Contrasting regimes for organic matter degradation in the East Siberian Sea and the Laptev Sea assessed through microbial incubations and molecular markers

• Two terrestrially dominated systems show contrasting degradation regimes.
• Degradation in SE Laptev Sea sediment driven by terrestrial matter
• Labile components derived from different sources

Compositional studies of organic matter on the East Siberian Arctic Shelf (ESAS) suggest that different terrestrial carbon pools have different propensities for transport and/or degradation. The current study combined laboratory-based microbial degradation experiments with earlier published degradation-diagnostic composition of several classes of terrestrial biomarkers on the same sediments to investigate differences and driving forces of terrestrial organic matter (TerrOM) degradation in two biogeochemically-contrasting regimes of the ESAS.The incubation-based anaerobic degradation rates were consistently higher (by average factor of 6) in the East Siberian Sea Kolyma Paleoriver Channel (ESS-KPC) (15 μmol CO2 g OC− 1 day− 1) compared to the Laptev Sea Buor-Khaya Bay (LS-BKB) (2.4 μmol CO2 g OC− 1 day− 1).The reported molecular markers show similarities between the terrestrial carbon pools in the two systems, but impose contrasting degradation regimes in combination with the incubation results. For the LS-BKB, there was a strong relationship between the degradation rates and the three lignin phenol-based degradation proxies (r2 = 0.93–0.96, p < 0.01, linear regression) and two wax lipid-based degradation proxies (r2 = 0.71 and 0.66, p < 0.05, linear regression). In contrast, for the ESS-KPC system, there was no relationship between incubation-based degradation rates and molecular marker-based degradation status of TerrOM. A principal component analysis indicated that short-chain fatty acids and dicarboxylic acids from CuO oxidation are mainly of terrestrial origin in the LS-BKB, but mainly of marine origin in the ESS-KPC. Hence, the microbial degradation in the western (LS-BKB) system appears to be fueled by TerrOM whereas the eastern (ESS-KPC) system degradation appears to be driven by MarOM. By combining molecular fingerprinting of TerrOM degradation state with laboratory-based degradation studies on the same ESAS sediments, a picture evolves of two distinctly different modes of TerrOM degradation in different parts of the ESAS system.