Thermal dissection and radiocarbon analysis of organic matter released from permafrost thaw slumps using online ramped oxidation-accelerator mass spectrometry (ORO-AMS).

The rapid warming of the Arctic is progressively thawing once-permanently frozen ground, known as permafrost. The destabilization of permafrost soils has far-reaching consequences, notably affecting drainage patterns, and subsequently inducing changes in downstream ecosystems. Furthermore, the soils of the permafrost region store nearly twice the amount of carbon currently present in the atmosphere. This extensive frozen reservoir of organic matter (OM) has been preserved for millennia. Upon thaw, microbial decomposition of OM held in permafrost soils can release greenhouse gases (GHGs) like carbon dioxide (CO2) and methane (CH4), thereby creating a positive feedback loop and exacerbating climate change. Retrogressive thaw slumps (RTS) are a striking example of landscape change and potential source of GHG emissions due to permafrost thaw. RTS result from thaw-driven erosion and are landslides expanding backwards as they thaw, creating large, teardrop-shaped scars on the landscape. Since the early 2000s, the Peel Plateau in the Northwest Territories, Canada, has experienced a significant increase in RTS activity. Large-scale mobilization of permafrost layers formed during the Pleistocene and the early Holocene, contributes to runoff with elevated amounts of old yet potentially labile OM. The reactivity of OM (and hence its susceptibility to conversion into GHGs) may be influenced by its chemical nature and physical environment (e.g. mineral protection), rendering it important to constrain these properties. Radiocarbon (14C) is a useful tool for tracing the sources and fate of organic matter, particularly in Arctic regions where the antiquity of permafrost carbon imparts a distinct signal. However, interpreting conventional bulk-level radiocarbon data is challenging due to the diverse components comprising OM. One approach to overcome this limitation involves serial oxidation of OM to CO2 at increasing temperatures, reflecting a gradient of thermal stability. Higher thermal stability is thought to also indicate greater resistance to microbial decomposition. This CO2, collected over specific temperature ranges (i.e., thermal fractions) is then analysed for its 14C content. This principle is here applied in an online ramped oxidation (ORO) setup, which is directly coupled via a double trap interface (DTI) to an accelerator mass spectrometry (AMS) system. This setup is utilized for analyzing samples collected from RTS features on the Peel Plateau, including the seasonally thawed active layer, Holocene and Pleistocene permafrost layers, recently thawed debris and exported particulate material. Earlier studies on the Peel Plateau's two largest RTS features revealed a mineral matrix primarily composed of silt, clay, and sand. Permafrost layer samples, runoff, and debris showed uniform grain size and carbon content, ranging from 1.2% to 1.5%, with F14C values ranging from 0.1530 to 0.0240, corresponding to the Holocene and Pleistocene Epochs (Bröder et al., 2021). In contrast, active layer samples exhibited higher carbon content, up to 16%, with F14C values ranging from 0.2912 to 0.7153, reflecting conventional 14C ages between approximately 10,000 and 2,600 years. Preliminary ORO analysis revealed comparable thermograms (CO2 concentration ppm vs temperature, °C) for permafrost samples, runoff, and debris and suggest a predominance of more resistant (recalcitrant) OM, in line with Bröder et al. (2021). In contrast, the active layer samples exhibited a thermal profile suggesting larger proportions of labile OM consistent with higher 14C (more modern organic carbon) content. Similarities in bulk F14C values and thermograms between debris and runoff suggest they primarily originate from the permafrost layers rather than the active layer, implying that some of the recalcitrant, permafrost OM could potentially persist during fluvial transport and export to the ocean. As part of this presentation, we will further examine the variability of F14C within the samples and the chemical fingerprinting of the distinct CO2 features observed in the thermograms.

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