ALBERT

Description: Withongoing climate warming, Arctic permafrost undergoes fast degradation, resulting inthe deepening of the seasonally unfrozen surface and deep permafrost thaw. As permafrost landscapes store huge amounts of carbon, they are becoming a source of greenhouse gases in the course of remobilization of former freeze-locked organic carbon. Large regions of the Arctic are coveredby ice-rich silt deposits with huge ice wedges, known as Yedoma. Due to their high ice content,these deposits are explicitly vulnerable to changing environmental conditions. One form of permafrost degradation is the formation of thermokarst lakes. These waterbodies play an important role regarding the thermal energy balance in the ground.Understanding the development of permafrost landscapes and the processes which cause their degradationis essential for the estimation of future changes of the permafrost-carbon feedback. The aim of this study was the reconstruction of the development of athermokarst affected late Quaternary landscape, by analyzing the deposits’ sedimentology to understand past depositional processes. Furthermore, I analysed the organic matter characteristics to identify the vulnerability of the organic carbon. Two sediment cores below two thermokarst lakes in the Lena Aldan interfluve region in Central Yakutia were investigated. One core originates from a Yedoma site (YUK15-YU-L15), where sediments accumulated during the late Pleistocene. The other core was retrievedfromthe bottom of an Alas lake (YUK15-YU-L7), consisting of diagenetically and thermal altered Yedoma deposits. Underneath both lakes, a talik (unfrozen ground or thaw bulb) was present. The talik was about 12 m deep for YUK15-YU-L15 and exceeded the core depth of YUK15-YU-L7. The fieldwork was conducted in March 2015 during a joined German-Russian expedition. The two cores were analysed for hydrochemical, biogeochemical and sedimentological parameters. The grain size distribution shows that the lake deposits of both cores were mainly accumulated by fluvial sedimentation processes. The very low organic carbon content in the deposits stand in contrast to other investigated Yedoma study sites. Reasons for the lack of carbon can be deep-thawing processes and related organic matter decomposition in the existing talik or the input of organic-poor sediments in the past. Water isotopes from pore water show a permanently frozen state for the lowerpartof the Yedoma lake core, thereby ruling out strong organic matter decomposition. For the Alas lake core,stable water isotopes reflect more recent precipitation values, resulting from rain and lake water infiltration in the unfrozen ground. An assumption is that the Alas lake deposits had characteristics similar tothe Yedoma lake deposits before permafrost degradation. This can help to estimate future developing stages of the study site. These findings indicate that Yedoma deposits are very heterogeneous on a global scale. The high water and ice content make these deposits vulnerableto fast permafrost degradationand ground subsidence. With the current warming scenarios, thermokarst activity will probably stay on a high level and an increase of thermokarst lake formation is likely in Central Yakutia.

Description: Permafrost thaw leads to thermokarst lake formation and talik growth tens of meters deep, enabling microbial decomposition of formerly frozen organic matter (OM). We analyzed two 17-m-long thermokarst lake sediment cores taken in Central Yakutia, Russia. One core was from an Alas lake in a Holocene thermokarst basin that underwent multiple lake generations, and the second core from a young Yedoma upland lake (formed ca. 70 years ago) whose sediments have thawed for the first time since deposition. This comparison provides a glance into OM fate in thawing Yedoma deposits. We analyzed total organic carbon (TOC) and dissolved organic carbon (DOC) content, n-alkanes concentrations, and bacterial and archaeal membrane markers. Furthermore, we conducted one-year-long incubations (4 °C, dark) and measured anaerobic carbon dioxide (CO2) and methane (CH4) production. The sediments from both cores contained little TOC (0.7±0.4 wt%), but DOC values were relatively high, with highest values in the frozen Yedoma lake sediments (1620 mg L-1). Cumulative GHG production after one year was highest in the Yedoma lake sediments (226±212 μg CO2-C gdw-1, 28±36 μg CH4-C gdw-1) and 3 and 1.5 times lower in the Alas lake sediments, respectively (75±76 μg CO2-C gdw-1, 19±29 μg CH4-C gdw-1). The highest CO2 production in the frozen Yedoma lake sediments likely results from decomposition of readily bioavailable OM, while highest CH4 production in the non-frozen top sediments of this core suggests that methanogenic communities established upon thaw. The lower GHG production in the non-frozen Alas lake sediments resulted from advanced OM decomposition during Holocene talik development. Furthermore, we found that drivers of CO2 and CH4 production differ following thaw. Our results suggest that GHG production from TOC-poor mineral deposits, which are widespread throughout the Arctic, can be substantial. Therefore, our novel data are relevant for vast ice-rich permafrost deposits vulnerable to thermokarst formation.

Description: 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.