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  • 1. Andersson, Per S.
    et al.
    Porcelli, Don
    Frank, Martin
    Bjork, Goran
    Dahlqvist, Ralf
    Gustafsson, Orjan
    Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic-Atlantic gateways2008In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 72, no 12, p. 2854-2867Article in journal (Refereed)
    Abstract [en]

    The neodymium concentration, C-Nd, and isotopic composition, epsilon(Nd), in seawater have been determined in the water column at five sites in the Barents Sea-Fram Strait area where most of the water exchange between the Arctic Ocean and the North Atlantic takes place. In the main Arctic Ocean inflow branch across the Barents Sea the concentration and isotopic composition (C-Nd = 15.5 pmol/kg and epsilon(Nd) = -10.8) are similar to those reported for the northeastern Nordic Seas, which is consistent with this region being a source area for the Arctic inflow. Due to the addition of Nd from Svalbard shelf sediments, the C-Nd in the surface waters above 150 m, in the Fram Strait inflow branch is higher by a factor of 2 and the epsilon(Nd) is shifted to lower values (-11.8). In the stratified Nansen Basin, where cold low salinity water overlies warmer Atlantic water the C-Nd and epsilon(Nd) do not vary with the vertical temperature-salinity structure but are essentially constant and similar to those of the Atlantic inflow throughout the entire water column, down to 3700 m depth, which indicates that the Nd is to a large extent of Atlantic origin. Compared to the Atlantic inflow water, the Nd in the major Arctic Ocean outflow, the Fram Strait, show higher C-Nd in the surface waters above 150 m, and a higher epsilon(Nd) (-9.8) throughout the entire water column down to 1300 m depth. Sources for the more radiogenic Nd isotopic composition in deep water of the Fram Strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the Canadian archipelago. River water is a possible source in the surface water but it also seems likely that Pacific water Nd, modified by interactions on the shelf, is an important component in the Fram Strait surface outflow. Changes in the relative proportions of inflow of river water and flow of Pacific water through the Arctic Ocean could thus influence the isotopic composition of Nd in the North Atlantic. (c) 2008 Elsevier Ltd. All rights reserved.

  • 2. Douglas, P. M. J.
    et al.
    Stolper, D. A.
    Smith, D. A.
    Walter Anthony, K. M.
    Paull, C. K.
    Dallimore, S.
    Wik, M.
    Crill, P. M.
    Winterdahl, M.
    Eiler, J. M.
    Sessions, A. L.
    Diverse origins of Arctic and Subarctic methane point source emissions identified with multiply-substituted isotopologues2016In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 188, p. 163-188Article in journal (Refereed)
    Abstract [en]

    Abstract Methane is a potent greenhouse gas, and there are concerns that its natural emissions from the Arctic could act as a substantial positive feedback to anthropogenic global warming. Determining the sources of methane emissions and the biogeochemical processes controlling them is important for understanding present and future Arctic contributions to atmospheric methane budgets. Here we apply measurements of multiply-substituted isotopologues, or clumped isotopes, of methane as a new tool to identify the origins of ebullitive fluxes in Alaska, Sweden and the Arctic Ocean. When methane forms in isotopic equilibrium, clumped isotope measurements indicate the formation temperature. In some microbial methane, however, non-equilibrium isotope effects, probably related to the kinetics of methanogenesis, lead to low clumped isotope values. We identify four categories of emissions in the studied samples: thermogenic methane, deep subsurface or marine microbial methane formed in isotopic equilibrium, freshwater microbial methane with non-equilibrium clumped isotope values, and mixtures of deep and shallow methane (i.e., combinations of the first three end members). Mixing between deep and shallow methane sources produces a non-linear variation in clumped isotope values with mixing proportion that provides new constraints for the formation environment of the mixing end-members. Analyses of microbial methane emitted from lakes, as well as a methanol-consuming methanogen pure culture, support the hypothesis that non-equilibrium clumped isotope values are controlled, in part, by kinetic isotope effects induced during enzymatic reactions involved in methanogenesis. Our results indicate that these kinetic isotope effects vary widely in microbial methane produced in Arctic lake sediments, with non-equilibrium Δ18 values spanning a range of more than 5‰.

