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Non-growing season CO2 emissions from Arctic tundra remain a major uncertainty in forecasting climate change consequences of permafrost thaw. We present the first time series of soil and microbial CO2 emissions from a graminoid tundra based on year-round in situ measurements of the radiocarbon content of soil CO2 (Δ14CO2) and of bulk soil C (Δ14C), microbial activity, and temperature. Combining these data with land-atmosphere CO2 exchange allows estimates of the proportion and mean age of microbial CO2 emissions year-round. We observe a seasonal shift in emission sources from fresh carbon during the growing season (August Δ14CO2 = 74 ± 4.7‰, 37% ± 3.4% microbial, mean ± se) to increasingly older soil carbon in fall and winter (March Δ14CO2 = 22 ± 1.3‰, 47% ± 8% microbial). Thus, rising soil temperatures and emissions during fall and winter are depleting aged soil carbon pools in the active layer and thawing permafrost and further accelerating climate change.

We developed a passive sampler for time-integrated collection and radiocarbon (14C) analysis of soil respiration, a major flux in the global C cycle. It consists of a permanent access well that controls the CO2 uptake rate and an exchangeable molecular sieve CO2 trap. We tested how access well dimensions and environmental conditions affect collected CO2, and optimized cleaning procedures to minimize 14CO2 memory. We also deployed two generations of the sampler in Arctic tundra for up to two years, collecting CO2 over periods of 3 days-2 months, while monitoring soil temperature, volumetric water content, and CO2 concentration. The sampler collects CO2 at a rate proportional to the length of a silicone tubing inlet (7-26 g CO2-C day-1·m Si-1). With constant sampler dimensions in the field, CO2 recovery is best explained by soil temperature. We retrieved 0.1-5.3 mg C from the 1st and 0.6-13 mg C from the 2nd generation samplers, equivalent to uptake rates of 2-215 (n=17) and 10-247 g CO2-C day-1 (n=20), respectively. The method blank is 8 ± 6 g C (mean ± sd, n=8), with a radiocarbon content (fraction modern) ranging from 0.5875-0.6013 (n=2). The sampler enables more continuous investigations of soil C emission sources and is suitable for Arctic environments.

The Arctic Ocean and adjacent seas are undergoing increased freshwater influx due to enhanced glacial and sea ice melt, precipitation, and runoff. Accurate delineation of these freshwater sources is vital as they critically modulate ocean composition and circulation with widespread and varied impacts. Despite this, the delineation of freshwater sources using physical oceanographic measurements (e.g., temperature, salinity) alone is challenging and there is a requirement to improve the partitioning of ocean water masses and their mixing relationships. Here, we complement traditional oceanographic measurements with continuous surface seawater isotopic analysis (δ18O and deuterium excess) across a transect extending from coastal Alaska to Baffin Bay and the Labrador Sea conducted from the US Coast Guard Cutter Healy in Autumn 2021. We find that the diverse isotopic signatures of Arctic freshwater sources, coupled with the high freshwater proportion in these marine systems, facilitates detailed fingerprinting and partitioning. We observe the highest freshwater composition in the Beaufort Sea and Amundsen Gulf regions, with heightened freshwater content in eastern Baffin Bay adjacent to West Greenland. We apply isotopic analysis to delineate freshwater sources, revealing that in the Western Arctic freshwater inputs are dominated by meteoric water inputs—specifically the Mackenzie River—with a smaller sea ice meltwater component and in Baffin Bay the primary sources are local precipitation and glacial meltwater discharge. We demonstrate that such freshwater partitioning cannot be achieved using temperature‐salinity relationships alone, and highlight the potential of seawater isotopic tracers to assess the roles and importance of these evolving freshwater sources. Plain Language Summary: Freshwater inputs to the Arctic seas, including glacial and sea ice meltwater, precipitation, and river runoff, are increasing as the Arctic warms. The impacts of these changing freshwater influxes are varied depending on the type of freshwater source, and thus it is important to delineate and trace these different freshwater sources, which represents a significant challenge using only traditional physical oceanographic measurements (e.g., temperature, salinity). In this study, we utilize a new approach to identify and trace freshwater sources using continuous seawater isotopic measurements during a cruise extending from coastal Alaska, through the Canadian Archipelago, and across Baffin Bay and the Labrador Sea. We show that these isotopic measurements, which have been commonly used in other media (e.g., precipitation, water vapor, ice cores), hold important and distinct information about the source and mixing of different freshwater sources. We use these measurements to identify the freshwater sources (e.g., Mackenzie vs. Yukon River) contributing to ocean surface waters across the Arctic region. Key Points: Seawater isotopic measurements (δ18O, δ2H, deuterium excess) show heightened freshwater content in the Beaufort Sea and Baffin BayIsotopic observations enable freshwater source delineation not feasible from traditional physical oceanographic methodsFreshwater source delineation includes the Mackenzie and Yukon Rivers around coastal Alaska and glacial meltwater in Baffin Bay [ABSTRACT FROM AUTHOR]

