Carbon cycling on the East Siberian Arctic Shelf – a change in air-sea CO₂ flux induced by mineralization of terrestrial organic carbon

Abstract. Measurements from the SWERUS-C3 and ISSS-08 Arctic expeditions were used to calibrate and validate a new physical-biogeochemical model developed to quantify key carbon cycling processes on the East Siberian Arctic Shelf (ESAS). The model was used in a series of experimental simulations with the specific aim to investigate the pathways of terrestrial dissolved and particulate organic carbon (DOC_ter and POC_ter) supplied to the shelf. Rivers supply on average 8.5 Tg C yr⁻¹ dissolved inorganic carbon (DIC), and further 8.5 and 1.1 Tg C yr⁻¹ DOC_ter and POC_ter respectively. Based on observed and simulated DOC concentrations and stable isotope values (δ¹³C_DOC) in shelf waters, we estimate that only some 20 % of the riverine DOC_ter is labile. According to our model results, an additional supply of approximately 14 Tg C yr⁻¹ eroded labile POC_ter is however required to describe the observed stable isotope values of DIC (δ¹³C_DIC). Degradation of riverine DOC_ter and POC_ter results in a 1.8 Tg C yr⁻¹ reduction in the uptake of atmospheric CO₂, while degradation of eroded POC_ter results in an additional 10 Tg C yr⁻¹ reduction. Our calculations indicate nevertheless that the ESAS is an overall small net sink for atmospheric CO₂ (1.7 Tg C yr⁻¹). The external carbon sources are largely compensated by a net export from the shelf to the Arctic Ocean (31 Tg C yr⁻¹), and to a smaller degree by a permanent burial in the sediments (2.7 Tg C yr⁻¹).