  • 3. Frank, M.
    et al.
    Porcelli, D.
    Andersson, P.
    Baskaran, M.
    Bjork, G.
    Kubik, P. W.
    Hattendorf, B.
    Guenther, D.
    The dissolved Beryllium isotope composition of the Arctic Ocean2009In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 73, no 20, p. 6114-6133Article in journal (Refereed)
    Abstract [en]

    We present the first comprehensive set of dissolved Be-10 and Be-9 concentrations in surface waters and vertical profiles of all major sub-basins of the Arctic Ocean, which are complemented by data from the major Arctic rivers Mackenzie, Lena, Yenisey and Ob. The results show that Be-10 and Be-9 concentrations in waters below 150 m depth are low and only vary within a factor of 2 throughout the Arctic Basin (350-750 atoms/g and 9-15 pmol/kg, respectively). In marked contrast, Be isotope compositions in the upper 150 m are highly variable and show systematic variations. Cosmogenic Be-10 concentrations range from 150 to 1000 atoms/g and concentrations of terrigenous Be-9 range from 7 to 65 pmol/kg, resulting in Be-10/Be-9 ratios (atom/atom) between 0.5 and 14 x 10(-8). In. owing Atlantic water masses in the Eurasian Basin are characterized by a Be-10/Be-9 signature of 7 x 10(-8). The inflow of Pacific water masses across the Bering Strait is characterized by lower ratios of 2-3 x 10(-8), which can be traced into the central Arctic Ocean, possibly as far as the Fram Strait. A comparison of the high dissolved surface Be-10 and Be-9 concentrations (corresponding to low Be-10/Be-9 signatures of similar to 2 x 10(-8)) in the Eurasian Basin with hydrographic parameters and river data documents efficient and rapid transport of Be with Siberian river waters across the Siberian Arctic shelves into the central Arctic Basin, although significant loss and exchange of Be on the shelves occurs. In contrast, fresh surface waters from the Canada Basin also show high cosmogenic Be-10 contents, but are not enriched in terrigenous Be-9 (resulting in high Be-10/Be-9 signatures of up to 14 x 10(-8)). This is explained by a combination of efficient scavenging of Be in the Mackenzie River estuary and the shelves and additional supply of cosmogenic Be-10 via atmospheric fallout and melting of old sea ice. The residence time of Be in the deep Arctic Ocean estimated from our data is 800 years and thus similar to the average Be residence time in the global ocean. (C) 2009 Elsevier Ltd. All rights reserved.

  • 4. Haley, B. A.
    et al.
    Frank, M.
    Moran, K.
    Backman, J.
    Cenozoic nedoymium isotope evolution of Arctic Ocean deep water2006In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 70, no 18, SArticle in journal (Refereed)
  • 5. Hirst, Catherine
    et al.
    Andersson, Per S.
    Shaw, Samuel
    Burke, Ian T.
    Kutscher, Liselott
    Murphy, Melissa J.
    Maximov, Trofim
    Pokrovsky, Oleg S.
    Mörth, Carl-Magnus
    Porcelli, Don
    Characterisation of Fe-bearing particles and colloids in the Lena River basin, NE Russia2017In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 213, p. 553-573Article in journal (Refereed)
    Abstract [en]

