| 1. |
- Brüchert, Volker, et al.
(author)
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Carbon mineralization in Laptev and East Siberian sea shelf and slope sediment
- 2018
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In: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 15:2, s. 471-490
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Journal article (peer-reviewed)abstract
- The Siberian Arctic Sea shelf and slope is a key region for the degradation of terrestrial organic material transported from the organic carbon-rich permafrost regions of Siberia. We report on sediment carbon mineralization rates based on O2 microelectrode profiling, intact sediment core incubations, 35 S-sulfate tracer experiments, porewater dissolved inorganic carbon (DIC), δ13 CDIC, and iron, manganese, and ammonium concentrations from 20 shelf and slope stations. This data set provides a spatial overview of sediment carbon mineralization rates and pathways over large parts of the outer Laptev and East Siberian Arctic shelf and slope, and allowed us to assess degradation rates and efficiency of carbon burial in these sediments. Rates of oxygen uptake and iron and manganese reduction were comparable to temperate shelf and slope environments, but bacterial sulfate reduction rates were comparatively low. In the topmost 20 to 50 cm of sediment, aerobic carbon mineralization dominated degradation and comprised on average 82% of the depthintegrated carbon mineralization. Oxygen uptake rates and 35 S-sulfate reduction rates were higher in the eastern East Siberian Sea shelf compared to the Laptev Sea shelf. DIC/NH4 + ratios in porewaters and the stable carbon isotope composition of remineralized DIC indicated that the degraded organic matter on the Siberian shelf and slope was a mixture of marine and terrestrial organic matter. Based on dual end member calculations, the terrestrial organic carbon contribution varied between 32% and 36%, with a higher contribution in the Laptev Sea than in the East Siberian Sea. Extrapolation of the measured degradation rates using isotope end member apportionment over the outer shelf of the Laptev and East Siberian Sea suggests that about 16 Tg C per year are respired in the outer shelf sea floor sediment. Of the organic matter buried below the oxygen penetration depth, between 0.6 and 1.3 Tg C per year are degraded by anaerobic processes, with a terrestrial organic carbon contribution ranging between 0.3 and 0.5 Tg per year.
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| 2. |
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| 3. |
- Bröder, Lisa, et al.
(author)
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Bounding cross-shelf transport time and degradation in Siberian-Arctic land-ocean carbon transfer
- 2018
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 9
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Journal article (peer-reviewed)abstract
- The burial of terrestrial organic carbon (terrOC) in marine sediments contributes to the regulation of atmospheric CO2 on geological timescales and may mitigate positive feedback to present-day climate warming. However, the fate of terrOC in marine settings is debated, with uncertainties regarding its degradation during transport. Here, we employ compound-specific radiocarbon analyses of terrestrial biomarkers to determine cross-shelf transport times. For the World's largest marginal sea, the East Siberian Arctic shelf, transport requires 3600 +/- 300 years for the 600 km from the Lena River to the Laptev Sea shelf edge. TerrOC was reduced by similar to 85% during transit resulting in a degradation rate constant of 2.4 +/- 0.6 kyr(-1). Hence, terrOC degradation during cross-shelf transport constitutes a carbon source to the atmosphere over millennial time. For the contemporary carbon cycle on the other hand, slow terrOC degradation brings considerable attenuation of the decadal-centennial permafrost carbon-climate feedback caused by global warming.
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| 4. |
- Bröder, Lisa, et al.
