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  • 1. Andersen, M. B.
    et al.
    Stirling, C. H.
    Porcelli, D.
    Halliday, A. N.
    Andersson, P. S.
    Baskaran, M.
    The tracing of riverine U in Arctic seawater with very precise U-234/U-238 measurements2007In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 259, no 1-2, p. 171-185Article in journal (Refereed)
    Abstract [en]

    The riverine flux of U that enters the deep oceans is not well constrained since the net losses during estuarine mixing are difficult to quantify. Riverine-dissolved U normally has a higher U-234/(238) U activity ratio (U-234/U-238(ar)) than the uniform value that characterizes open ocean seawater and could be used as a tracer of riverine inputs if one could resolve subtle variations in seawater composition. Using new mass spectrometry techniques we achieve a long-term reproducibility +/- 0.3 parts per thousand on U-234/U-238(ar) which permits the tracing of riverine U in seawater samples from the Arctic - a partially restricted basin that is ideal for such a study. We find that surface waters from the Arctic basins carry elevated U-234/(238)Uar when compared with deep ocean seawater. Samples from the Canada Basin have a significant freshwater component and provide evidence that the Mackenzie River loses similar to 65% of its U in the Mackenzie shelf/estuary zone before entering the deeper basin. This is in contrast to samples from the Makarov Basin, which provide evidence that all of the freshwater input is derived from the major Yenisey River alone, despite the proximity of the Lena and Ob Rivers. The differing behaviour of U between the Mackenzie and Yenisey Rivers is most likely a consequence of the strong binding of U to dissolved organic matter (DOC) or secondary phases in these rivers. The Yenisey River appears to transport the majority of the DOC through the shelf and into the Makarov Basin. In contrast, the Mackenzie River appears to lose a significant amount of DOC (> 50%) in the estuary/shelf zone, which may lead to loss of associated U. These findings offer a more detailed picture of the fresh riverine water flow patterns in the Arctic Ocean when compared to other geochemical proxies. The non-conservative behaviour of U in the Mackenzie River through the shelf/estuaries has important implications for U input into oceans and the total marine budget. (c) 2007 Elsevier B.V. All rights reserved.

  • 2. Andersen, M. B.
    et al.
    Stirling, C.H.
    Porcelli, D.
    Halliday, A.N.
    Andersson, P. S.
    Baskaran, M.
    The tracing of riverine U in Arctic seawater with very precise 234U / 238U measurements2007In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 259, p. 171-185Article in journal (Refereed)
  • 3. Bennike, O
    et al.
    Björck, S
    Lambeck, K
    Estimates of South Greenland late-glacial ice limits from a new relative sea level curve2002In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 197, no 3-4, p. 171-186, article id PII S0012-821X(02)00478-8Article in journal (Refereed)
    Abstract [en]

    Marine-lacustrine isolation contacts from seven basins in the Nanortalik area, South Greenland have been analysed and dated. The basins were isolated from the sea as a consequence of isostatic rebound following deglaciation. The isolation contacts were identified with litho- and biostratigraphical analyses, especially sedimentary changes, grey scale analyses and analyses of macroscopical remains of plants and animals. Dating was performed by analytical mass spectroscopy radiocarbon dating of macrofossils and bulk sediment samples. A slow initial relative sea level fall that begins at 13.8 cal ka BP changes to a rapid relative sea level fall before the sea level fell below the present-day sea level just prior to 10 cal ka BP. The emergence curve goes further back in time than any previous emergence curve constructed from Greenland, which reflects the early deglaciation of the studied region. The glacio-isostatic crustal rebound following deglaciation was around 110 in. The sea level history indicates that the margin of the Greenland ice sheet probably extended out to the shelf margin during the Last Glacial Maximum, and that the ice thickness must have been at least 1500 in over the outer coast. Thus the highest coastal mountains would have been ice-covered, which is surprising given their alpine character. In addition, the major part of the recession of the ice must have occurred relatively late and quickly, maybe from 14 to 12 cal ka BP. The late Holocene transgression may, at least in part, be due to increased isostatic loading as a consequence of advancing glaciers during the Neoglaciation. (C) 2002 Elsevier Science B.V. All rights reserved.