    Rivers are significant contributors of Fe to the ocean. However, the characteristics of chemically reactive Fe remain poorly constrained, especially in large Arctic rivers, which drain landscapes highly susceptible to climate change and carbon cycle alteration. The aim of this study was a detailed characterisation (size, mineralogy, and speciation) of riverine Fe-bearing particles (>0.22µm) and colloids (1kDa–0.22µm) and their association with organic carbon (OC), in the Lena River and tributaries, which drain a catchment almost entirely underlain by permafrost. Samples from the main channel and tributaries representing watersheds that span a wide range in topography and lithology were taken after the spring flood in June 2013 and summer baseflow in July 2012. Fe-bearing particles were identified, using Transmission Electron Microscopy, as large (200nm–1µm) aggregates of smaller (20–30nm) spherical colloids of chemically-reactive ferrihydrite. In contrast, there were also large (500nm–1µm) aggregates of clay (illite) particles and smaller (100–200nm) iron oxide particles (dominantly hematite) that contain poorly reactive Fe. TEM imaging and Scanning Transmission X-ray microscopy (STXM) indicated that the ferrihydrite is present as discrete particles within networks of amorphous particulate organic carbon (POC) and attached to the surface of primary produced organic matter and clay particles. Together, these larger particles act as the main carriers of nanoscale ferrihydrite in the Lena River basin. The chemically reactive ferrihydrite accounts for on average 70±15% of the total suspended Fe in the Lena River and tributaries. These observations place important constraints on Fe and OC cycling in the Lena River catchment area and Fe-bearing particle transport to the Arctic Ocean.

  • 6.
    Hodgkins, Suzanne B.
    et al.
    Florida State Univ, Dept Earth Ocean & Atmospher Sci, Tallahassee, FL 32306 USA..
    Tfaily, Malak M.
    Pacific Northwest Natl Lab, Environm Mol Sci Lab, Richland, WA 99354 USA.;Pacific Northwest Natl Lab, Div Biol Sci, Richland, WA 99354 USA..
    Podgorski, David C.
    Florida State Univ, Future Fuels Inst, Natl High Magnet Field Lab, Tallahassee, FL 32310 USA..
    McCalley, Carmody K.
    Rochester Inst Technol, Thomas H Gosnell Sch Life Sci, Rochester, NY 14623 USA..
    Saleska, Scott R.
    Univ Arizona, Dept Ecol & Evolutionary Biol, Tucson, AZ 85716 USA..
    Crill, Patrick M.
    Stockholm Univ, Dept Geol Sci, S-10691 Stockholm, Sweden..
    Rich, Virginia I.
    Ohio State Univ, Dept Microbiol, Columbus, OH 43210 USA..
    Chanton, Jeffrey P.
    Florida State Univ, Dept Earth Ocean & Atmospher Sci, Tallahassee, FL 32306 USA..
    Cooper, William T.
    Florida State Univ, Dept Chem & Biochem, Tallahassee, FL 32306 USA..
    Elemental composition and optical properties reveal changes in dissolved organic matter along a permafrost thaw chronosequence in a subarctic peatland2016In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 187, p. 123-140Article in journal (Refereed)
    Abstract [en]

    The fate of carbon stored in permafrost-zone peatlands represents a significant uncertainty in global climate modeling. Given that the breakdown of dissolved organic matter (DOM) is often a major pathway for decomposition in peatlands, knowledge of DOM reactivity under different permafrost regimes is critical for determining future climate feedbacks. To explore the effects of permafrost thaw and resultant plant succession on DOM reactivity, we used a combination of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), UV/Vis absorbance, and excitation-emission matrix spectroscopy (EEMS) to examine the DOM elemental composition and optical properties of 27 pore water samples gathered from various sites along a permafrost thaw sequence in Stordalen Mire, a thawing subarctic peatland in northern Sweden. The presence of dense Sphagnum moss, a feature that is dominant in the intermediate thaw stages, appeared to be the main driver of variation in DOM elemental composition and optical properties at Stordalen. Specifically, DOM from sites with Sphagnum had greater aromaticity, higher average molecular weights, and greater O/C, consistent with a higher abundance of phenolic compounds that likely inhibit decomposition. These compounds are released by Sphagnum and may accumulate due to inhibition of phenol oxidase activity by the acidic pH at these sites. In contrast, sites without Sphagnum, specifically fully-thawed rich fens, had more saturated, more reduced compounds, which were high in N and S. Optical properties at rich fens indicated the presence of microbially-derived DOM, consistent with the higher decomposition rates previously measured at these sites. These results indicate that Sphagnum acts as an inhibitor of rapid decomposition and CH4 release in thawing subarctic peatlands, consistent with lower rates of CO2 and CH4 production previously observed at these sites. However, this inhibitory effect may disappear if Sphagnum-dominated bogs transition to more waterlogged rich fens that contain very little to no living Sphagnum. Release of this inhibition allows for higher levels of microbial activity and potentially greater CH4 release, as has been observed in these fen sites. (C) 2016 Elsevier Ltd. All rights reserved.

