| 1. |
- Sun, Xiaole, 1983-, et al.
(författare)
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Climate Dependent Diatom Production is Preserved in Biogenic Si Isotope Signatures
- 2011
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 8:11, s. 3491-3499
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to reconstruct diatom production in the subarctic northern tip of the Baltic Sea, Bothnian Bay, based on down-core analysis of Si isotopes in biogenic silica (BSi). Dating of the sediment showed that the samples covered the period 1820 to 2000. The sediment core record can be divided into two periods, an unperturbed period from 1820 to 1950 and a second period affected by human activities (from 1950 to 2000). This has been observed elsewhere in the Baltic Sea. The shift in the sediment core record after 1950 is likely caused by large scale damming of rivers. Diatom production was inferred from the Si isotope composition which ranged between δ30Si −0.18‰ and +0.58‰ in BSi, and assuming fractionation patterns due to the Raleigh distillation, the production was shown to be correlated with air and water temperature, which in turn were correlated with the mixed layer (ML) depth. The sedimentary record showed that the deeper ML depth observed in colder years resulted in less production of diatoms. Pelagic investigations in the 1990's have clearly shown that diatom production in the Baltic Sea is controlled by the ML depth. Especially after cold winters and deep water mixing, diatom production was limited and dissolved silicate (DSi) concentrations were not depleted in the water column after the spring bloom. Our method corroborates these findings and offers a new method to estimate diatom production over much longer periods of time in diatom dominated aquatic systems, i.e. a large part of the world's ocean and coastal seas.
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| 2. |
- Swärd, Henrik, 1975-, et al.
(författare)
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Mineral and isotopic (Nd, Sr) signature of fine-grained deglacial and Holocene sediments from the Mackenzie Trough, Arctic Canada
- 2022
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Ingår i: Arctic, Antarctic and Alpine research. - : Informa UK Limited. - 1523-0430 .- 1938-4246. ; 54:1, s. 346-367
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Tidskriftsartikel (refereegranskat)abstract
- Dolomites in Arctic Ocean sediments are widely attributed to erosion and transport of sediments from northern Canada and Greenland. Coarse-grained dolomite-rich ice-rafted debris is often linked to iceberg transport, but the origin of fine-grained dolomite is less well constrained. A presumed source is the Mackenzie River. In this article, we fingerprint the minerogenic and isotopic (Nd and Sr) composition of deglacial and Holocene fine-grained sediments (<38 μm) from the shallow Mackenzie Trough. Sediments from an 81.5-m borehole (MTW01) were analyzed. The borehole is composed of a progradational (deltaic), transitional (transgressive), and marine unit (<9.4 cal. ka BP). The average dolomite content (~7 percent) and ɛNd signals (−13.3) are surprisingly constant in the progradational and marine units. The isotopic signature is inherited from the Interior Platform, the major underlying bedrock region of the Mackenzie River mainstream. The transitional unit contains fluctuations in ɛNd (−11.0 and −14.6), reflecting enhanced input from the North American Cordillera and Canadian Shield that are not associated with elevated amounts of dolomite. Additional studies combining minerogenic and detrital ɛNd analyses from sites proximal to the paleo-icestreams draining the Canadian Arctic are required to ascertain the origin of dolomite enrichment in central Arctic Ocean sediments.
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| 3. |
- Conrad, Sarah, et al.
(författare)
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Distribution of Fe isotopes in particles and colloids in the salinity gradient along the Lena River plume, Laptev Sea
- 2019
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Ingår i: Biogeosciences. - : European Geosciences Union (EGU). - 1726-4170 .- 1726-4189. ; 16:6, s. 1305-1319
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Tidskriftsartikel (refereegranskat)abstract
- Riverine Fe input is the primary Fe source for the ocean. This study is focused on the distribution of Fe along the Lena River freshwater plume in the Laptev Sea using samples from a 600 km long transect in front of the Lena River mouth. Separation of the particulate ( > 0.22 μm), colloidal (0.22 μm–1 kDa), and truly dissolved (< 1 kDa) fractions of Fe was carried out. The total Fe concentrations ranged from 0.2 to 57μM with Fe dominantly as particulate Fe. The loss of > 99% of particulate Fe and about 90% of the colloidal Fe was observed across the shelf, while the truly dissolved phase was almost constant across the Laptev Sea. Thus, the truly dissolved Fe could be an important source of bioavailable Fe for plankton in the central Arctic Ocean, together with the colloidal Fe. Fe-isotope analysis showed that the particulate phase and the sediment below the Lena River freshwater plume had negative δ56Fe values (relative to IRMM-14). The colloidal Fe phase showed negative δ56Fe values close to the river mouth (about -0.20 ‰) and positive δ56Fe values in the outermost stations (about +0.10 ‰). We suggest that the shelf zone acts as a sink for Fe particles and colloids with negative δ56Fe values, representing chemically reactive ferrihydrites. The positive δ56Fe values of the colloidal phase within the outer Lena River freshwater plume might represent Fe oxyhydroxides, which remain in the water column, and will be the predominant δ56Fe composition in the Arctic Ocean.
