Abstract
Abstract. Fluvial discharge and coastal erosion of the permafrost-dominated East Siberian Arctic delivers large quantities of terrigenous organic carbon (Terr-OC) to marine waters. The composition and fate of the remobilized Terr-OC needs to be better constrained as it impacts the potential for a climate–carbon feedback. In the present study, the bulk isotope (δ13C and Δ14C) and macromolecular (lignin-derived phenols) composition of the cross-shelf exported organic carbon (OC) in different marine pools is evaluated. For this purpose, as part of the SWERUS-C3 expedition (July–September 2014), sediment organic carbon (SOC) as well as water column (from surface and near-bottom seawater) dissolved organic carbon (DOC) and particulate organic carbon (POC) samples were collected along the outer shelves of the Kara Sea, Laptev Sea and East Siberian Sea. The results show that the Lena River and the DOC may have a preferential role in the transport of Terr-OC to the outer shelf. DOC concentrations (740–3600 µg L−1) were 1 order of magnitude higher than POC (20–360 µg L−1), with higher concentrations towards the Lena River plume. The δ13C signatures in the three carbon pools varied from −23.9 ± 1.9 ‰ in the SOC, −26.1 ± 1.2 ‰ in the DOC and −27.1 ± 1.9 ‰ in the POC. The Δ14C values ranged between −395 ± 83 (SOC), −226 ± 92 (DOC) and −113 ± 122 ‰ (POC). These stable and radiocarbon isotopes were also different between the Laptev Sea and the East Siberian Sea. Both DOC and POC showed a depleted and younger trend off the Lena River plume. Further, the Pacific inflow and the sea-ice coverage, which works as a barrier preventing the input of “young” DOC and POC, seem to have a strong influence in these carbon pools, presenting older and more enriched δ13C signatures under the sea-ice extent. Lignin phenols exhibited higher OC-normalized concentrations in the SOC (0.10–2.34 mg g−1 OC) and DOC (0.08–2.40 mg g−1 OC) than in the POC (0.03–1.14 mg g−1 OC). The good relationship between lignin and Δ14C signatures in the DOC suggests that a significant fraction of the outer-shelf DOC comes from “young” Terr-OC. By contrast, the slightly negative correlation between lignin phenols and Δ14C signatures in POC, with higher lignin concentrations in older POC from near-bottom waters, may reflect the off-shelf transport of OC from remobilized permafrost in the nepheloid layer. Syringyl ∕ vanillyl and cinnamyl ∕ vannillyl phenol ratios presented distinct clustering between DOC, POC and SOC, implying that those pools may be carrying different Terr-OC of partially different origin. Moreover, 3,5-dihydroxybenzoic acid to vanillyl phenol ratios and p-coumaric acid to ferulic acid ratios, used as a diagenetic indicators, enhanced in POC and SOC, suggesting more degradation within these pools. Overall, the key contrast between enhanced lignin yields both in the youngest DOC and the oldest POC samples reflects a significant decoupling of terrestrial OC sources and pathways.
Highlights
Studies of terrestrial organic carbon (Terr-OC) in the Arctic Ocean are receiving increasing interest due to concerns about the consequences on the carbon cycle by amplified climate change
Whereas dissolved organic carbon (DOC) showed the highest values in surface waters of the Laptev Sea (2000 ± 1100 μg L−1), off the Lena River mouth, particulate organic carbon (POC) concentrations were slightly higher in the Kara Sea (290 ± 86 μg L−1) and the E-ESS (150 ± 92 μg L−1) with no significant differences between surface and near-bottom waters
sediment organic carbon (SOC) values in surface sediments from the same stations presented higher concentrations in the E-ESS (1.32 ± 0.42 %), and exhibited an increase in the Laptev Sea (1.21 ± 0.26 %; Fig. 3). This is in the lower range of what was previously reported in the inner shelf of the East Siberian Arctic Shelf (ESAS) (Charkin et al, 2011; Karlsson et al, 2011, 2015; Tesi et al, 2014; Vonk et al, 2012), suggesting either degradation of terrigenous organic carbon (Terr-OC) or sediment sorting during the across-shelf transport as discussed in Tesi et al (2014) and Bröder et al (2016)
Summary
Studies of terrestrial organic carbon (Terr-OC) in the Arctic Ocean are receiving increasing interest due to concerns about the consequences on the carbon cycle by amplified climate change. Fluvial and erosional processes are expected to increase, as well as biomass cover, resulting in higher input fluxes and changing composition of Terr-OC to the continental shelf (Lantuit et al, 2013; Peterson et al, 2002; Sanchez-Garcia et al, 2014; Serreze et al, 2002). Those mechanisms would enhance the remobilization of permafrost carbon, potentially constituting a climate–carbon positive feedback, in terms of CO2 outgassing from degradation of thawing permafrost. It is essential to understand the sources, dynamical fate and composition of exported Terr-OC in order to assess its impact within the carbon cycle
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