  • 7. Murphy, Melissa J.
    et al.
    Porcelli, Don
    von Strandmann, Philip A. E. Pogge
    Hirst, Catherine A.
    Kutscher, Liselott
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Katchinoff, Joachim A.
    Mörth, Carl-Magnus
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Maximov, Trofim
    Andersson, Per S.
    Tracing silicate weathering processes in the permafrost-dominated Lena River watershed using lithium isotopes2019In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 245, p. 154-171Article in journal (Refereed)
    Abstract [en]

    Increasing global temperatures are causing widespread changes in the Arctic, including permafrost thawing and altered freshwater inputs and trace metal and carbon fluxes into the ocean and atmosphere. Changes in the permafrost active layer thickness can affect subsurface water flow-paths and water-rock interaction times, and hence weathering processes. Riverine lithium isotope ratios (reported as delta Li-7) are tracers of silicate weathering that are unaffected by biological uptake, redox, carbonate weathering and primary lithology. Here we use Li isotopes to examine silicate weathering processes in one of the largest Russian Arctic rivers: the Lena River in eastern Siberia. The Lena River watershed is a large multi-lithological catchment, underlain by continuous permafrost. An extensive dataset of dissolved Li isotopic compositions of waters from the Lena River main channel, two main tributaries (the Aldan and Viliui Rivers) and a range of smaller sub-tributaries are presented from the post-spring flood/early-summer period at the onset of active layer development and enhanced water-rock interactions. The Lena River main channel (average delta Li-7(diss) similar to 19 parts per thousand) has a slightly lower isotopic composition than the mean global average of 23 parts per thousand (Huh et al., 1998a). The greatest range of [Li] and delta Li-7(diss) are observed in catchments draining the south-facing slopes of the Verkhoyansk Mountain Range. South-facing slopes in high-latitude, permafrost-dominated regions are typically characterised by increased summer insolation and higher daytime temperatures relative to other slope aspects. The increased solar radiation on south-facing catchments promotes repeated freeze-thaw cycles, and contributes to more rapid melting of snow cover, warmer soils, and increased active layer thaw depths. The greater variability in delta Li-7 and [Li] in the south-facing rivers likely reflect the greater infiltration of melt water and enhanced water-rock interactions within the active layer. A similar magnitude of isotopic fractionation is observed between the low-lying regions of the Central Siberian Plateau (and catchments draining into the Viliui River), and catchments draining the Verkhoyansk Mountain Range into the Aldan River. This is in contrast to global rivers in non-permafrost terrains that drain high elevations or areas of rapid uplift, where high degrees of physical erosion promote dissolution of freshly exposed primary rock typically yielding low delta Li-7(diss), and low-lying regions exhibit high riverine delta Li-7 values resulting from greater water-rock interaction and formation of secondary mineral that fractionates Li isotopes. Overall, the range of Li concentrations and delta 7Lidiss observed within the Lena River catchment are comparable to global rivers located in temperate and tropical regions. This suggests that cryogenic weathering features specific to permafrost regions (such as the continual exposure of fresh primary minerals due to seasonal freeze-thaw cycles, frost shattering and salt weathering), and climate (temperature and runoff), are not a dominant control on delta Li-7 variations. Despite vastly different climatic and weathering regimes, the same range of riverine delta Li-7 values globally suggests that the same processes govern Li geochemistry - that is, the balance between primary silicate mineral dissolution and the formation (or exchange with) secondary minerals. This has implications for the use of delta Li-7 as a palaeo-weathering tracer for interpreting changes in past weathering regimes.

