Contrasting composition of terrigenous organic matter in the DOC, POC and sedimentary organic carbon (SOC) pools on the outer East Siberian Arctic Shelf (ESAS)

Fluvial and erosional discharge of the thawing permafrost in the East Siberian Arctic delivers large quantities of terrigenous organic carbon (Terr-OC) into marine waters. The fate of the remobilized Terr-OC shelf export needs to be better constrained as it impacts the potential for a climate-carbon feedback. In the present study, the composition of the cross-shelf exported organic carbon in different marine pools is evaluated.The results show DOC values (737-3598 μg/L) one order of magnitude higher than POC (20-356 μg/L) with higher concentrations closer to the Lena River, particularly for the DOC. δ13C and Δ14C signatures and lignin phenols present good correlations with DOC levels. In contrast, POC may have a preferential marine origin, as concentrations do not correlate with isotope and terrestrial biomarker proxies. The δ13C signatures in the three carbon pools vary from -23.9±1.9‰ in the SOC, -26.1±1.2‰ in the DOC and -27.1±1.9‰ in the POC. And the Δ14C values range between -356±64‰ (SOC), -226±92‰ (DOC) and -113±122‰ (POC). Those stable and radiocarbon isotopes also present contrasting offsets between the Laptev Sea and the East Siberian Sea. DOC and POC show a depleted and younger trend off the Lena River plume. The older and enriched signatures in the outer-shelf of the ESS suggest the influence of the sea ice coverage and the Pacific inflow from the East.Lignin phenols exhibit higher concentration in the SOC (0.1-2.3 mg/g OC) and DOC (0.1-2.4 mg/g OC) than in the POC (0.03-1.1 mg/g OC). The good correlation between lignin and Δ14C signatures in the DOC confirms that a significant fraction of the outer-shelf DOC comes from Terr-OC. On the other hand, lignin phenols in the POC do not correlate with Δ14C values suggesting a predominant marine origin of this carbon pool. Ratios of syringyl to vanillyl phenols increase in DOC suggesting either molecular degradation or fractionations during leaching. Regarding lignin provenance, syringil/vanillyl and cinnamyl/vannillyl phenols ratios present clear boundaries showing that SOC, DOC and POC come from different Terr-OC sources. Overall, it seems that terrigenous DOM is just partially removed by microbial degradation along the shelf transport and most of the terr-OC in the Siberian Arctic is exported off-shelf within this carbon pool.