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Transport, degradation and burial of organic matter released from permafrost to the East Siberian Arctic Shelf
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.ORCID iD: 0000-0002-5454-7883
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Department of Environmental Science and Analytical Chemistry, Stockholm University , 2016.
Keyword [en]
organic carbon, marine sediments, East Siberian Sea, Laptev Sea, cross-shelf transport, degradation rate constants, carbon isotopes, terrestrial biomarkers, HMW wax lipids, lignin phenols, compound-specific radiocarbon analysis
National Category
Environmental Sciences
Research subject
Applied Environmental Science
Identifiers
URN: urn:nbn:se:su:diva-136380ISBN: 978-91-7649-571-1 (print)ISBN: 978-91-7649-572-8 (print)OAI: oai:DiVA.org:su-136380DiVA: diva2:1052179
Public defence
2017-02-03, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.

Available from: 2017-01-11 Created: 2016-12-05 Last updated: 2017-03-14Bibliographically approved
List of papers
1. Historical records of organic matter supply and degradation status in the East Siberian Sea
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2016 (English)In: Organic Geochemistry, ISSN 0146-6380, E-ISSN 1873-5290, Vol. 91, 16-30 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Arctic, East Siberian Arctic Shelf, Pb-210, Monte Carlo, delta C-13, Delta C-14, Lignin, HMW wax lipids
National Category
Earth and Related Environmental Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-126195 (URN)10.1016/j.orggeochem.2015.10.008 (DOI)000366706100003 ()
Available from: 2016-01-28 Created: 2016-01-26 Last updated: 2016-12-22Bibliographically approved
2. Fate of terrigenous organic matter across the Laptev Sea from the mouth of the Lena River to the deep sea of the Arctic interior
Open this publication in new window or tab >>Fate of terrigenous organic matter across the Laptev Sea from the mouth of the Lena River to the deep sea of the Arctic interior
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2016 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 13, no 17, 5003-5019 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Biological Sciences Earth and Related Environmental Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-135200 (URN)10.5194/bg-13-5003-2016 (DOI)000383963700003 ()
Available from: 2016-11-16 Created: 2016-11-01 Last updated: 2016-12-22Bibliographically approved
3. Bounding cross-shelf transport time and degradation in land-ocean carbon transfer
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(English)Article in journal (Refereed) Submitted
National Category
Climate Research
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-136376 (URN)
Available from: 2016-12-05 Created: 2016-12-05 Last updated: 2016-12-22Bibliographically approved
4. The integrated flux of terrigenous organic carbon degradation in surface sediments on the Laptev and East Siberian Sea Shelves
Open this publication in new window or tab >>The integrated flux of terrigenous organic carbon degradation in surface sediments on the Laptev and East Siberian Sea Shelves
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(English)Manuscript (preprint) (Other academic)
National Category
Climate Research
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-136378 (URN)
Available from: 2016-12-05 Created: 2016-12-05 Last updated: 2016-12-22Bibliographically approved

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