(author)
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Fate of terrigenous organic matter across the Laptev Sea from the mouth of the Lena River to the deep sea of the Arctic interior
- 2016
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In: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 13:17, s. 5003-5019
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Journal article (peer-reviewed)abstract
- Ongoing global warming in high latitudes may cause an increasing supply of permafrost-derived organic carbon through both river discharge and coastal erosion to the Arctic shelves. Mobilized permafrost carbon can be either buried in sediments, transported to the deep sea or degraded to CO2 and outgassed, potentially constituting a positive feedback to climate change. This study aims to assess the fate of terrigenous organic carbon (TerrOC) in the Arctic marine environment by exploring how it changes in concentration, composition and degradation status across the wide Laptev Sea shelf. We analyzed a suite of terrestrial biomarkers as well as source-diagnostic bulk carbon isotopes (delta C-13, Delta C-14) in surface sediments from a Laptev Sea transect spanning more than 800 km from the Lena River mouth (< 10m water depth) across the shelf to the slope and rise (2000-3000m water depth). These data provide a broad view on different TerrOC pools and their behavior during cross-shelf transport. The concentrations of lignin phenols, cutin acids and high-molecular-weight (HMW) wax lipids (tracers of vascular plants) decrease by 89-99% along the transect. Molecular-based degradation proxies for TerrOC (e.g., the carbon preference index of HMW lipids, the HMW acids / alkanes ratio and the acid / aldehyde ratio of lignin phenols) display a trend to more degraded TerrOC with increasing distance from the coast. We infer that the degree of degradation of permafrost-derived TerrOC is a function of the time spent under oxic conditions during protracted cross-shelf transport. Future work should therefore seek to constrain cross-shelf transport times in order to compute a TerrOC degradation rate and thereby help to quantify potential carbon-climate feedbacks.
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| 5. |
- Bröder, Lisa, et al.
(author)
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Historical records of organic matter supply and degradation status in the East Siberian Sea
- 2016
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In: Organic Geochemistry. - : Elsevier BV. - 0146-6380 .- 1873-5290. ; 91, s. 16-30
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Journal article (peer-reviewed)abstract
- Destabilization and degradation of permafrost carbon in the Arctic regions could constitute a positive feedback to climate change. A better understanding of its fate upon discharge to the Arctic shelf is therefore needed. In this study, bulk carbon isotopes as well as terrigenous and marine biomarkers were used to construct two centennial records in the East Siberian Sea. Differences in topsoil and Pleistocene Ice Complex Deposit permafrost concentrations, modeled using delta C-13 and Delta C-14, were larger between inner and outer shelf than the changes over time. Similarly, lignin-derived phenol and cutin acid concentrations differed by a factor of ten between the two stations, but did not change significantly over time, consistent with the dual-carbon isotope model. High molecular weight (HMW) n-alkane and n-alkanoic acid concentrations displayed a smaller difference between the two stations (factor of 3-6). By contrast, the fraction for marine OC drastically decreased during burial with a half-life of 19-27 years. Vegetation and degradation proxies suggested supply of highly degraded gymnosperm wood tissues. Lipid Carbon Preference Index (CPI) values indicated more extensively degraded HMW n-alkanes on the outer shelf with no change over time, whereas n-alkanoic acids appeared to be less degraded toward the core top with no large differences between the stations. Taken together, our results show larger across-shelf changes than down-core trends. Further investigation is required to establish whether the observed spatial differences are due to different sources for the two depositional settings or, alternatively, a consequence of hydrodynamic sorting combined with selective degradation during cross-shelf transport.