  • 8. Naafs, B. D. A.
    et al.
    Inglis, G. N.
    Zheng, Y.
    Amesbury, M. J.
    Biester, H.
    Bindler, R.
    Blewett, J.
    Burrows, M. A.
    Torres, D. del Castillo
    Chambers, F. M.
    Cohen, A. D.
    Evershed, R. P.
    Feakins, S. J.
    Galka, M.
    Gallego-Sala, A.
    Gandois, L.
    Gray, D. M.
    Hatcher, P. G.
    Coronado, E.N. Honorio
    Hughes, P. D. M.
    Huguet, A.
    Könönen, M.
    Laggoun-Defarge, F.
    Lähteenoja, O.
    Lamentowicz, M.
    Marchant, R.
    McClymont, E.
    Pontevedra-Pombal, X.
    Ponton, C.
    Pourmand, A.
    Rizzuti, A. M.
    Rochefort, L.
    Schellekens, J.
    Vleeschouwer, F. De
    Pancost, R. D.
    Introducing global peat-specific temperature and pH calibrations based on brGDGT bacterial lipids2017In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 208, no Supplement C, p. 285-301Article in journal (Refereed)
    Abstract [en]

    Abstract Glycerol dialkyl glycerol tetraethers (GDGTs) are membrane-spanning lipids from Bacteria and Archaea that are ubiquitous in a range of natural archives and especially abundant in peat. Previous work demonstrated that the distribution of bacterial branched GDGTs (brGDGTs) in mineral soils is correlated to environmental factors such as mean annual air temperature (MAAT) and soil pH. However, the influence of these parameters on brGDGT distributions in peat is largely unknown. Here we investigate the distribution of brGDGTs in 470 samples from 96 peatlands around the world with a broad mean annual air temperature (-8 to 27 °C) and pH (3-8) range and present the first peat-specific brGDGT-based temperature and pH calibrations. Our results demonstrate that the degree of cyclisation of brGDGTs in peat is positively correlated with pH, pH=2.49xCBTpeat+8.07 (n=51, R2=0.58, RMSE=0.8) and the degree of methylation of brGDGTs is positively correlated with MAAT, MAATpeat (°C)=52.18xMBT5me'-23.05 (n=96, R2=0.76, RMSE=4.7°C). These peat-specific calibrations are distinct from the available mineral soil calibrations. In light of the error in the temperature calibration ( 4.7°C), we urge caution in any application to reconstruct late Holocene climate variability, where the climatic signals are relatively small, and the duration of excursions could be brief. Instead, these proxies are well-suited to reconstruct large amplitude, longer-term shifts in climate such as deglacial transitions. Indeed, when applied to a peat deposit spanning the late glacial period ( 15.2kyr), we demonstrate that MAATpeat yields absolute temperatures and relative temperature changes that are consistent with those from other proxies. In addition, the application of MAATpeat to fossil peat (i.e. lignites) has the potential to reconstruct terrestrial climate during the Cenozoic. We conclude that there is clear potential to use brGDGTs in peats and lignites to reconstruct past terrestrial climate.

  • 9. Pearson, Emma J.
    et al.
    Juggins, Steve
    Talbot, Helen M.
    Weckström, Jan
    Rosèn, Peter
    Ryves, David B.
    Roberts, Stephen J.
    Schmidt, Roland
    A lacustrine GDGT-temperature calibration from the Scandinavian Arctic to Antarctic: Renewed potential for the application of GDGT-paleothermometry in lakes2011In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 75, no 20, p. 6225-6238Article in journal (Refereed)
    Abstract [en]

    Quantitative climate reconstructions are fundamental to understand long-term trends in natural climate variability and to test climate models used to predict future climate change. Recent advances in molecular geochemistry have led to calibrations using glycerol dialkyl glycerol tetraethers (GDGTs), a group of temperature-sensitive membrane lipids found in Archaea and bacteria. GDGTs have been used to construct temperature indices for oceans (TEX86 index) and soils (MBT/CBT index). The aim of this study is to examine GDGT-temperature relationships and assess the potential of constructing a GDGT-based palaeo-thermometer for lakes. We examine GDGT-temperature relationships using core top sediments from 90 lakes across a north–south transect from the Scandinavian Arctic to Antarctica including sites from Finland, Sweden, Siberia, the UK, Austria, Turkey, Ethiopia, Uganda, Chile, South Georgia and the Antarctic Peninsula. We examine a suite of 15 GDGTs, including compounds used in the TEX86 and MBT/CBT indices and reflecting the broad range of GDGT inputs to small lake systems.