Non‐growing season CO2 emissions from Arctic tundra remain a major uncertainty in forecasting climate change consequences of permafrost thaw. We present the first time series of soil and microbial CO2 emissions from a graminoid tundra based on year‐round in situ measurements of the radiocarbon content of soil CO2 (Δ14CO2) and of bulk soil C (Δ14C), microbial activity, and temperature. Combining these data with land‐atmosphere CO2 exchange allows estimates of the proportion and mean age of microbial CO2 emissions year‐round. We observe a seasonal shift in emission sources from fresh carbon during the growing season (August Δ14CO2 = 74 ± 4.7‰, 37% ± 3.4% microbial, mean ± se) to increasingly older soil carbon in fall and winter (March Δ14CO2 = 22 ± 1.3‰, 47% ± 8% microbial). Thus, rising soil temperatures and emissions during fall and winter are depleting aged soil carbon pools in the active layer and thawing permafrost and further accelerating climate change. Plain Language Summary: The Arctic is warming and large quantities of organic matter in permafrost soils are at risk of becoming decomposed to carbon dioxide (CO2) by microbes. Measurements of soil CO2 emissions do not provide direct estimates of permafrost emissions, however, because CO2 is produced by microbes and plant roots. Using a new sampler, we collected soil CO2 in northern Alaska over 3–6 weeks for 2 years and analyzed its radiocarbon content to distinguish CO2 sources. To calculate CO2 budgets, we added observations of soil and microbial CO2 emissions and soil temperature. We find that microbes rely on fresh plant matter in summer but older organic matter from fall to spring; a process not captured by standard laboratory experiments. Thus, warming permafrost soils are not just more rapidly cycling fresh plant litter but losing organic matter that accumulated over decades to millennia. Key Points: Combining continuous soil CO2 flux with Δ14CO2 observations enables a systematic evaluation of carbon cycling in Arctic soils year‐roundOutside the growing season, soil microorganisms rely on older, local soil carbon pools not captured in short‐term incubation experimentsPermafrost warming and thaw is depleting soil carbon pools; fluxes of older carbon are greatest in summer but dominate emissions in winter [ABSTRACT FROM AUTHOR]

We developed a passive sampler for time-integrated collection and radiocarbon (14C) analysis of soil respiration, a major flux in the global C cycle. It consists of a permanent access well that controls the CO2 uptake rate and an exchangeable molecular sieve CO2 trap. We tested how access well dimensions and environmental conditions affect collected CO2, and optimized cleaning procedures to minimize 14CO2 memory. We also deployed two generations of the sampler in Arctic tundra for up to two years, collecting CO2 over periods of 3 days–2 months, while monitoring soil temperature, volumetric water content, and CO2 concentration. The sampler collects CO2 at a rate proportional to the length of a silicone tubing inlet (7–26 µg CO2-C day-1·m Si-1). With constant sampler dimensions in the field, CO2 recovery is best explained by soil temperature. We retrieved 0.1–5.3 mg C from the 1st and 0.6–13 mg C from the 2nd generation samplers, equivalent to uptake rates of 2–215 (n=17) and 10–247 µg CO2-C day-1 (n=20), respectively. The method blank is 8 ± 6 µg C (mean ± sd, n=8), with a radiocarbon content (fraction modern) ranging from 0.5875–0.6013 (n=2). The sampler enables more continuous investigations of soil C emission sources and is suitable for Arctic environments. [ABSTRACT FROM AUTHOR]