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| 4. |
- Fehr, Manuela A., et al.
(författare)
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Iron isotope variations in Holocene sediments of the Gotland Deep, Baltic Sea
- 2008
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 72:3, s. 807-826
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Forskningsöversikt (refereegranskat)abstract
- Holocene sediments from the Gotland Deep basin in the Baltic Sea were investigated for their Fe isotopic composition in order to assess the impact of changes in redox conditions and a transition from freshwater to brackish water on the isotope signature of iron. The sediments display variations in delta Fe-56 (differences in the Fe-56/Fe-54 ratio relative to the IRMM-14 standard) from -0.27 +/- 0.09 parts per thousand to +0.21 +/- 0.08 parts per thousand. Samples deposited in a mainly limnic environment with oxygenated bottom water have a mean delta Fe-56 of +0.08 +/- 0.13 parts per thousand, which is identical to the mean Fe isotopic composition of igneous rocks and oxic marine sediments. In contrast, sediments that formed in brackish water under periodically euxinic conditions display significantly lighter Fe isotope signatures with a mean delta Fe-56 of -0.14 +/- 0.19 parts per thousand. Negative correlations of the delta Fe-56 values with the Fe/Al ratio and S content of the samples suggest that the isotopically light Fe in the periodically euxinic samples is associated with reactive Fe enrichments and sulfides. This is supported by analyses of pyrite separates from this unit that have a mean Fe isotopic composition of -1.06 +/- 0.20 parts per thousand for delta Fe-56. The supply of additional Fe with a light Fe isotopic signature can be explained with the shelf to basin Fe shuttle model. According to the Fe shuttle model, oxides and benthic ferrous Fe that is derived from dissimilatory iron reduction from shelves is transported and accumulated in euxinic basins. The data furthermore suggest that the euxinic water has a negative dissolved delta Fe-56 value of about -1.4 parts per thousand to -0.9 parts per thousand. If negative Fe isotopic signatures are characteristic for euxinic sediment formation, widespread euxinia in the past might have shifted the Fe isotopic composition of dissolved Fe in the ocean towards more positive delta Fe-56 values.
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| 5. |
- Krall, Lindsay, et al.
(författare)
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Submarine groundwater discharge at Forsmark, Gulf of Bothnia, providedby Ra isotopes
- 2017
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Ingår i: Marine Chemistry. - Amsterdam : Elsevier. - 0304-4203 .- 1872-7581. ; 196, s. 162-172
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Tidskriftsartikel (refereegranskat)abstract
- Submarine groundwater discharge (SGD) to the Gulf of Bothnia, Baltic Sea, has been believed to be insignificant from hydrological models, yet geochemical investigations of SGD in this basin are limited. In this study, 223Ra, 224Ra, 226Ra, and 228Ra have been complemented by stable δ18O and d2H isotopes to characterize and quantify SGD rates from the coast of Forsmark, Sweden to Öregrundsgrepen Strait (Gulf of Bothnia). In shallow bays, SGD has been traced using relations between Ra, d2H and d18O isotopes and salinity. Zonal SGD from deformation zones, expressed as hydraulically conductive fractures, overlain by the Börstilåsen esker, a strip of conductive gravel sediment, have been traced through offshore 224Raxs and 226Ra trends. On the basis of a 224Raxs mass balance, the SGD flow rate ranges from (5.5 ± 3.0) · 103 m3 d-1 to (950 ± 520) · 103 m3 d-1. These rates are up to two orders of magnitude higher than those determined from local hydrological models, which consider only the fresh component of SGD. From the divergence between the hydrological and 224Raxs models, it is inferred that the site is influenced by a component of recirculated seawater.
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| 6. |
- Horst, Axel, et al.
