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

Abstract:

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.