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| 6. |
- Bröder, Lisa-Marie, 1985-
(author)
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Transport, degradation and burial of organic matter released from permafrost to the East Siberian Arctic Shelf
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Permafrost soils in the Arctic store large quantities of organic matter, roughly twice the amount of carbon that was present in the atmosphere before the industrial revolution. This freeze-locked carbon pool is susceptible to thawing caused by amplified global warming at high latitudes. The remobilization of old permafrost carbon facilitates its degradation to carbon dioxide and methane, thereby providing a positive feedback to climate change.Accelerating coastal erosion in addition to projected rising river discharge with enhancing sediment loads are anticipated to transport increasing amounts of land-derived organic carbon (OC) to the Arctic Ocean. On its shallow continental shelves, this material may be remineralized in the water column or in the sediments, transported without being altered off shelf towards the deep sea of the Arctic Interior or buried in marine sediments and hence sequestered from the contemporary carbon cycle. The fate of terrigenous material in the marine environment, though offering potentially important mechanisms to either strengthen or attenuate the permafrost-carbon climate feedback, is so far insufficiently understood.In this doctoral thesis, sediments from the wide East Siberian Arctic Shelf, the world’s largest shelf-sea system, were used to investigate some of the key processes for OC cycling. A range of bulk sediment properties, carbon isotopes and molecular markers were employed to elucidate the relative importance of different organic matter sources, the role of cross-shelf transport and the relevance of degradation during transport and after burial.Overall, OC released from thawing permafrost constitutes a significant proportion of the sedimentary organic matter on the East Siberian Arctic Shelf. Two sediment cores from the inner and outer East Siberian Sea recorded no substantial changes in source material or clear trends in degradation status for the last century. With increasing distance from the coast, however, strong gradients were detected towards lower concentrations of increasingly reworked land-derived OC. The time spent during cross-shelf transport was consequently found to exert first-order control on degradation. Compound-specific radiocarbon dating on terrigenous biomarkers revealed a net transport time of ~4 000 years across the 600 km wide Laptev Sea shelf, yielding degradation rate constants for bulk terrigenous OC and specific biomarkers on the order of 2-4 kyr-1.From these results, the carbon flux released by degradation of terrigenous OC in surface sediments was estimated to be ~1.7 Gg yr-1, several orders of magnitude lower than what had been quantified earlier for dissolved and particulate OC in the water column. Lower oxygen availability and close associations with the mineral matrix may protect sedimentary OC from remineralization and thereby weaken the permafrost-carbon feedback to present climate change.
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| 7. |
- Bröder, Lisa, et al.
(author)
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Quantifying Degradative Loss of Terrigenous Organic Carbon in Surface Sediments Across the Laptev and East Siberian Sea
- 2019
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In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 33:1, s. 85-99
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Journal article (peer-reviewed)abstract
- Ongoing permafrost thaw in the Arctic may remobilize large amounts of old organic matter. Upon transport to the Siberian shelf seas, this material may be degraded and released to the atmosphere, exported off-shelf, or buried in the sediments. While our understanding of the fate of permafrost-derived organic matter in shelf waters is improving, poor constraints remain regarding degradation in sediments. Here we use an extensive data set of organic carbon concentrations and isotopes (n=109) to inventory terrigenous organic carbon (terrOC) in surficial sediments of the Laptev and East Siberian Seas (LS + ESS). Of these similar to 2.7 Tg terrOC about 55% appear resistant to degradation on a millennial timescale. A first-order degradation rate constant of 1.5 kyr(-1) is derived by combining a previously established relationship between water depth and cross-shelf sediment-terrOC transport time with mineral-associated terrOC loadings. This yields a terrOC degradation flux of similar to 1.7Gg/year from surficial sediments during cross-shelf transport, which is orders of magnitude lower than earlier estimates for degradation fluxes of dissolved and particulate terrOC in the water column of the LS + ESS. The difference is mainly due to the low degradation rate constant of sedimentary terrOC, likely caused by a combination of factors: (i) the lower availability of oxygen in the sediments compared to fully oxygenated waters, (ii) the stabilizing role of terrOC-mineral associations, and (iii) the higher proportion of material that is intrinsically recalcitrant due to its chemical/molecular structure in sediments. Sequestration of permafrost-released terrOC in shelf sediments may thereby attenuate the otherwise expected permafrost carbon-climate feedback. Plain language summary Frozen soils in the Arctic contain large amounts of old organic matter. With ongoing climate change this previously freeze-locked carbon storage becomes vulnerable to transport and decay. Upon delivery to the shallow nearshore seas, it may either be directly degraded to carbon dioxide or methane and thereby fuel further warming or get buried and stored in sediments on the sea floor. Our understanding of the fate of carbon released from permafrost soils is increasing, yet uncertainties remain regarding its degradation in the sediment. Here we constrain how much land-derived organic carbon is deposited in the top layer of the sediment (the part that is prone to transport and exposed to oxygen-stimulated degradation) in the Laptev and East Siberian Seas. We find that more than half of this stock likely resists degradation, while the rest decays relatively slowly. Therefore, the amount of carbon released annually from degradation in surface sediments is much smaller than what was found to be emitted from overlying waters in earlier studies. We suspect that this difference is caused by a combination of mechanisms hindering degradation in sediments and thus conclude that the burial of land-derived carbon may help to dampen the climate impact of thawing permafrost.
