Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from δ13C values of DIC

Alling, V., Porcelli, D., Mörth, C. M., Anderson, L. G., Sánchez García, L., Gustafsson, Ö., Andersson, P. S., Humborg, C. 2012. Degradation of terrestrial organic carbon, primary production and out-gassing of CO₂ in the Laptev and East Siberian Seas as inferred from δ¹³C values of DIC. Geochimica et Cosmochimica Acta 95, 143-159 https://doi.org/10.1016/j.gca.2012.07.028

The cycling of carbon on the Arctic shelves, including outgassing of CO₂ to the atmosphere, is not clearly understood. Degradation of terrestrial organic carbon (OC_ter) has recently been shown to be pronounced over the East Siberian Arctic Shelf (ESAS), i.e. the Laptev and East Siberian Seas, producing dissolved inorganic carbon (DIC). To further explore the processes affecting DIC, an extensive suite of shelf water samples were collected during the summer of 2008, and assessed for the stable carbon isotopic composition of DIC (δ¹³C_DIC). The δ¹³C_DIC values varied between −7.2‰ to +1.6‰ and strongly deviated from the compositions expected from only mixing between river water and seawater. Model calculations suggest that the major processes causing these deviations from conservative mixing were addition of (DIC) by degradation of OC_ter, removal of DIC during primary production, and outgassing of CO₂. All waters below the halocline in the ESAS had δ¹³C_DIC values that appear to reflect mixing of river water and seawater combined with additions of on average 70 ± 20 μM of DIC, originating from degradation of OC_ter in the coastal water column. This is of the same magnitude as the recently reported deficits of DOC_ter and POC_ter for the same waters. The surface waters in the East Siberian Sea had higher δ¹³C_DIC values and lower DIC concentrations than expected from conservative mixing, consistent with additions of DIC from degradation of OC_ter and outgassing of CO₂. The outgassing of CO₂ was equal to loss of 123 ± 50 μM DIC. Depleted δ¹³C_POC values of −29‰ to −32‰ in the mid to outer shelf regions are consistent with POC from phytoplankton production. The low δ¹³C_POC values are likely due to low δ¹³C_DIC of precursor DIC, which is due to degradation of OC_ter, rather than reflecting terrestrial input compositions. Overall, the δ¹³C_DIC values confirm recent suggestions of substantial degradation of OC_ter over the ESAS, and further show that a large part of the CO₂ produced from degradation has been outgassed to the atmosphere.