    GDGTs are present in varying proportions in all lakes examined. The TEX86 index is not applicable to our sites because of the large relative proportions of soil derived and methanogenic components. Similarly, the MBT/CBT index is also not applicable and predicts temperatures considerably lower than those measured. We examine relationships between individual GDGT compounds and temperature, pH, conductivity and water depth. Temperature accounts for a large and statistically independent fraction of variation in branched GDGT composition. We propose a GDGT-temperature regression model with high accuracy and precision (R2 = 0.88; RMSE = 2.0 °C; RMSEP = 2.1 °C) for use in lakes based on a subset of branched GDGT compounds and highlight the potential of this new method for reconstructing past temperatures using lake sediments.

  • 10. Porcelli, Don
    et al.
    Andersson, Per S.
    Baskaran, M.
    Frank, Martin
    Bjork, Goran
    Semiletov, Igor
    The distribution of neodymium isotopes in Arctic Ocean basins2009In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 73, no 9, p. 2645-2659Article in journal (Refereed)
    Abstract [en]

    Nd concentration and isotope data have been obtained for the Canada, Amundsen, and Makarov Basins of the Arctic Ocean. A pattern of high Nd concentrations (up to 58 pM) at shallow depths is seen throughout the Arctic, and is distinct from that generally seen in other oceans where surface waters are relatively depleted. A range of isotopic variations across the Arctic and within individual depth profiles reflects the different sources of waters. The dominant source of water, and so Nd, is the Atlantic Ocean, with lesser contributions from the Pacific and Arctic Rivers. Radiogenic isotope Nd signatures (up to epsilon(Nd) = -6.5) can be traced in Pacific water flowing into the Canada Basin. Waters from rivers draining older terrains provide very unradiogenic Nd (down to epsilon(Nd) = -14.2) that can be traced in surface waters across much of the Eurasian Basin. A distinct feature of the Arctic is the general influence of the shelves on the Nd concentrations of waters flowing into the basins, either from the Pacific across the Chukchi Sea, or from across the extensive Siberian shelves. Water-shelf interaction results in an increase in Nd concentration without significant changes in salinity in essentially all waters in the Arctic, through processes that are not yet well understood. In estuarine regions other processes modify the Nd signal of freshwater components supplied into the Arctic Basin, and possibly also contribute to sedimentary Nd that may be subsequently involved in sediment-water interactions. Mixing relationships indicate that in estuaries, Nd is removed from major river waters to different degrees. Deep waters in the Arctic are higher in Nd than the inflowing Atlantic waters, apparently through enrichments of waters on the shelves that are involved in ventilating the deep basins. These enrichments generally have not resulted in major shifts in the isotopic compositions of the deep waters in the Makarov Basin (epsilon(Nd) similar to -10.5), but have created distinctive Nd isotope signatures that were found near the margin of the Canada Basin (with epsilon(Nd) similar to -9.0). The deep waters of the Amundsen Basin are also distinct from the Atlantic waters (with epsilon(Nd) = -12.3), indicating that there has been limited inflow from the adjacent Makarov Basin through the Lomonosov Ridge. (C) 2009 Elsevier Ltd. All rights reserved.

