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Contrasting sources of dissolved and particulate organic matter along 62N-72N in the Siberian-Arctic Lena River
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Manchester, UK.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The Lena River transports large amounts of sediment and dissolved organic matter to the shallow Laptev Sea, where it may be subject to degradation and potential release of OC. We studied organic matter collected in summer 2008, along a 1450 km section of the Lena River, from near Yakutsk at 62°N to the deltaic region at 72°N, to better understand potential in-river processing of the terrestrial particulate and dissolved fractions in the river surface water.

Carbon isotopes (δ13C and Δ14C) and plant wax lipid markers combine to reveal two distinct OC pools with different behavior in the river. The molar OC/TN ratios for POC were low (6-13) which suggests contribution from (freshwater) plankton, but most of the POC was of old age (770-4500 14C years) which rather suggests a pre-aged origin - perhaps from erosion of riverbank permafrost material. Much in contrast, COC was young (20-440 14C years) and displayed a high OC/TN composition (23-56) with a steady δ13C signal along the river course (-26.7 to -27.7). There was an apparent absence of ice complex deposit permafrost (mineral soil/yedoma OC) in the COC fraction, and only small contributions to POC. The COC signal suggest contribution from contemporary plant detritus/surface soil OC. It seems as if pre-aged permafrost OC, potentially from riverbank erosion, partitions into the particulate pool and almost not at all to the DOC/COC pool.

Degradation markers indicate a highly degraded COC lipid pool and a less degraded POC - the n-alkane carbon preference index (CPI, C24-C34) was 1.0-1.3 for COC and 1.2-4.9 (on average 3.3) for POC.

Taken together DOC/COC and POC have clearly different terrestrial sources and different fates on its way to the shelf waters. Previously freeze-locked old permafrost OC remobilizes into the Lena River in particulate form which (at least temporarily) escapes degradation as it follows the river course seawards in a less degraded state.

Keyword [en]
organic carbon, stabel carbon, radiocarbon, isotopes, Lena River, colloidal matter, particulate matter
National Category
Earth and Related Environmental Sciences
Research subject
Applied Environmental Science
Identifiers
URN: urn:nbn:se:su:diva-116872OAI: oai:DiVA.org:su-116872DiVA: diva2:809315
Available from: 2015-05-01 Created: 2015-05-01 Last updated: 2016-01-29Bibliographically approved
In thesis
1. Compositional clues to sources and sinks of terrestrial organic matter transported to the Eurasian Arctic shelf
Open this publication in new window or tab >>Compositional clues to sources and sinks of terrestrial organic matter transported to the Eurasian Arctic shelf
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The amount of organic carbon (OC) present in Siberian Arctic permafrost soils is estimated at twice the amount of carbon currently in the atmosphere. The shelf seas of the Arctic Ocean receive large amounts of this terrestrial OC from Eurasian Arctic rivers and from coastal erosion. Degradation of this land-derived material in the sea would result in the production of dissolved carbon dioxide and may then add to the atmospheric carbon dioxide reservoir. Observations from the Siberian Arctic suggest that transfer of carbon from land to the marine environment is accelerating. However, it is not clear how much of the transported OC is degraded and oxidized, nor how much is removed from the active carbon cycle by burial in marine sediment.

Using bulk geochemical parameters, total OC, d13C and D14C isotope composition, and specific molecular markers of plant wax lipids and lignin phenols, the abundance and composition of OC was determined in both dissolved and particulate carrier phases: the colloidal OC (COC; part of the dissolved OC), particulate OC (POC), and sedimentary OC (SOC). Statistical modelling was used to quantify the relative contribution of OC sources to these phases. Terrestrial OC is derived from the seasonally thawing top layer of permafrost soil (topsoil OC) and frozen OC derived from beneath the active layer eroded at the coast, commonly identified as yedoma ice complex deposit OC (yedoma ICD-OC). These carbon pools are transported differently in the aquatic conduits. Topsoil OC was found in young DOC and POC, in the river water, and the shelf water column, suggesting long-distance transport of this fraction. The yedoma ICD-OC was found as old particulate OC that settles out rapidly to the underlying sediment and is laterally transported across the shelf, likely dispersed by bottom nepheloid layer transport or via ice rafting.

These two modes of OC transport resulted in different degradation states of topsoil OC and yedoma ICD-OC. Terrestrial CuO oxidation derived biomarkers indicated a highly degraded component in the COC. In contrast, the terrestrial component of the SOC was much less degraded. In line with earlier suggestions the mineral component in yedoma ICD functions as weight and surface protection of the associated OC, which led to burial in the sediment, and limited OC degradation. The degradability of the terrestrial OC in shelf sediment was also addressed in direct incubation studies. Molecular markers indicate marine OC (from primary production) was more readily degraded than terrestrial OC. Degradation was also faster in sediment from the East Siberian Sea, where the marine contribution was higher compared to the Laptev Sea. Although terrestrial carbon in the sediment was degraded slower, the terrestrial component also contributed to carbon dioxide formation in the incubations of marine sediment.

These results contribute to our understanding of the marine fate of land-derived OC from the Siberian Arctic. The mobilization of topsoil OC is expected to grow in magnitude with climate warming and associated active layer deepening. This translocated topsoil OC component was found to be highly degraded, which suggests degradation during transport and a possible contribution to atmospheric carbon dioxide. Similarly, the yedoma ICD-OC (and or old mineral soil carbon) may become a stronger source with accelerated warming, but slow degradation may limit its impact on active carbon cycling in the Siberian Shelf Seas.

Place, publisher, year, edition, pages
Stockholm: Department of Environmental Science and Analytical Chemistry, Stockholm University, 2015. 40 p.
Keyword
organic carbon, degradation, microcosm, incubation, terrestrial biomarkers, acyl lipids, lignin phenols, radiocarbon, Eurasian Arctic shelf, East Siberian Sea, Laptev Sea, Lena River, colloidal matter, particulate matter, sedimentary matter
National Category
Earth and Related Environmental Sciences
Research subject
Applied Environmental Science
Identifiers
urn:nbn:se:su:diva-116876 (URN)978-91-7649-195-9 (ISBN)
Public defence
2015-06-11, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
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Supervisors
Note

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

Available from: 2015-05-20 Created: 2015-05-01 Last updated: 2015-05-29Bibliographically approved

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