polarity record. Availability of a fresh 50-m sediment core enabled us to test this possibility. Our magnetostratigraphic record is similar to that of previous studies, but it is also complex above the Olduvai subchron. We confirm abundant occurrences of authigenic greigite and pyrrhotite, along with detrital titanomagnetite. Authigenic monoclinic pyrrhotite indicates growth significantly later than deposition, and greigite can grow at any time during diagenesis, depending on the availability of dissolved iron and sulphide. The spatially variable magnetic polarity pattern at Vrica is therefore interpreted to have resulted from post-depositional magnetic iron sulphide formation at variable times. Tectonism along the Calabrian arc provides a plausible mechanism for forcing reducing fluids through the sediments, thereby supplying the dissolved ions needed to produce late diagenetic sulphide growth and remagnetization. The complex magnetostratigraphic record at Vrica was taken into account when the APTS was developed, and alternative interpretations result in a maximum age difference of 50 kyr for the upper Olduvai reversal. Our results therefore do not undermine the APTS. Rather, they explain the complex magnetic polarity pattern at this globally important location and highlight the importance of remagnetization processes in such sediments.

  • 30. Schaller, M.
    et al.
    Ehlers, T. A.
    Lang, Karl A. H.
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Tuebingen, Germany.
    Schmid, M.
    Fuentes-Espoz, J. P.
    Addressing the contribution of climate and vegetation cover on hillslope denudation, Chilean Coastal Cordillera (26 degrees-38 degrees S)2018In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 489, p. 111-122Article in journal (Refereed)
    Abstract [en]

    The Earth surface is modulated by interactions among tectonics, climate, and biota. The influence of each of these factors on hillslope denudation rates is difficult to disentangle. The Chilean Coastal Cordillera offers a strong climate and vegetation gradient from arid and unvegetated in the North to humid and vegetated in the South. A similar (convergent) plate tectonic boundary lies to the West of the Coastal Cordillera. We present eight depth profiles analyzed for in situ-produced cosmogenic Be-10 in four study areas. These profiles reveal denudation rates of soil-mantled hillslopes and the depth of mobile layers. Depth profiles were investigated from both S- and N-facing mid-slope positions. Results indicate the depth of the mobile layers in the four study areas increase from N to S in latitude. When mixing is present in the mobile layers they are completely mixed. In the S- and N-facing hillslopes of each study area, mid-slope positions do not show a systematic change in depth of the mobile layers nor in denudation rates based on cosmogenic depth profiles. From N to S in latitude, modelled denudation rates of hillslopes increase from similar to 0.46 to similar to 5.65 cm/kyr and then decrease to similar to 3.22 cm/kyr in the southernmost, highest vegetation cover, study area. Calculated turnover times of soils decrease from 30 to similar to 11 kyr and then increase to similar to 22 kyr. In this work, the increasing denudation rates are attributed to increasing mean annual precipitation from N to S. However, despite the ongoing increase in precipitation from N to S, the denudation rate in the southernmost location does not continue to increase due to the protective nature of increasing vegetation cover. This indicates a vegetation induced non-linear relationship with denudation rates.

  • 31. Snape, Joshua F.
    et al.
    Nemchin, Alexander A.
    Bellucci, Jeremy J.
    Whitehouse, Martin J.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Tartèse, Romain
    Barnes, Jessica J.
    Anand, Mahesh
    Crawford, Ian A.
    Joy, Katherine H.
    Lunar basalt chronology, mantle differentiation and implications for determining the age of the Moon2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 451, p. 149-158Article in journal (Refereed)
    Abstract [en]

    Despite more than 40 years of studying Apollo samples, the age and early evolution of the Moon remain contentious. Following the formation of the Moon in the aftermath of a giant impact, the resulting Lunar Magma Ocean (LMO) is predicted to have generated major geochemically distinct silicate reservoirs, including the sources of lunar basalts. Samples of these basalts, therefore, provide a unique opportunity to characterize these reservoirs. However, the precise timing and extent of geochemical fractionation is poorly constrained, not least due to the difficulty in determining accurate ages and initial Pb isotopic compositions of lunar basalts. Application of an in situ ion microprobe approach to Pb isotope analysis has allowed us to obtain precise crystallization ages from six lunar basalts, typically with an uncertainty of about +/- 10 Ma, as well as constrain their initial Pb-isotopic compositions. This has enabled construction of a two-stage model for the Pb-isotopic evolution of lunar silicate reservoirs, which necessitates the prolonged existence of high-mu reservoirs in order to explain the very radiogenic compositions of the samples. Further, once firm constraints on U and Pb partitioning behaviour are established, this model has the potential to help distinguish between conflicting estimates for the age of the Moon. Nonetheless, we are able to constrain the timing of a lunar mantle reservoir differentiation event at 4376 +/- 18 Ma, which is consistent with that derived from the Sm-Nd and Lu-Hf isotopic systems, and is interpreted as an average estimate of the time at which the high-mu, urKREEP reservoir was established and the Ferroan Anorthosite (FAN) suite was formed.

  • 32.
    Zhao, Zhihong
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Simultaneous calculation of metamorphic fluid fluxes, reaction rates and fluid-rock interaction timescales using a novel inverse modeling framework2013In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 373, p. 217-227Article in journal (Refereed)
    Abstract [en]

    To study metamorphic carbonation at greenschist facies conditions in the SW Scottish Highlands, a novel inverse modeling framework, which combines solutions of the transport equation with a global optimization method of differential evolution, was developed. Using this framework, we calculated simultaneously time-integrated and time-averaged metamorphic fluid fluxes of 83.4 +/- 35.4 m(3) m(-2) and 10(-10.1) (+/-) (0.5) m(3) m(-2) s(-1), respectively, a time-averaged reaction rate constant of 10(-12.7--10.2) m s(-1) and comparable timescales for fluid flow and fluid-driven reaction of 10(4.3 +/- 0.5) yr and 10(2.7-5.2) yr, respectively. These parameters were calculated using an empirical estimate of the coefficient of molecular diffusion and a calculated value for metamorphic porosity. Our estimates are (1) consistent with single pass flow of fluid released by metamorphic devolatilization, (2) within the range where heat is transported by conduction and matter is transported by advection, (3) in agreement with an emerging consensus that metamorphic events are relatively short-lived, and (4) supportive of applying laboratory-based estimates of kinetic parameters to metamorphic systems. Based on a sensitivity analysis, we show that (1) selecting the diffusion coefficient (rather than fluid velocity, reaction rate or flow duration) as an input parameter yields more robust estimates of metamorphic fluid flow parameters, and (2) ignoring reaction-dependent porosity and reaction rates can result in an order-of-magnitude uncertainty in best-fit flow parameters, evaluated from concentration profiles. Finally, similarity between our calculated time-averaged metamorphic fluid fluxes which were obtained numerically and those which were obtained analytically confirms the validity of using the 'quasi-stationary state' assumption to quantify metamorphic fluid flow parameters.

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