Carbon mineralization in Laptev and East Siberian sea shelf and slope sediment

Volker Brüchert,Lisa Bröder,Tommaso Tesi,Joanna E. Sawicka,Xiaole Sun,Igor P. Semiletov,Samantha P. Joye,Vladimir A. Samarkin

doi:10.5194/bg-15-471-2018

Abstract

Abstract. The Siberian Arctic Sea shelf and slope is a key region for the degradation of terrestrial organic material transported from the organic-carbon-rich permafrost regions of Siberia. We report on sediment carbon mineralization rates based on O2 microelectrode profiling; intact sediment core incubations; 35S-sulfate tracer experiments; pore-water dissolved inorganic carbon (DIC); δ13CDIC; and iron, manganese, and ammonium concentrations from 20 shelf and slope stations. This data set provides a spatial overview of sediment carbon mineralization rates and pathways over large parts of the outer Laptev and East Siberian Arctic shelf and slope and allows us to assess degradation rates and efficiency of carbon burial in these sediments. Rates of oxygen uptake and iron and manganese reduction were comparable to temperate shelf and slope environments, but bacterial sulfate reduction rates were comparatively low. In the topmost 50 cm of sediment, aerobic carbon mineralization dominated degradation and comprised on average 84 % of the depth-integrated carbon mineralization. Oxygen uptake rates and anaerobic carbon mineralization rates were higher in the eastern East Siberian Sea shelf compared to the Laptev Sea shelf. DIC ∕ NH4+ ratios in pore waters and the stable carbon isotope composition of remineralized DIC indicated that the degraded organic matter on the Siberian shelf and slope was a mixture of marine and terrestrial organic matter. Based on dual end-member calculations, the terrestrial organic carbon contribution varied between 32 and 36 %, with a higher contribution in the Laptev Sea than in the East Siberian Sea. Extrapolation of the measured degradation rates using isotope end-member apportionment over the outer shelf of the Laptev and East Siberian seas suggests that about 16 Tg C yr−1 is respired in the outer shelf seafloor sediment. Of the organic matter buried below the oxygen penetration depth, between 0.6 and 1.3 Tg C yr−1 is degraded by anaerobic processes, with a terrestrial organic carbon contribution ranging between 0.3 and 0.5 Tg yr−1.

Highlights

The biogeochemical fate of terrestrial organic carbon deposited on the Arctic shelf and slope is one of the most important open questions for the marine Arctic carbon cycle (e.g., Tesi et al, 2014; Macdonald et al, 2015; McGuire et al, 2009; Vonk et al, 2012)
Bottom-water salinity varied between 34.9 ‰ in the outer Laptev Sea at 3146 m depth (Station 1) and 29.1 ‰ in the East Siberian Sea at 40 m (Station 45)
The lower salinity in the East Siberian Sea can be attributed to longshore transport of freshwater eastward from the Lena River

Summary

Introduction

The biogeochemical fate of terrestrial organic carbon deposited on the Arctic shelf and slope is one of the most important open questions for the marine Arctic carbon cycle (e.g., Tesi et al, 2014; Macdonald et al, 2015; McGuire et al, 2009; Vonk et al, 2012). A key problem in this context is the considerable uncertainty regarding the mineralization of terrestrial organic matter exported by rivers and coastal erosion to the Siberian shelf and slope (Tesi et al, 2014; Karlsson et al 2015; Semiletov et al, 2011; Salvado et al, 2015)

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Conclusion