(författare)
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Stable bromine isotopic composition of methyl bromide released from plant matter
- 2014
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 125, s. 186-195
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Tidskriftsartikel (refereegranskat)abstract
- Methyl bromide (CH3Br) emitted from plants constitutes a natural source of bromine to the atmosphere, and is a component in the currently unbalanced global CH3Br budget. In the stratosphere, CH3Br contributes to ozone loss processes. Studies of stable isotope composition may reduce uncertainties in the atmospheric CH3Br budget, but require well-constrained isotope fingerprints of the source end members. Here we report the first measurements of stable bromine isotopes (delta Br-81) in CH3Br from abiotic plant emissions. Incubations of both KBr-fortified pectin, a ubiquitous cell-stabilizing macromolecule, and of a natural halophyte (Salicornia fruticosa), yielded an enrichment factor (epsilon) of -2.00 +/- 0.23 parts per thousand (1 sigma, n = 8) for pectin and -1.82 +/- 0.02 parts per thousand (1 sigma, n = 4) for Salicornia (the relative amount of the heavier Br-81 was decreased in CH3Br compared to the substrate salt). For short incubations, and up to 10% consumption of the salt substrate, this isotope effect was similar for temperatures from 30 up to 300 degrees C. For longer incubations of up to 90 h at 180 degrees C the delta Br-81 values increased from -2 parts per thousand to 0 parts per thousand for pectin and to -1 parts per thousand for Salicornia. These delta Br-81 source signatures of CH3Br formation from plant matter combine with similar data for carbon isotopes to facilitate multidimensional isotope diagnostics of the CH3Br budget.
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| 7. |
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| 8. |
- Chi Fru, Ernest, et al.
(författare)
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Cu isotopes in marine black shales record the Great Oxidation Event
- 2016
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 113:18, s. 4941-4946
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Tidskriftsartikel (refereegranskat)abstract
- The oxygenation of the atmosphere similar to 2.45-2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth's redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (delta Cu-65(ERM-AE633)) in organic carbon-rich shales spanning the period 2.66-2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in Cu-65, along with the preferential removal of Cu-65 by iron oxides, left seawater and marine biomass depleted in Cu-65 but enriched in Cu-63. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy delta Cu-65 values coincides with a shift to negative sedimentary delta Fe-56 values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary delta Cu-65 values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.
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| 9. |
- Hirst, Catherine, et al.
(författare)
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Characterisation of Fe-bearing particles and colloids in the Lena River basin, NE Russia
- 2017
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Ingår i: Geochimica et Cosmochimica Acta. - Cambridge : Elsevier. - 0016-7037 .- 1872-9533. ; 213, s. 553-573
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Tidskriftsartikel (refereegranskat)abstract
- Rivers are significant contributors of Fe to theocean. 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 (> 0.22 µm) and colloids (1 kDa – 0.22 µm) and their association with organic carbon (OC), in the Lena River and tributaries, which drain a catchment almost entirely underlain by permafrost. Samples fromthe main channel and tributaries representing watersheds that span a wide rangein topography and lithology were taken after the spring flood in June 2013 and summer baseflow in July 2012. Fe-bearing particles were identified, usingTransmission Electron Microscopy, as large (200 nm – 1 µm) aggregates of smaller (20 nm - 30 nm) spherical colloids of chemically-reactive ferrihydrite.In contrast, there were also large (500 nm – 1 µm) aggregates of clay (illite) particles and smaller (100 - 200 nm) iron oxide particles (dominantly hematite) that contain poorly reactive Fe. TEM imaging and Scanning Transmission X-raymicroscopy (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.
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| 10. |
- Kutscher, Liselott, et al.
(författare)
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Spatial variation in concentration and sources of organic carbon in the Lena River, Siberia
- 2017
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Ingår i: Journal of Geophysical Research - Biogeosciences. - New York. - 2169-8953 .- 2169-8961. ; 122
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Tidskriftsartikel (refereegranskat)abstract
- Global warming in permafrost areas is expected to change fluxes of riverine organic carbon (OC) 22 to the Arctic Ocean. Here OC concentrations, stable carbon isotope signatures (δ13C) and carbon-23 nitrogen ratios (C/N) are presented from 22 sampling stations in the Lena River and 40 of its 24 tributaries. Sampling was conducted during two expeditions: the first in July 2012 in the south 25 and southeastern region and the second in June 2013 in the northern region of the Lena basin. 26 The data showed significant spatial differences in concentrations and major sources of OC. Mean 27 sub-catchment slopes were correlated with OC concentrations, implying that mountainous areas 28 in general had lower concentrations than lowland areas. δ13C and C/N data from tributaries 29 originating in mountainous areas indicated that both dissolved and particulate OC (DOC and 30 POC) were mainly derived from soil organic matter (SOM). In contrast, tributaries originating in 31 lowland areas had larger contributions from fresh vegetation to DOC, while aquatically produced 32 OC was the major source of POC. We suggest that these differences in dominant sources 33 indicated differences in dominant flow pathways. Tributaries with larger influence of fresh 34 vegetation probably had surficial flow pathways, while tributaries with more SOM influence had 35 deeper water flow pathways. Thus, the future export of OC to the Arctic Ocean will likely be 36 controlled by changes in spatial patterns in hydroclimatology and the depth of the active layers 37 influencing the dominant water flow pathways in Arctic river basins.
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