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Terrestrial organic carbon dynamics in Arctic coastal areas: budgets and multiple stable isotope approaches
Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Arctic rivers transport 31-42 Tg organic carbon (OC) each year to the Arctic Ocean, which is equal to 10% of the global riverine OC discharge. Since the Arctic Ocean only holds approximately 1% of the global ocean volume, the influence of terrestrially derived organic carbon (OCter) in the Arctic Ocean is relatively high. Despite the global importance of this region the behavior of the, by far largest fraction of the OCter, the dissolved organic carbon (DOC) in Arctic and sub-arctic estuaries is still a matter of debate. This thesis describes data originating from field cruises in Arctic and sub-arctic estuaries and coastal areas with the aim to improve the understanding of the fate of OCter in these areas, with specific focus on DOC. All presented studies indicate that DOCter and terrestrially derived particulate organic carbon (POCter) are subjected to substantial degradation in high-latitude estuaries, as shown by the non-conservative behavior of DOC in the East Siberian Arctic Shelf Seas (ESAS) (paper I) and the even more rapid degradation of POC in the same region (paper II). The removals of OCter in Arctic shelf seas were further supported by multiple isotope studies (paper III and IV), which showed that a use of 13C/12C in both OC and DIC, together with 34S/32S is a powerful tool to describe the sources and fate of OCter in estuaries and coastal seas. High-latitude estuaries play a key role in the coupling between terrestrial and marine carbon pools. In contrast to the general perception, this thesis shows that they are not only transportation areas for DOCter from rivers to the ocean, but are also active sites for transformation, degradation and sedimentation of DOCter, as well as for POCter. In a rapidly changing climate, the importance of these areas for the coupling between inorganic and organic carbon pools cannot be underestimated.

Place, publisher, year, edition, pages
Stockholm: Department of Applied Environmental Science (ITM) , 2010. , 51 p.
Keyword [en]
organic carbon, DOC, POC, multiple stable isotopes, Laptev Sea, East Siberian Sea, Lena River, Arctic, residence times, degradation
National Category
Earth and Related Environmental Sciences
Research subject
Applied Environmental Science
Identifiers
URN: urn:nbn:se:su:diva-43455ISBN: 978-91-7447-119-9 (print)OAI: oai:DiVA.org:su-43455DiVA: diva2:356862
Public defence
2010-11-12, 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 1: In press. Paper 2: Submitted. Paper 4: Manuscript.

Available from: 2010-10-21 Created: 2010-10-14 Last updated: 2017-08-25Bibliographically approved
List of papers
1. Non-conservative behavior of dissolved organic carbon across the Laptev and East Siberian Seas
Open this publication in new window or tab >>Non-conservative behavior of dissolved organic carbon across the Laptev and East Siberian Seas
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2010 (English)In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 24, GB4033- p.Article in journal (Refereed) Published
Abstract [en]

Climate change is expected to have a strong effect on the Eastern Siberian Arctic Shelf (ESAS) region, which includes 40% of the Arctic shelves and comprises the Laptev and East Siberian seas. The largest organic carbon pool, the dissolved organic carbon (DOC), may change significantly due to changes in both riverine inputs and transformation rates; however, the present DOC inventories and transformation patterns are poorly understood. Using samples from the International Siberian Shelf Study 2008, this study examines for the first time DOC removal in Arctic shelf waters with residence times that range from months to years. Removals of up to 10%–20% were found in the Lena River estuary, consistent with earlier studies in this area, where surface waters were shown to have a residence time of approximately 2 months. In contrast, the DOC concentrations showed a strong nonconservative pattern in areas with freshwater residence times of several years. The average losses of DOC were estimated to be 30%–50% during mixing along the shelf, corresponding to a first-order removal rate constant of 0.3 yr−1. These data provide the first observational evidence for losses of DOC in the Arctic shelf seas, and the calculated DOC deficit reflects DOC losses that are higher than recent model estimates for the region. Overall, a large proportion of riverine DOC is removed from the surface waters across the Arctic shelves. Such significant losses must be included in models of the carbon cycle for the Arctic Ocean, especially since the breakdown of terrestrial DOC to CO2 in Arctic shelf seas may constitute a positive feedback mechanism for Arctic climate warming. These data also provide a baseline for considering the effects of future changes in carbon fluxes, as the vast northern carbon-rich permafrost areas draining into the Arctic are affected by global warming.

National Category
Biochemistry and Molecular Biology
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-43437 (URN)10.1029/2010GB003834 (DOI)
Available from: 2010-10-14 Created: 2010-10-13 Last updated: 2017-12-12Bibliographically approved
2. Inventories and behavior of particulate organic carbon in the Laptev and East Siberian seas
Open this publication in new window or tab >>Inventories and behavior of particulate organic carbon in the Laptev and East Siberian seas
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2011 (English)In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 25, GB2007- p.Article in journal (Refereed) Published
Abstract [en]

Fluvial and erosional release processes in permafrost-dominated Eurasian Arctic cause transport of large amounts of particulate organic carbon (POC) to coastal waters. The marine fate of this terrestrial POC (terr-POC), water column degradation, burial in shelf sediments, or export to depth, impacts the potential for climate-carbon feedback. As part of the International Siberian Shelf Study (ISSS-08; August-September 2008), the POC distribution, inventory, and fate in the water column of the extensive yet poorly studied Eurasian Arctic Shelf seas were investigated. The POC concentration spanned 1-152 mu M, with highest values in the SE Laptev Sea. The POC inventory was constrained for the Laptev (1.32 +/- 0.09 Tg) and East Siberian seas (2.85 +/- 0.20 Tg). A hydraulic residence time of 3.5 +/- 2 years for these Siberian shelf seas yielded a combined annual terr-POC removal flux of 3.9 +/- 1.4 Tg yr(-1). Accounting for sediment burial and shelf-break exchange, the terr-POC water column degradation was similar to 2.5 +/- 1.6 Tg yr(-1), corresponding to a first-order terr-POC degradation rate constant of 1.4 +/- 0.9 yr(-1), which is 5-10 times faster than reported for terr-DOC degradation in the Arctic Ocean. This terr-POC degradation flux thus contributes substantially to the dissolved inorganic carbon excess of 10 Tg C observed during ISSS-08 for these waters. This evaluation suggests that extensive decay of terr-POC occurs already in the water column and contributes to outgassing of CO(2). This process should be considered as a geographically dislocated carbon-climate coupling where thawing of vulnerable permafrost carbon on land is eventually adding CO(2) above the ocean.

