Release of Black Carbon From Thawing Permafrost Estimated by Sequestration Fluxes in the East Siberian Arctic Shelf Recipient

Abstract

Black carbon (BC) plays an important role in carbon burial in marine sediments globally. Yet the sequestration of BC in the Arctic Ocean is poorly understood. Here we assess the concentrations, fluxes, and sources of soot BC (SBC)—the most refractory component of BC—in sediments from the East Siberian Arctic Shelf (ESAS), the World's largest shelf sea system. SBC concentrations in the contemporary shelf sediments range from 0.1 to 2.1 mg g−1 dw, corresponding to 2–12% of total organic carbon. The 210Pb‐derived fluxes of SBC (0.42–11 g m−2 yr−1) are higher or in the same range as fluxes reported for marine surface sediments closer to anthropogenic emissions. The total burial flux of SBC in the ESAS (~4,000 Gg yr−1) illustrates the great importance of this Arctic shelf in marine sequestration of SBC. The radiocarbon signal of the SBC shows more depleted yet also more uniform signatures (−721 to −896‰; average of −774 ± 62‰) than of the non‐SBC pool (−304 to −728‰; average of −491 ± 163‰), suggesting that SBC is coming from an, on average, 5,900 ± 300 years older and more specific source than the non‐SBC pool. We estimate that the atmospheric BC input to the ESAS is negligible (~0.6% of the SBC burial flux). Statistical source apportionment modeling suggests that the ESAS sedimentary SBC is remobilized by thawing of two permafrost carbon (PF/C) systems: surface soil permafrost (topsoil/PF; 25 ± 8%) and Pleistocene ice complex deposits (ICD/PF; 75 ± 8%). The SBC contribution to the total mobilized permafrost carbon (PF/C) increases with increasing distance from the coast (from 5 to 14%), indicating that the SBC is more recalcitrant than other forms of translocated PF/C. These results elucidate for the first time the key role of permafrost thaw in the transport of SBC to the Arctic Ocean. With ongoing global warming, these findings have implications for the biogeochemical carbon cycle, increasing the size of this refractory carbon pool in the Arctic Ocean.

Carbonaceous material export from Siberian permafrost tracked across the Arctic Shelf using Raman spectroscopy

“…The results of this study indicate that terrestrially sourced CM in the East Siberian Arctic region can be found in sediments hundreds of kilometres offshore. Whilst direct quantification of CM as a fraction of total OC is not possible using Raman spectroscopy, chemical analyses show that SBC could account for 14 % of terrestial OC in distal sediments (Salvadó et al, 2017). Highly Graphitised CM is structurally similar to SBC and the chemothermal treatment used in these studies likely removes low-grade CM, implying that CM concentrations could be similar to, if not higher than, SBC offshore.…”

“…Soot black carbon (SBC) is a complementary portion of the recalcitrant OC load in the ESAS. Salvadó et al (2017) studied SBC using chemical methods (chemothermal oxidation to remove non-SBC, followed by elemental analysis and stable/radiocarbon isotope mass spectrometry), which allowed for quantification of 'SBC' and source apportionment. This method does not directly measure the nature of SBC particles, in contrast to Raman analysis, but allows for quatification rather than investigating relative changes.…”

Answer to reviewer #1

“…In the outer plume only about 2 % of the total OC was present as POC. It has been suggested that POC in the Lena River freshwater plume is transported in different forms, including large particles, which can sink, and almost neutrally buoyant flocculates of humic substances (Gustafsson and Gschwend, 1997;Gustafsson et al, 2000;Sánchez-García et al, 2011). The POC, which is as- sociated with larger particles (> 0.7 µm), will settle close to land, whereas the humic substance flocculates will travel further out .…”

“…High-purity Suprapure ® acids were used throughout sample treatment and analysis. Organic carbon analyses were carried out at Stockholm University (for analytical details; see Alling et al, 2010;Sánchez-García et al, 2011). For element analysis, the water samples (colloidal: 1 kDa to 0.22 µm; truly dissolved: < 1 kDa) were diluted (2-200 fold) with 10 % HNO 3 .…”

“…1). Samples from this region collected during this cruise have also been studied for DOC (Alling et al, 2010;Bröder et al, 2016;Karlsson et al, 2016;Salvadó et al, 2017), dissolved inorganic carbon , POC (Karlsson et al, 2016;Sánchez-García et al, 2011), nutrients and alkalinity (Anderson et al, 2009;Pipko et al, 2017), and stable O isotopes (Rosén et al, 2015).…”