Subsea permafrost as a potential major source of dissolved organic matter to the East Siberian Arctic Shelf

Abstract

Arctic subsea permafrost contains more organic carbon than the terrestrial counterpart (similar to 1400 Pg C vs. similar to 1000 Pg C) and is undergoing fast degradation (at rates of similar to 10 to 30 cm yr(-1) over the past 3 decades) in response to climate warming. Yet the flux of organic carbon sequestered in the sediments of subsea permafrost to overlying water column, which can trigger enormous positive carbon-climate feedbacks, remain unclear. In this study, we examined the dissolved organic matter (DOM) diffusion to bottom seawaters from East Siberian Sea (ESS) sediments, which was estimated at about 943-2240 g C m(-2) yr(-1) and 10-55 g C m(-2) yr(-1) at the continuous-discontinuous transition zone of subsea permafrost and the remainder shelf and slope sites, respectively. The released DOM is characterized by prevailing dominance (>= 98%) of low molecular weight (M-D < 350 Da) fractions. A red-shifted (emission wavelength >500 nm) fluorescence fingerprint, a typical feature of sediment/soil DOM, accounts for 4-6% and 7-8% in the fluorescence distributions of seawaters and pore waters, respectively, on ESS shelf. Statistical analysis revealed that seawaters and pore waters possessed similar DOM composition. The estimated total benthic efflux of dissolved organic carbon (DOC) was similar to 0.7-1.0 Pg C yr(-1) when the estimate was scaled up to the entire Arctic shelf underlain with subsea permafrost assuming the width of continuous-discontinuous transition zone is 1 to 10 m. This estimation is consistent with the established similar to 10-30 cm yr(-1) degradation rates of subsea permafrost by estimating its thaw-out time. Compiled observation data suggested that subsea permafrost might be a major DOM source to the Arctic Ocean, which could release tremendous carbon upon remineralization via its degradation to CO2 and CH4 in the water column. (C) 2021 Elsevier B.V. All rights reserved.