Rapid permafrost thaw by thermokarst mobilizes previously frozen organic matter (OM) down to tens of meters deep within decades to centuries, leading to microbial degradation and greenhouse gas release. Late Pleistocene ice-rich Yedoma deposits that thaw underneath thermokarst lakes and refreeze after lake drainage are called taberal sediments. Although widespread, these have not been the subject of many studies. To study OM characteristics and degradability in thawed Yedoma, we obtained a 31.5 m long core from beneath a thermokarst lake on the Bykovsky Peninsula, northeastern Siberia. We reported radiocarbon ages, biogeochemical parameters [organic carbon (OC) content and bulk carbon isotopes] and n-alkane distributions. We found the most degraded OM in frozen, fluvial sediments at the bottom of the core, as indicated by the lowest n-alkane odd-over-even predominance (OEP; 2.2). Above this, the thawed Yedoma sediments had an n-alkane distribution typical of emergent vegetation, suggesting a landscape dominated by low-centered polygons. These sediments were OC poor (OC content: 0.8 wt%, 60% of samples < 0.1 wt%), but the OM (OEP∼5.0) was better preserved than in the fluvial sediments. The upper part of the Yedoma reflected a transition to a drier, grass dominated environment. Furthermore, this unit’s OM was least degraded (OEP∼9.4). The thermokarst lake that formed about 8 cal ka BP thawed the Yedoma in the talik and deposited Holocene lake sediments containing well-preserved OM (OEP∼8.4) with the highest n-alkane concentrations (20.8 μg g–1 sediment). Old, allochthonous OM was found in the thawed Yedoma and frozen fluvial deposits. Using an n-alkane endmember model, we identified a mixed OM input in all units. In our study, the thawed Yedoma sediments contained less OC than reported in other studies for still frozen Yedoma. The Yedoma OM was more degraded compared to previous biomarker research on frozen Yedoma. However, this signal is overprinted by the input signal. The fluvial deposits below the Yedoma contained more OM, but this OM was more degraded, which can be explained by the OM input signal. Continued talik deepening and expansion of this thermokarst lake and others similar to it will expose OM with heterogeneous properties to microbial degradation.

中文翻译：

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西伯利亚东北部 Bykovsky 半岛热喀斯特湖下融化的永久冻土沉积物的正烷烃特征

热岩溶导致的快速永久冻土融化在几十到几个世纪内将先前冻结的有机物质 (OM) 移动到几十米深，导致微生物降解和温室气体释放。晚更新世富含冰的 Yedoma 沉积物在热岩溶湖泊下方解冻并在湖泊排水后重新冻结，这些沉积物被称为表层沉积物。虽然很普遍，但这些都不是许多研究的主题。为了研究解冻的 Yedoma 的 OM 特征和可降解性，我们从西伯利亚东北部 Bykovsky 半岛的一个热岩溶湖下方获得了一个 31.5 m 长的岩心。我们报告了放射性碳年龄、生物地球化学参数 [有机碳 (OC) 含量和本体碳同位素] 和正烷烃分布。我们在核心底部的冰冻河流沉积物中发现了最严重的 OM，如最低的正构烷烃奇数对偶优势 (OEP; 2.2) 所示。在此之上，解冻的 Yedoma 沉积物具有典型的新兴植被的正烷烃分布，表明景观以低中心多边形为主。这些沉积物的 OC 较差（OC 含量：0.8 wt%，60% 的样品 < 0.1 wt%），但 OM（OEP∼5.0）比河流沉积物中保存得更好。Yedoma 的上部反映了向干燥、以草为主的环境的过渡。此外，该单元的 OM 退化最少（OEP∼9.4）。形成约 8 cal ka BP 的热岩溶湖解冻了 talik 中的 Yedoma 并沉积了全新世湖沉积物，其中含有保存完好的 OM（OEP∼8.4），其中正烷烃浓度最高（20.8 μg g-1 沉积物）。在解冻的 Yedoma 和冰冻的河流沉积物中发现了古老的外来 OM。使用正烷烃端元模型，我们确定了所有单元中的混合 OM 输入。在我们的研究中，解冻的 Yedoma 沉积物含有的 OC 比其他研究中报告的仍然冻结的 Yedoma 少。与之前对冷冻 Yedoma 的生物标志物研究相比，Yedoma OM 的降解程度更高。然而，该信号被输入信号叠印。Yedoma 下方的河流沉积物含有更多的 OM，但这种 OM 退化程度更高，这可以通过 OM 输入信号来解释。这个热岩溶湖和其他类似的持续深化和扩张将使具有异质特性的 OM 暴露于微生物降解。与之前对冷冻 Yedoma 的生物标志物研究相比，Yedoma OM 的降解程度更高。然而，该信号被输入信号叠印。Yedoma 下方的河流沉积物含有更多的 OM，但这种 OM 退化程度更高，这可以通过 OM 输入信号来解释。这个热岩溶湖和其他类似的持续深化和扩张将使具有异质特性的 OM 暴露于微生物降解。与之前对冷冻 Yedoma 的生物标志物研究相比，Yedoma OM 的降解程度更高。然而，该信号被输入信号叠印。Yedoma 下方的河流沉积物含有更多的 OM，但这种 OM 退化程度更高，这可以通过 OM 输入信号来解释。这个热岩溶湖和其他类似的持续深化和扩张将使具有异质特性的 OM 暴露于微生物降解。