Title screen

[References: p. 1189-1191 (56 tit.)]

This large-scale quasi-synoptic study gives a comprehensive picture of sea-air CO2 fluxes during the melt season in the central and outer Laptev Sea (LS) and East Siberian Sea (ESS). During a 7 week cruise we compiled a continuous record of both surface water and air CO2 concentrations, in total 76,892 measurements. Overall, the central and outer parts of the ESAS constituted a sink for CO2, and we estimate a median uptake of 9.4 g C m-2 yr-1 or 6.6 Tg C yr-1. Our results suggest that while the ESS and shelf break waters adjacent to the LS and ESS are net autotrophic systems, the LS is a net heterotrophic system. CO2 sea-air fluxes for the LS were 4.7 g C m-2 yr-1, and for the ESS we estimate an uptake of 7.2 g C m-2 yr-1. Isotopic composition of dissolved inorganic carbon ([sigma]13CDIC and [sigma]13CCO2) in the water column indicates that the LS is depleted in [sigma]13CDICcompared to the Arctic Ocean (ArcO) and ESS with an offset of 0.5‰ which can be explained by mixing of [sigma]13CDIC-depleted riverine waters and 4.0 Tg yr-1 respiration of OCter; only a minor part (0.72 Tg yr-1) of this respired OCter is exchanged with the atmosphere. Property-mixing diagrams of total organic carbon and isotope ratio ([sigma]13CSPE-DOC) versus dissolved organic carbon (DOC) concentration diagram indicate conservative and nonconservative mixing in the LS and ESS, respectively. We suggest land-derived particulate organic carbon from coastal erosion as an additional significant source for the depleted [sigma]13CDIC.