Isotopic Seasonality of Fluvial-Derived Greenhouse Gases Implies Active Layer Deepening

AbstractIn the northern circumpolar regions, continued climate change promotes abrupt and pervasive thawing of organic-rich permafrost soils, rendering any associated carbon susceptible to mobilization and degradation. Permafrost-derived dissolved and particulate organic carbon entrained into aquatic systems is highly degradable and rapidly mineralized to carbon dioxide (CO2). Thus, signals of permafrost disturbances are short-lived and easily missed. Here, we explore the isotopic composition of dissolved CO2 and methane (CH4) to fingerprint the presence of widespread permafrost thaw in Arctic watersheds. We investigate spatial and seasonal variations in the stable (13C) and radiocarbon (F14C) isotopic signatures of dissolved gases in two river networks in Northern Alaska. Isotopic evidence suggests that the CO2 compositions are homogenous along the fluvial continuum and integrate geogenic and biogenic sources across the watershed. From spring to fall, Bayesian mixing modeling predicts a systematic depletion in the stable and F14C isotopic signatures of dissolved CO2 sources. We attribute this pattern to increasing contributions of aged carbon in response to a deepening of the active layer. As percolating groundwater accesses deeper soil horizons, CO2 produced by heterotrophic oxidation of organic matter is leached and transported to streams and rivers. In contrast, we observe no clear relationships between CH4 compositions and landscape properties. Stream CH4 is likely influenced by local inputs from streambeds, adjacent water saturated soils, and lake outflows. Our findings demonstrate that greenhouse gases dissolved in fluvial systems are sensitive to the release of aged carbon from thawing permafrost. 1 IntroductionRivers serve as crucial conduits between terrestrial and marine ecosystems (Ripl 2003, Milliman and Farnsworth 2013). They are effective integrators across drainage basins (Cole et al 2007, Ward et al 2017) and sensitive to internal landscape dynamics, as they adeptly capture alterations in geomorphology, lithology, and vegetation, discernible in the particulate and dissolved loads (e.g., Gaillardet et al 1999, Syvitski and Milliman 2007, Galy et al 2011). Additionally, rivers archive the impact of both natural disturbances (e.g., fires, landslides, floods) (Hilton et al 2011, Li et al 2016, Rathburn et al 2017, Jones et al 2020) and anthropogenic activities (e.g., land use change, fertilization, sewage discharge, fossil fuel combustion) (Butman et al 2015, Drake et al 2019) in their sedimentary records. This role is especially critical in the Arctic, where the remoteness of the region limits extensive area campaigns and climate change disproportionately impacts the landscape, e.g., through permafrost thaw and thermokarsts (Vonk et al 2015a, Biskaborn et al 2019). These mechanisms lead to the exposure of previously sequestered carbon in permafrost soils to decomposition and the subsequent release of carbon dioxide (CO2), thereby accelerating greenhouse gas feedback mechanisms (Schuur et al 2015, Turetsky et al 2020, Miner et al 2022).Rivers in Arctic regions provide a unique lens through which to observe the effects of climate change on landscape and biogeochemistry. Concentrations and isotopic compositions of carbon (both organic and inorganic) (Hilton et al 2015, Drake et al 2018, Schwab et al 2020), major ions (e.g., nitrate, sulfate) (Harms and Jones 2012, Treat et al 2016, Lafrenière et al 2017), and molecular biomarkers (van Dongen et al 2008, Feng et al 2015a, 2015b) transported in Arctic rivers offer insights into the shifts in biogeochemical processes and landscape dynamics. Such studies underscore the importance of monitoring Arctic rivers not only as indicators of current environmental changes but also as predictors of future shifts in the global carbon cycle and climate system.The direct observation of greenhouse gases CO2 and methane (CH4) emerges as another critical tool for monitoring biogeochemical alterations within drainage watersheds. While the tracking of concentrations and evasion rates of these gases in Arctic ecosystems is extensive (Rasilo et al 2017, Miner et al 2022, Lauerwald et al 2023), investigations into their carbon isotopic compositions are notably sparse. This scarcity is implicitly linked to the technical challenges of collecting adequate samples, leading researchers to focus predominantly on supersaturated environments like soil pore waters and lakes where sampling is more feasible (Czimczik and Welker 2010, Walter Anthony et al 2016, Estop-Aragonés et al 2020, Kuhn et al 2021). In such settings, studies have primarily targeted gas ebullition from underlying sediments or bed rock (Zimov et al 1997, Bouchard et al 2015, Walter Anthony et al 2016), thus neglecting the dissolved states of CO2 and CH4 (Elder et al 2018, 2019, Garcia-Tigreros et al 2023). The absence of isotopic studies on dissolved greenhouse gases in rivers highlights a significant research gap. Such analyses could provide key insights into the sources and transformations of CO2 and CH4, improving our understanding the cycling and fate of carbon in Arctic regions.Here, we aim to address this critial knowledge gap by elucidating the factors controlling sources and compositions of dissolved greenhouse gases within Arctic river networks. Our study represents the first collection of dissolved CO2 and CH4 stable (δ13C) and radiocarbon (14C) isotopic data during the ice-free period. By examining temporal and geomorphological gradients, we discern patterns and pinpoint source contributions of these gases, thereby enhancing our understanding of Arctic fluvial biogeochemistry.