National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-68135 (URN)10.1029/2010GB003862 (DOI)000290873000004 ()
Note
authorCount :9Available from: 2012-01-03 Created: 2012-01-03 Last updated: 2017-12-08Bibliographically approved
3. Tracing terrestrial organic matter by delta34S and delta13C signatures in a subarctic estuary
Open this publication in new window or tab >>Tracing terrestrial organic matter by delta34S and delta13C signatures in a subarctic estuary
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2008 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 53, no 6, 2594-2602 p.Article in journal (Refereed) Published
Abstract [en]

A key issue to understanding the transformations of terrestrial organic carbon in the ocean is to disentangle the latter from marine-produced organic matter. We applied a multiple stable isotope approach using 34S and 13C isotope signatures from estuarine dissolved organic matter (DOM), enabling us to constrain the contribution of terrestrial-derived DOM in an estuarine gradient of the northern Baltic Sea. The stable isotope signatures for dissolved organic sulfur (34SDOS) have twice the range between terrestrial and marine end members compared to the stable isotope signatures for dissolved organic carbon (13CDOC); hence, the share of terrestrial DOM in the total estuarine DOM can be calculated more precisely. DOM samples from the water column were collected using ultrafiltration on board the German RV Maria S Merian during a winter cruise, in the Bothnian Bay, Bothnian Sea, and Baltic proper. We calculated the terrestrial fraction of the estuarine DOC (DOCter) from both 13CDOC and 34SDOS signatures and applying fixed C: S ratios for riverine and marine end members to convert S isotope signatures into DOC concentrations. The 34SDOS signature of the riverine end member was +7.02‰, and the mean signatures from Bothnian Bay, Bothnian Sea, and Baltic proper were +10.27, +12.51, and +13.67‰, respectively, showing an increasing marine signal southwards (34SDOS marine end member 5 18.1‰). These signatures indicate that 87‰, 75‰, and 67‰, respectively, of the water column DOC is of terrestrial origin (DOCter) in these basins. Comparing the fractions of DOCter in each basin—that are still based on few winter values only—with the annual river input of DOC, it appears that the turnover time for DOCter in the Gulf of Bothnia is much shorter than the hydraulic turnover time, suggesting that high-latitude estuaries might be efficient sinks for DOCter.

National Category
Biochemistry and Molecular Biology
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-43431 (URN)10.4319/lo.2008.53.6.2594 (DOI)000261355500021 ()
Available from: 2010-10-14 Created: 2010-10-13 Last updated: 2017-12-12Bibliographically approved
4. Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from delta C-13 values of DIC
Open this publication in new window or tab >>Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from delta C-13 values of DIC
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2012 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 95, 143-159 p.Article in journal (Refereed) Published
Abstract [en]

The cycling of carbon on the Arctic shelves, including outgassing of CO2 to the atmosphere, is not clearly understood. Degradation of terrestrial organic carbon (OCter) has recently been shown to be pronounced over the East Siberian Arctic Shelf (ESAS), i.e. the Laptev and East Siberian Seas, producing dissolved inorganic carbon (DIC). To further explore the processes affecting DIC, an extensive suite of shelf water samples were collected during the summer of 2008, and assessed for the stable carbon isotopic composition of DIC (delta C-13(DIC)). The delta C-13(DIC) values varied between -7.2 parts per thousand to +1.6 parts per thousand and strongly deviated from the compositions expected from only mixing between river water and seawater. Model calculations suggest that the major processes causing these deviations from conservative mixing were addition of (DIC) by degradation of OCter, removal of DIC during primary production, and outgassing of CO2. All waters below the halocline in the ESAS had delta C-13(DIC) values that appear to reflect mixing of river water and seawater combined with additions of on average 70 +/- 20 mu M of DIC, originating from degradation of OCter in the coastal water column. This is of the same magnitude as the recently reported deficits of DOCter and POCter for the same waters. The surface waters in the East Siberian Sea had higher delta C-13(DIC) values and lower DIC concentrations than expected from conservative mixing, consistent with additions of DIC from degradation of OCter and outgassing of CO2. The outgassing of CO2 was equal to loss of 123 +/- 50 mu M DIC. Depleted delta C-13(POC) values of -29 parts per thousand to -32 parts per thousand in the mid to outer shelf regions are consistent with POC from phytoplankton production. The low delta C-13(POC) values are likely due to low delta C-13(DIC) of precursor DIC, which is due to degradation of OCter, rather than reflecting terrestrial input compositions. Overall, the delta C-13(DIC) values confirm recent suggestions of substantial degradation of OCter over the ESAS, and further show that a large part of the CO2 produced from degradation has been outgassed to the atmosphere.

National Category
Geochemistry Geophysics
Identifiers
urn:nbn:se:su:diva-82969 (URN)10.1016/j.gca.2012.07.028 (DOI)000309591500012 ()
Note

AuthorCount:8;

Available from: 2012-12-10 Created: 2012-12-03 Last updated: 2017-12-07Bibliographically approved

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