Table1_Geochemistry and Weathering Indices of Yedoma and Alas Deposits beneath Thermokarst Lakes in Central Yakutia.DOCX

Ice- and organic-rich deposits of late Pleistocene age, known as Yedoma Ice Complex (IC), are widespread across large permafrost regions in Northeast Siberia. To reconstruct Yedoma IC formation in Central Yakutia, we analyzed the geochemistry, sedimentology, and stratigraphy of thawed and frozen deposits below two thermokarst lakes in different evolutionary stages (a mature alas lake and a initial Yedoma lake) from the Yukechi site in the Lena-Aldan interfluve. We focused on inorganic geochemical characteristics and mineral weathering in two ∼17 m long sediment cores to trace syngenetic permafrost aggradation and degradation over time. Geochemical properties, element ratios, and specific weathering indices reflect varying sedimentation processes and seasonal thaw depths under variable environmental conditions. Deeper thaw during the interstadial Marine Isotope Stage (MIS) 3 enabled increasing mineral weathering and initial thermokarst processes. Sedimentological proxies reflect high transport energy and short transport paths and mainly terrestrial sediment supply. The Yedoma formation resulted from fluvial, alluvial and aeolian processes. Low mean TOC contents in both cores contrast with Yedoma deposits elsewhere. Likely, this is a result of the very low organic matter content of the source material of the Yukechi Yedoma. Pronounced cryostructures and strongly depleted pore water stable isotopes show a perennially frozen state and preserved organic matter for the lower part of the Yedoma lake core, while changing permafrost conditions, conditions promoting weathering, and strong organic matter decomposition are suggested by our proxies for its middle and upper parts. For the alas lake core, less depleted water stable isotopes reflect the influence of recent precipitation, i.e. the infiltration of rain and lake water into the unfrozen ground. The FENG, MIA(R), and ICV weathering indices have proven to be promising proxies for the identification of conditions that promote mineral weathering to different degrees in the stratigraphy of the thawed and frozen Yedoma deposits, for which we assume a rather homogeneous chemical composition of the parent material. Our study highlights that the understanding of environmental conditions during Yedoma formation and degradation processes by specific geochemical proxies is crucial for assessing the potential decomposition and preservation of the frozen and unfrozen Yedoma inventories.

晚更新世时期富含冰与有机质的沉积物，被称为Yedoma冰复合体（Yedoma Ice Complex, IC），广泛分布于西伯利亚东北部的大片多年冻土区域。为重建雅库特中部Yedoma冰复合体的形成过程，我们对勒拿-阿尔丹河间地（Lena-Aldan interfluve）Yukechi遗址处两处处于不同演化阶段的热喀斯特湖（thermokarst lake）——一处为成熟阿拉斯湖（alas lake）、另一处为初始Yedoma湖——之下的解冻与冻结沉积物的地球化学、沉积学及地层学特征展开了分析。我们聚焦于两根约17米长的沉积岩芯的无机地球化学特征与矿物风化情况，以追溯同生多年冻土的沉积与随时间推移的退化过程。地球化学性质、元素比值与特定风化指数，能够反映不同环境条件下的沉积过程与季节解冻深度。间冰阶海洋同位素阶段3（Marine Isotope Stage, MIS 3）时期的更强解冻，推动了矿物风化程度提升与初始热喀斯特过程的发展。沉积学代用指标反映出较高的搬运能量与较短的搬运路径，且沉积物主要来源于陆源供给。Yedoma冰复合体的形成源于河流、冲积与风成作用过程。两根岩芯的总有机碳（Total Organic Carbon, TOC）平均含量较低，与其他地区的Yedoma沉积物存在显著差异。这一现象大概率源于Yukechi地区Yedoma的物源有机质含量极低。Yedoma湖岩芯下部发育有清晰的冻融构造与极度亏损的孔隙水稳定同位素特征，表明其处于长期冻结状态且有机质得以保存；而岩芯中上部的代用指标则显示，多年冻土条件发生变化、存在促进风化的环境条件以及有机质发生了强烈分解。对于阿拉斯湖岩芯的孔隙水稳定同位素亏损程度更低，反映出近期降水的影响，即雨水与湖水渗透进入未冻结土层。经验证表明，FENG、MIA(R)以及化学变异指数（Chemical Index of Variability, ICV）等风化指数，可有效识别Yedoma解冻与冻结沉积物地层中不同程度促进矿物风化的环境条件——我们假设其母质的化学组成相对均一。本研究强调，借助特定地球化学代用指标解析Yedoma形成与退化过程中的环境条件，对于评估Yedoma冻结与未冻结储量的潜在分解与保存潜力具有关键意义。