  • 11.
    Sun, Xiaole
    et al.
    Baltic Sea Center SU.
    Mörth, Carl-Magnus
    Porcelli, Don
    Kutscher, Liselott
    Naturhistoriska riksmuseet, Enheten för geovetenskap.
    Hirst, Catherine
    Naturhistoriska riksmuseet, Enheten för geovetenskap.
    Murphy, Melissa J.
    Maximov, Trofim
    Petrov, Roman E.
    Humborg, Christoph
    Schmitt, Melanie
    Naturhistoriska riksmuseet, Enheten för geovetenskap.
    Andersson, Per
    Naturhistoriska riksmuseet, Enheten för geovetenskap.
    Stable silicon isotopic compositions of the Lena River and its tributaries: Implications for silicon delivery to the Arctic Ocean2018In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 241, p. 120-133Article in journal (Refereed)
    Abstract [en]

    Silicon isotope values (d30SiDSi) of dissolved silicon (DSi) have been analyzed in the Lena River and its tributaries, one ofthe largest Arctic watersheds in the world. The geographical and temporal variations of d30SiDSi range from +0.39 to +1.86‰with DSi concentrations from 34 to 121 lM. No obvious patterns of DSi concentrations and d30SiDSi values were observedalong over 200 km of the two major tributaries, the Viliui and Aldan Rivers. In summer, the variations of DSi concentrationsand d30SiDSi values in the water are either caused by biological uptake by higher plants and phytoplankton or by mixing ofwater masses carrying different DSi concentrations and d30SiDSi values. DSi in tributaries from the Verkhoyansk MountainRange seems to be associated with secondary clay formation that increased the d30SiDSi values, while terrestrial biological productionis likely more prevalent in controlling d30SiDSi values in Central Siberian Plateau and Lena Amganski Inter-RiverArea. In winter, when soils were frozen, the d30SiDSi values in the river appeared to be controlled by weathering and clay formationin deep intrapermafrost groundwater. During the spring flood, dissolved silicate materials and phytoliths were flushedfrom the upper thawed soils into rivers, which reset d30SiDSi values to the values observed prior to the biological bloom insummer. The results indicate that the Si isotope values reflect the changing processes controlling Si outputs to the Lena Riverand to the Arctic Ocean between seasons. The annual average d30SiDSi value of the Lena Si flux is calculated to be +0.86± 0.3‰ using measured d30SiDSi values from each season. Combined with the estimate of +1.6 ± 0.25‰ for the Yenisey River,an updated d30SiDSi value of the major river Si inputs to the Arctic Ocean is estimated to be +1.3 ± 0.3‰. This value isexpected to shift towards higher values in the future because of the impacts from a variety of biological and geochemicalprocesses and sources under global warming.

  • 12. Tesi, T.
    et al.
    Semiletov, I.
    Hugelius, G.
    Dudarev, O.
    Kuhry, P.
    Gustafsson, Ö.
    Composition and fate of terrigenous organic matter along the Arctic land-ocean continuum in East Siberia: Insights from biomarkers and carbon isotopes2014In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 133Article in journal (Refereed)
    Abstract [en]

