Carbon isotopes and lipid biomarker investigation of sources, transport and degradation of terrestrial organic matter in the Buor-Khaya Bay, SE Laptev Sea

Abstract

The world's largest continental shelf, the East Siberian Shelf Sea, receives substantial input of terrestrial organic carbon (terr-OC) from both large rivers and erosion of its coastline. Degradation of organic matter from thawing permafrost in the Arctic is likely to increase, potentially creating a positive feedback mechanism to climate warming. This study focuses on the Buor-Khaya Bay (SE Laptev Sea), an area with strong terr-OC input from both coastal erosion and the Lena river. To better understand the fate of this terr-OC, molecular (acyl lipid biomarkers) and isotopic tools (stable carbon and radiocarbon isotopes) have been applied to both particulate organic carbon (POC) in surface water and sedimentary organic carbon (SOC) collected from the underlying surface sediments. <br><br> Clear gradients in both extent of degradation and differences in source contributions were observed both between surface water POC and surface sediment SOC as well as over the 100 s km investigation scale (about 20 stations). Depleted δ<sup>13</sup>C-OC and high HMW/LMW <i>n</i>-alkane ratios signaled that terr-OC was dominating over marine/planktonic sources. <br><br> Despite a shallow water column (10–40 m), the isotopic shift between SOC and POC varied systematically from +2 to +5 per mil for δ<sup>13</sup>C and from +300 to +450 for Δ<sup>14</sup>C from the Lena prodelta to the Buor-Khaya Cape. At the same time, the ratio of HMW <i>n</i>-alkanoic acids to HMW <i>n</i>-alkanes as well as HMW <i>n</i>-alkane CPI, both indicative of degradation, were 5–6 times greater in SOC than in POC. This suggests that terr-OC was substantially older yet less degraded in the surface sediment than in the surface waters. This unusual vertical degradation trend was only recently found also for the central East Siberian Sea. <br><br> Numerical modeling (Monte Carlo simulations) with δ<sup>13</sup>C and Δ<sup>14</sup>C in both POC and SOC was applied to deduce the relative contribution of – plankton OC, surface soil layer OC and yedoma/mineral soil OC. This three end-member dual-carbon-isotopic mixing model suggests quite different scenarios for the POC vs SOC. Surface soil is dominating (63 ± 10 %) the suspended organic matter in the surface water of SE Laptev Sea. In contrast, the yedoma/mineral soil OC is accounting for 60 ± 9 % of the SOC. We hypothesize that yedoma-OC, associated with mineral-rich matter from coastal erosion is ballasted and thus quickly settles to the bottom. The mineral association may also explain the greater resistance to degradation of this terr-OC component. In contrast, more amorphous humic-like and low-density terr-OC from surface soil and recent vegetation represents a younger but more bioavailable and thus degraded terr-OC component held buoyant in surface water. Hence, these two terr-OC components may represen...

Temperature and moisture effects on greenhouse gas emissions from deep active-layer boreal soils

“…Deep active-layer soils store large quantities of soil C (Mueller et al, 2015) but are not subject to abundant inputs of fresh C from vegetation. Therefore, the starting quality of the native soil C in active-layer soils is older, more microbially processed, and dominated by more stable "heavy" organic C (Karlsson et al, 2011). Thus, it may not be surprising that these more stable C compounds would be metabolized by processes that have been reported to be less temperature-sensitive.…”

Sources and characteristics of terrestrial carbon in Holocene-scale sediments of the East Siberian Sea

“…4). Earlier studies demonstrated that the decay of fresh marine organic matter is more rapid compared to the degradation of terrestrial organic matter (Karlsson et al, 2011(Karlsson et al, , 2015Salvadó et al, 2016;Vonk et al, 2010). This may lead to the selective preservation of terrestrial organic matter in the sediments of the ESAS (Karlsson et al, 2011(Karlsson et al, , 2015Vonk et al, 2010).…”

“…These different sources have a natural variability in their isotopic composition (endmembers). This variability needs to be taken into account to correctly estimate the relative source contributions and the associated uncertainties (e.g., Andersson, 2011). In previous studies a Bayesian Markov chain Monte Carlo (MCMC) approach has been used to estimate the relative source contributions for individual data points (Andersson et al, 2015;Tesi et al, 2016a).…”

Space–time dynamics of carbon and environmental parameters related to carbon dioxide emissions in the Buor-Khaya Bay and adjacent part of the Laptev Sea

“…The BKB is a hotspot for erosion input from coastal ice complexes (called "yedoma" by local people) and was reported to be an important region for land-seaatmosphere carbon exchanges (Semiletov, 1999a, b;Charkin et al, 2011;Karlsson et al, 2011;Semiletov et al, 2011). Erosion input stems from rapidly collapsing ice complexes, which compose up to 30 % of the BKB coastline.…”

“…POC represents an earlier phase of OM transformation associated with poorly defined compositional changes such as aggregation/disaggregation of particles, incorporation of older and more degraded material, and recombination of organic compounds (Cooper et al, 2005). In the BKB, major POC sources include soil and yedoma- borne terrestrial OC and OC mobilized to the water column by coastal and bottom erosion Karlsson et al, 2011;Feng et al, 2013).…”