  • 4. Dahlqvist, R
    et al.
    Andersson, P S
    Ingri, J
    The concentration and isotopic composition of diffusible Nd in fresh and marine waters2005In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 233, no 1-2, p. 9-16Article in journal (Refereed)
    Abstract [en]

    Variations in the Nd isotopic composition of ocean water through time, recorded in marine deposits, are suggested to document changes in erosional input and ocean circulation. Thus characterization of the dissolved Nd component in seawater is important. But few attempts have been made to directly measure truly dissolved Nd. Here we report Nd data, obtained using the technique of diffusive gradients in thin-films (DGT) designed to measure in situ, diffusible labile trace metals in aqueous solutions. The method samples free ions and possibly very small complexes. The concentration and isotopic composition of Nd in fresh, brackish and seawater have been determined with DGT, analyzed with thermal ionization mass-spectrometry, and compared with filtered and bulk water samples. Concentrations measured in water samples and with DGT show that the relative amount of diffusible Nd increases with salinity, from being about 10% in the fresh water to 42% in seawater. At each sampled site, the isotopic composition of Nd in the water shows a similar isotopic composition, within errors, with Nd sampled using the DGT method. These results indicate that there is a complete exchange between the particulate/colloidal fraction and the truly dissolved phase. Therefore our findings suggest that solute Nd reflects both the isotopic composition of the marine sediments and the bulk water. (c) 2005 Elsevier B.V. All rights reserved.

  • 5. Lambelet, M.
    et al.
    Rehkamper, M.
    de Flierdt, T. V.
    Xue, Z. C.
    Kreissig, K.
    Coles, B.
    Porcelli, D.
    Andersson, P.
    Isotopic analysis of Cd in the mixing zone of Siberian rivers with the Arctic Ocean-New constraints on marine Cd cycling and the isotope composition of riverine Cd2013In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 361Article in journal (Refereed)
    Abstract [en]

    The Cd concentrations and isotopic compositions were determined for 19 water samples which cover the mixing zones of four major rivers with the coastal seas of the Siberian Shelf. The waters span salinities from about 1-32, with Cd concentrations of about 0.02-0.46 nmol/kg and small but resolvable Cd isotope fractionations, with epsilon Cd-114/110 values of between +1.4 and +5.7. The data for the majority of the samples are in accord with the systematics expected for quasi-binary mixing of Arctic seawater (epsilon Cd-114/110 approximate to 5.5 +/- 0.5 and Cd concentration between 0.1 and 0.25 nmol/kg) with river waters characterized by epsilon Cd-114/110 approximate to 2 +/- 1 and low pristine Cd contents of about 0.02-0.06 nmol/kg. The river values are similar to the inferred composition of the continental crust, which implies that weathering produces no or only limited Cd isotope fractionation. The results for five samples provide clear evidence for non-conservative behaviour of Cd, as the element is released from suspended riverine particles during mixing with seawater. The isotopic data furthermore show that the desorbed Cd is characterized by epsilon Cd-114/110 approximate to +3 in accord with a natural origin. This implies that the (natural) net riverine Cd fluxes of the Siberian rivers to seawater are also likely to be characterized by epsilon Cd-114/110 approximate to 2 +/- 1, a value that is either identical to or intermediate between the composition of the continental crust and marine deep waters. Additional data for the boreal Kalix River in Sweden contrasts with the results obtained for the Siberian rivers, as the former exhibits a much lighter Cd isotope composition of epsilon Cd-114/110 = -3.8 coupled with a much higher Cd content of similar to 0.24 nmol/kg. These characteristics appear to be a consequence of the distinct hydro-geological setting of the Kalix drainage basin, which suggests that the riverine input of Cd isotopes to the oceans might display significant regional variability. In summary, our study underlines the important role that stable isotope analyses can play in biogeochemical investigations of trace metals. Here, the Cd isotope results provide important constraints, which are not available from concentration data alone, on the cycling of Cd in riverine and shelf environments. (C) 2012 Elsevier B.V. All rights reserved.

