Quantifying Degradative Loss of Terrigenous Organic Carbon in Surface Sediments Across the Laptev and East Siberian Sea

Abstract

Ongoing permafrost thaw in the Arctic may remobilize large amounts of old organic matter. Upon transport to the Siberian shelf seas, this material may be degraded and released to the atmosphere, exported off‐shelf, or buried in the sediments. While our understanding of the fate of permafrost‐derived organic matter in shelf waters is improving, poor constraints remain regarding degradation in sediments. Here we use an extensive data set of organic carbon concentrations and isotopes ( n = 109) to inventory terrigenous organic carbon (terrOC) in surficial sediments of the Laptev and East Siberian Seas (LS + ESS). Of these ~2.7 Tg terrOC about 55% appear resistant to degradation on a millennial timescale. A first‐order degradation rate constant of 1.5 kyr −1 is derived by combining a previously established relationship between water depth and cross‐shelf sediment‐terrOC transport time with mineral‐associated terrOC loadings. This yields a terrOC degradation flux of ~1.7 Gg/year from surficial sediments during cross‐shelf transport, which is orders of magnitude lower than earlier estimates for degradation fluxes of dissolved and particulate terrOC in the water column of the LS + ESS. The difference is mainly due to the low degradation rate constant of sedimentary terrOC, likely caused by a combination of factors: (i) the lower availability of oxygen in the sediments compared to fully oxygenated waters, (ii) the stabilizing role of terrOC‐mineral associations, and (iii) the higher proportion of material that is intrinsically recalcitrant due to its chemical/molecular structure in sediments. Sequestration of permafrost‐released terrOC in shelf sediments may thereby attenuate the otherwise expected permafrost carbon‐climate feedback.

Rapid Fluvio-Thermal Erosion of a Yedoma Permafrost Cliff in the Lena River Delta

“…However, Bröder et al (2019) reported on slow C degradation rates of terrestrial C in the top layer of seafloor sediments, of which a majority was considered even resistant to degradation. In addition, a large amount of the permafrost C might become re-buried in marine sediments of the Laptev and East Siberian Sea shelf (Vonk et al, 2012;Vonk and Gustafsson, 2013;Bröder et al, 2019).…”

“…The deep mobilization of permafrost C caused by thermo-erosional processes along shores might add C to the atmosphere, which is not yet taken into account in current Earth System Models that include only topdown thaw (Turetsky et al, 2020). However, how much of the C released through shore erosion is re-buried on the Arctic shelf or in the deep Arctic Ocean versus is being mineralized and released into the atmosphere is still a matter of discussion (Vonk et al, 2012;Bröder et al, 2019;Grotheer et al, 2020).…”

Dissolved organic matter at the fluvial–marine transition in the Laptev Sea using in situ data and ocean colour remote sensing

“…In offshore resuspension areas with very high TSM concentration, DOC and TSM do not necessarily co-vary. Large amounts of terrigenous organic matter can be mineralized on short timescales (about 50 % within a year; Kaiser et al, 2017) and strongly degraded when deposited in sediments (Bröder et al, 2016(Bröder et al, , 2019Brüchert et al, 2018).…”

The Late Mesozoic‐Cenozoic Arctic Ocean Climate and Sea Ice History: A Challenge for Past and Future Scientific Ocean Drilling

“…Huge amounts of soil organic carbon are currently stored frozen in permafrost soils (e.g., Tarnocai et al, 2009;Zimov et al, 2009), and vast amounts of methane, in a solid form as gas hydrates, are trapped in permafrost and at shallow depths in cold ocean sediments (e.g., Romanovskii et al, 2005). With increasing temperature of the permafrost (Romanovsky et al, 2010) and the water at the seafloor, as a result of increased surface warming in Arctic regions (Stocker et al, 2013), a widespread increase in the thickness of the thawed layer and the decomposition of hydrates could lead to the release of large quantities of CH 4 (and CO 2 ) to the atmosphere (ACIA, 2004(ACIA, , 2005Anisimov et al, 1997;Goulden et al, 1998;Michaelson et al, 1996) as well as an enhanced mobilization and export of old, previously frozen soil-derived organic carbon (e.g., Bröder et al, 2018;Bröder et al, 2019;Schuur et al, 2008;Tesi et al, 2016;Vonk et al, 2012;Winterfeld et al, 2015Winterfeld et al, , 2018. The release of these greenhouse gases in turn would create a positive feedback mechanism that can amplify regional and global warming.…”