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| 8. |
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| 9. |
- Keskitalo, Kirsi, et al.
(author)
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Sources and characteristics of terrestrial carbon in Holocene-scale sediments of the East Siberian Sea
- 2017
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In: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 13:9, s. 1213-1226
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Journal article (peer-reviewed)abstract
- Thawing of permafrost carbon (PF-C) due to climate warming can remobilise considerable amounts of terrestrial carbon from its long-term storage to the marine environment. PF-C can be then be buried in sediments or remineralised to CO2 with implications for the carbon-climate feedback. Studying historical sediment records during past natural climate changes can help us to understand the response of permafrost to current climate warming. In this study, two sediment cores collected from the East Siberian Sea were used to study terrestrial organic carbon sources, composition and degradation during the past similar to 9500 cal yrs BP. CuO-derived lignin and cutin products (i.e., compounds solely biosynthesised in terrestrial plants) combined with delta C-13 suggest that there was a higher input of terrestrial organic carbon to the East Siberian Sea between similar to 9500 and 8200 cal yrs BP than in all later periods. This high input was likely caused by marine transgression and permafrost destabilisation in the early Holocene climatic optimum. Based on source apportionment modelling using dual-carbon isotope (Delta C-14, Delta C-13) data, coastal erosion releasing old Pleistocene permafrost carbon was identified as a significant source of organic matter translocated to the East Siberian Sea during the Holocene.
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| 10. |
- Martens, Jannik, et al.
(author)
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Remobilization of Old Permafrost Carbon to Chukchi Sea Sediments During the End of the Last Deglaciation
- 2019
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In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 33:1, s. 2-14
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Journal article (peer-reviewed)abstract
- Climate warming is expected to destabilize permafrost carbon (PF-C) by thaw-erosion and deepening of the seasonally thawed active layer and thereby promote PF-C mineralization to CO2 and CH4. A similar PF-C remobilization might have contributed to the increase in atmospheric CO2 during deglacial warming after the last glacial maximum. Using carbon isotopes and terrestrial biomarkers (Delta C-14, delta C-13, and lignin phenols), this study quantifies deposition of terrestrial carbon originating from permafrost in sediments from the Chukchi Sea (core SWERUS-L2-4-PC1). The sediment core reconstructs remobilization of permafrost carbon during the late Allerod warm period starting at 13,000 cal years before present (BP), the Younger Dryas, and the early Holocene warming until 11,000 cal years BP and compares this period with the late Holocene, from 3,650 years BP until present. Dual-carbon-isotope-based source apportionment demonstrates that Ice Complex Deposit-ice- and carbon-rich permafrost from the late Pleistocene (also referred to as Yedoma)-was the dominant source of organic carbon (66 +/- 8%; mean +/- standard deviation) to sediments during the end of the deglaciation, with fluxes more than twice as high (8.0 +/- 4.6 g.m(-2).year(-1)) as in the late Holocene (3.1 +/- 1.0 g.m(-2).year(-1)). These results are consistent with late deglacial PF-C remobilization observed in a Laptev Sea record, yet in contrast with PF-C sources, which at that location were dominated by active layer material from the Lena River watershed. Release of dormant PF-C from erosion of coastal permafrost during the end of the last deglaciation indicates vulnerability of Ice Complex Deposit in response to future warming and sea level changes.
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