  • 6. März, Christian
    et al.
    Vogt, Christoph
    Schnetger, Bernhard
    Brumsack, Hans-Jürgen
    Variable Eocene-Miocene sedimentation processes and bottom water redox conditions in the Central Arctic Ocean (IODP Expedition 302)2011In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 310, p. -537Article in journal (Refereed)
  • 7. Ripperger, S.
    et al.
    Rehkaemper, M.
    Porcelli, D.
    Halliday, A. N.
    Cadmium isotope fractionation in seawater - A signature of biological activity2007In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 261, no 3-4, p. 670-684Article in journal (Refereed)
    Abstract [en]

    Investigations of cadmium isotope variations in the oceans may provide new insights into the factors that control the marine distribution and cycling of this element. Here we present the results of Cd isotope and concentration analyses for 22 seawater samples from the Atlantic, Southern, Pacific, and Arctic Oceans. The results reveal, for the first time, large and well resolved Cd isotope fractionations in the marine environment. The majority of the seawater samples display an inverse relationship between dissolved Cd contents and isotope compositions, which range from epsilon(114/110)Cd approximate to +3 +/- 0.5 for Cd-rich waters (0.8-1.0 nmol/kg) to epsilon(114/110)Cd approximate to 38 +/- 6 for surface water with a Cd concentration of only 0.003 nmol/kg (all epsilon(114/110)Cd data are reported relative to the JMC Cd Munster standard). This suggests that the Cd isotope variations reflect kinetic isotope effects that are generated during closed system uptake of dissolved seawater Cd by phytoplankton. A few samples do not follow this trend, as they exhibit extremely low Cd contents (<0.008 nmol/kg) and nearly un-fractionated Cd isotope compositions. Such complexities, which are not revealed by concentration data alone, require that the Cd distribution at the respective sites was affected by additional processes, such as water mass mixing, atmospheric inputs of Cd and/or adsorption. Uniform isotope compositions of epsilon(114/110)Cd = + 3.3 +/- 0.5 (1 S.D.) were determined for seawater from >= 900 in depth, despite of Cd concentrations that display the expected increase along the global deep-water pathway from the Atlantic (similar to 0.3 nmol/kg) to the Pacific Ocean (similar to 0.9 nmol/kg). This indicates that the biomass, which is remineralized in the deeper ocean, is also characterized by a very constant Cd isotope composition. This observation is in accord with the interpretation that the Cd distribution in surface waters is primarily governed by Rayleigh fractionation during near-quantitative uptake of dissolved seawater Cd. (C) 2007 Elsevier B.V. All rights reserved.

  • 8. Shaw, Alison M.
    et al.
    Behn, Mark D.
    Humphris, Susan E.
    Sohn, Robert A.
    Gregg, Patricia M.
    Deep pooling of low degree melts and volatile fluxes at the 85 degrees E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses2010In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 289, no 3-4, p. 311-322Article in journal (Refereed)
    Abstract [en]

    We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85 degrees E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO2-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO2 contents in the melt inclusions extend to higher values (167-1596 ppm) than in the co-existing glasses (187-227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO2 melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (similar to 4 km) and similar to 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25-40 km) and cold (1240 degrees-1325 degrees C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous: this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9-20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO2 is perfectly incompatible, we show that the highest CO2 concentrations of the melt inclusions (similar to 1600 ppm) are consistent with the calculated CO2 concentrations of primary undegassed melts. The highest measured CO2/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO2 content of 134 ppm and would result in a global ridge flux of 2.0 x 10(12) mol CO2/yr. (C) 2009 Elsevier B.V. All rights reserved.

  • 9. Watson, E. J.
    et al.
    Swindles, G. T.
    Savov, I. P.
    Lawson, I. T.
    Connor, C. B.
    Wilson, J. A.
    Estimating the frequency of volcanic ash clouds over northern Europe2017In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 460, no Supplement C, p. 41-49Article in journal (Refereed)
    Abstract [en]

    Abstract Fine ash produced during explosive volcanic eruptions can be dispersed over a vast area, where it poses a threat to aviation, human health and infrastructure. Here, we focus on northern Europe, which lies in the principal transport direction for volcanic ash from Iceland, one of the most active volcanic regions in the world. We interrogate existing and newly produced geological and written records of past ash fallout over northern Europe in the last 1000 years and estimate the mean return (repose) interval of a volcanic ash cloud over the region to be 44±7 years. We compare tephra records from mainland northern Europe, Great Britain, Ireland and the Faroe Islands, with records of proximal Icelandic volcanism and suggest that an Icelandic eruption with a Volcanic Explosivity Index rating (VEI) ≥ 4 and a silicic magma composition presents the greatest risk of producing volcanic ash that can reach northern Europe. None of the ash clouds in the European record which have a known source eruption are linked to a source eruption with VEI < 4. Our results suggest that ash clouds are more common over northern Europe than previously proposed and indicate the continued threat of ash deposition across northern Europe from eruptions of both Icelandic and North American volcanoes.

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