Abstract
The release of greenhouse gases from the large organic carbon stock in permafrost deposits in the circumarctic regions may accelerate global warming upon thaw. The extent of this positive climate feedback is thought to be largely controlled by the microbial degradability of the organic matter preserved in these sediments. In addition, weathering and oxidation processes may release inorganic carbon preserved in permafrost sediments as CO2, which is generally not accounted for. We used 13C and 14C analysis and isotopic mass balances to differentiate and quantify organic and inorganic carbon released as CO2 in the field from an active retrogressive thaw slump of Pleistocene-age Yedoma and during a 1.5-years incubation experiment. The results reveal that the dominant source of the CO2 released from freshly thawed Yedoma exposed as thaw mound is Pleistocene-age organic matter (48–80%) and to a lesser extent modern organic substrate (3–34%). A significant portion of the CO2 originated from inorganic carbon in the Yedoma (17–26%). The mixing of young, active layer material with Yedoma at a site on the slump floor led to the preferential mineralization of this young organic carbon source. Admixtures of younger organic substrates in the Yedoma thaw mound were small and thus rapidly consumed as shown by lower contributions to the CO2 produced during few weeks of aerobic incubation at 4°C corresponding to approximately one thaw season. Future CO2 fluxes from the freshly thawed Yedoma will contain higher proportions of ancient inorganic (22%) and organic carbon (61–78%) as suggested by the results at the end, after 1.5 years of incubation. The increasing contribution of inorganic carbon during the incubation is favored by the accumulation of organic acids from microbial organic matter degradation resulting in lower pH values and, in consequence, in inorganic carbon dissolution. Because part of the inorganic carbon pool is assumed to be of pedogenic origin, these emissions would ultimately not alter carbon budgets. The results of this study highlight the preferential degradation of younger organic substrates in freshly thawed Yedoma, if available, and a substantial release of CO2 from inorganic sources.
Highlights
Permafrost deposits in the northern circumpolar regions contain about 1,300 to 1,600 Gt of organic carbon (OC) that accumulated over thousands of years and was stored at sub-zero temperatures (Schuur et al, 2015)
The undisturbed Yedoma exposed as thaw mound (TM2) had the lowest 14C contents in the range of 0.023–0.109 F14COC in the thaw layer (0–70 cm depth) corresponding to conventional 14C ages of 17,830 to 29,790 years before present (BP) (Supplementary Table S1)
This result agrees well with high respiration rates of Pleistocene-age Yedoma measured in previous incubation experiments (Dutta et al, 2006; Zimov et al, 2006; Lee et al, 2012) that were related to higher amounts of labile organic matter (OM) than in Holocene deposits (Walz et al, 2018)
Summary
Permafrost deposits in the northern circumpolar regions contain about 1,300 to 1,600 Gt of organic carbon (OC) that accumulated over thousands of years and was stored at sub-zero temperatures (Schuur et al, 2015). About 327–466 Gt OC, is stored in the loess-like Yedoma sediments that were deposited during the late Pleistocene and early Holocene in unglaciated areas of the Arctic region (Schirrmeister, 2011; Strauss et al, 2017). Most studies assessing OM quality and degradability, respectively rely on incubation experiments at different temperatures (Schädel et al, 2014, Schädel et al, 2016). The results of these laboratory studies may not necessarily apply to natural, more complex conditions. Chemical characterizations of the OM have been used to differentiate potentially labile and more recalcitrant OC pools based on OC/N ratios (e.g., Schädel et al, 2014; Kuhry et al, 2020) and characteristic organic compounds used as indicators for OM bioavailability and stage of degradation, respectively (e.g., Routh et al, 2014; Strauss et al, 2015; Stapel et al, 2016; Tanski et al, 2017; Jongejans et al, 2021)
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