Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from delta C-13 values of DIC

The cycling of carbon on the Arctic shelves, including outgassing of CO to the atmosphere, is not clearly understood. Degradation of terrestrial organic carbon (OC ) 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 (δ C ). The δ C 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 , removal of DIC during primary production, and outgassing of CO . All waters below the halocline in the ESAS had δ C values that appear to reflect mixing of river water and seawater combined with additions of on average 70±20μM of DIC, originating from degradation of OC in the coastal water column. This is of the same magnitude as the recently reported deficits of DOC and POC for the same waters. The surface waters in the East Siberian Sea had higher δ C values and lower DIC concentrations than expected from conservative mixing, consistent with additions of DIC from degradation of OC and outgassing of CO . The outgassing of CO was equal to loss of 123±50μM DIC. Depleted δ C values of -29‰ to -32‰ in the mid to outer shelf regions are consistent with POC from phytoplankton production. The low δ C values are likely due to low δ C of precursor DIC, which is due to degradation of OC , rather than reflecting terrestrial input compositions. Overall, the δ C values confirm recent suggestions of substantial degradation of OC over the ESAS, and further show that a large part of the CO produced from degradation has been outgassed to the atmosphere. © 2012 Elsevier Ltd