    Climate warming is predicted to translocate terrigenous organic carbon (TerrOC) to the Arctic Ocean and affect the marine biogeochemistry at high latitudes. The magnitude of this translocation is currently unknown, so is the climate response. The fate of the remobilized TerrOC across the Arctic shelves represents an unconstrained component of this feedback. The present study investigated the fate of permafrost carbon along the land-ocean continuum by characterizing the TerrOC composition in three different terrestrial carbon pools from Siberian permafrost (surface organic rich horizon, mineral soil active layer, and Ice Complex deposit) and marine sediments collected on the extensive East Siberian Arctic Shelf (ESAS). High levels of lignin phenols and cutin acids were measured in all terrestrial samples analyzed indicating that these compounds can be used to trace the heterogeneous terrigenous material entering the Arctic Ocean. In ESAS sediments, comparison of these terrigenous biomarkers with other TerrOC proxies (bulk delta C-13/delta C-14 and HMW lipid biomarkers) highlighted contrasting across-shelf trends. These differences could indicate that TerrOC in the ESAS is made up of several pools that exhibit contrasting reactivity toward oxidation during the transport. In this reactive spectrum, lignin is the most reactive, decreasing up to three orders of magnitude from the inner-to the outer-shelf while the decrease of HMW wax lipid biomarkers was considerably less pronounced. Alternatively, degradation might be negligible while sediment sorting during the across-shelf transport could be the major physical forcing that redistributes different TerrOC pools characterized by different matrix-association. Despite the marked decrease shown by lignin, the fingerprint of lignin phenols such as the acid: aldehyde ratio of vanillyl and syringyl phenols showed a lack of any across-shelf trends and exhibited an extremely wide range of values in all terrestrial samples. By contrast, the 3,5-dihydroxybenzoic: vanillyl phenols ratio exhibited a clear across-shelf trend suggesting either increasing degradation with distance from the coast or TerrOC sorting along the sediment dispersal system. The ratio of syringyl: vanillyl phenols indicated that gymnosperm tissues are more important than angiosperm tissues in surface sediments, in particular off the Lena River mouth, consistent with the vegetation in its watershed. Conversely, the fingerprint of p-hydroxybenzenes suggests lack of substantial input of moss-derived material. Finally, autochthonous lipid-and protein-derived CuO reaction products displayed a strong along-shelf gradient likely reflecting the inflow of nutrient-rich Pacific waters from the Bering Strait that stimulate primary productivity in the eastern ESAS. In particular short-chain fatty acids showed a clear frontal/transition zone between Pacific-influenced and river-influenced waters approximately along the 160 degrees E longitude. Considering the labile nature of phytoplankton, priming and co-metabolism processes might stimulate degradation of TerrOC in the easternmost region of the Siberian shelf. This study demonstrated the need to consider multiple TerrOC proxies at isotopic/molecular levels to differentiate the fate for different allocthonous components in Arctic sediments and the need to assess how these TerrOC pools are distributed in different density, size, and settling fractions to better discriminate between the extent of hydrodynamic sorting versus degradation. (C) 2014 Elsevier Ltd. All rights reserved.

  • 13. Zimmermann, Bettina
    et al.
    Porcelli, Don
    Frank, Martin
    Andersson, Per S.
    Baskaran, M.
    Lee, Der-Chuen
    Halliday, Alex N.
    Hafnium isotopes in Arctic Ocean water2009In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 73, no 11, p. 3218-3233Article in journal (Refereed)
    Abstract [en]

    The first isotopic compositions of dissolved hafnium in seawater from across the Arctic Ocean are reported. Most samples from the four sub-basins of the Arctic Ocean have values within error of an average of epsilon(Hf) = +0.8. Combined Hf-Nd isotope compositions do not fall on the well-established positive correlation for mantle and crustal rocks. Instead, Arctic waters have Hf that is more radiogenic than that typically found in rocks with similar Nd isotope compositions, a feature previously found in ferromanganese crusts and waters from the Pacific Ocean. Arctic seawater samples generally fall on the lower part of the ferromanganese crust array, reflecting influences of inputs from Arctic rivers and interactions of shelf waters with underlying sediments. Arctic rivers have much higher Hf concentrations (7-30 pM) than Arctic seawater (0.36-4.2 pM). Water from the Mackenzie River has the least radiogenic Hf, with epsilon(Hf) = -7.1 +/- 1.7, and plots furthest away from the ferromanganese crust array, while waters from the Ob, Yenisey, and Lena Rivers have values that are indistinguishable from most Arctic waters. In the Amundsen, Makarov, and Canada basins, Hf concentrations are highest at the surface and lowest in the deeper waters, reflecting the influences of riverine inputs and of waters that have flowed over the extensive Siberian continental shelves and have Nd and Hf characteristics that reflect water-sediment interactions. This is in contrast to the relatively low near surface Hf concentrations reported for locations elsewhere. The Pacific water layer in the Canada Basin exhibits the highest value of epsilon(Hf) = +6.8 +/- 1.8, reflecting the Hf isotopic composition of waters entering the Arctic from the Pacific Ocean. Mixing relationships indicate that a substantial fraction of the Hf in the Mackenzie River is lost during estuarine mixing; the behaviour of Hf from other rivers is less constrained. (C) 2009 Elsevier Ltd. All rights reserved.

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