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We apply a recently developed measurement technique for methane (CH4) isotopologues* (isotopic variants of CH4—13CH4, 12CH3D, 13CH3D, and 12CH2D2) to identify contributions to the atmospheric burden from fossil fuel and microbial sources. The aim of this study is to constrain factors that ultimately control the concentration of this potent greenhouse gas on global, regional, and local levels. While predictions of atmospheric methane isotopologues have been modeled, we present direct measurements that point to a different atmospheric methane composition and to a microbial flux with less clumping (greater deficits relative to stochastic) in both 13CH3D and 12CH2D2 than had been previously assigned. These differences make atmospheric isotopologue data sufficiently sensitive to variations in microbial to fossil fuel fluxes to distinguish between emissions scenarios such as those generated by different versions of EDGAR (the Emissions Database for Global Atmospheric Research), even when existing constraints on the atmospheric CH4 concentration profile as well as traditional isotopes are kept constant. 

A large volume of freshwater is incorporated in the relatively fresh (salinity ~32–33) Pacific Ocean waters that are transported north through the Bering Strait relative to deep Atlantic salinity in the Arctic Ocean (salinity ~34.8). These freshened waters help maintain the halocline that separates cold Arctic surface waters from warmer Arctic Ocean waters at depth. The stable oxygen isotope composition of the Bering Sea contribution to the upper Arctic Ocean halocline was established as early as the late 1980’s as having a δ 18 O V - SMOW value of approximately -1.1‰. More recent data indicates a shift to an isotopic composition that is more depleted in 18 O (mean δ 18 O value ~-1.5‰). This shift is supported by a data synthesis of >1400 water samples (salinity from 32.5 to 33.5) from the northern Bering and Chukchi seas, from the years 1987–2020, which show significant year-to-year, seasonal and regional variability. This change in the oxygen isotope composition of water in the upper halocline is consistent with observations of added freshwater in the Canada Basin, and mooring-based estimates of increased freshwater inflows through Bering Strait. Here, we use this isotopic time-series as an independent means of estimating freshwater flux changes through the Bering Strait. We employed a simple end-member mixing model that requires that the volume of freshwater (including runoff and other meteoric water, but not sea ice melt) flowing through Bering Strait has increased by ~40% over the past two decades to account for a change in the isotopic composition of the 33.1 salinity water from a δ 18 O value of approximately -1.1‰ to a mean of -1.5‰. This freshwater flux change is comparable with independent published measurements made from mooring arrays in the Bering Strait (freshwater fluxes rising from 2000–2500 km 3 in 2001 to 3000–3500 km 3 in 2011). 

The expected reduction of ice algae with declining sea ice may prove to be detrimental to the Pacific Arctic ecosystem. Benthic organisms that rely on sea ice organic carbon (iPOC) sustain benthic predators such as the Pacific walrus ( Odobenus rosmarus divergens ). The ability to track the trophic transfer of iPOC is critical to understanding its value in the food web, but prior methods have lacked the required source specificity. We analyzed the H-Print index, based on biomarkers of ice algae versus phytoplankton contributions to organic carbon in marine predators, in Pacific walrus livers collected in 2012, 2014 and 2016 from the Northern Bering Sea (NBS) and Chukchi Sea. We paired these measurements with stable nitrogen isotopes ( δ 15 N) to estimate trophic position. We observed differences in the contribution of iPOC in Pacific walrus diet between regions, sexes, and age classes. Specifically, the contribution of iPOC to the diet of Pacific walruses was higher in the Chukchi Sea (52%) compared to the NBS (30%). This regional difference is consistent with longer annual sea ice persistence in the Chukchi Sea. Within the NBS, the contribution of iPOC to walrus spring diet was higher in females (~45%) compared to males (~30%) for each year (p < 0.001), likely due to specific foraging behavior of females to support energetic demands associated with pregnancy and lactation. Within the Chukchi Sea, the iPOC contribution was similar between males and females, yet higher in juveniles than in adults. Despite differences in the origin of organic carbon fueling the system (sea ice versus pelagic derived carbon), the trophic position of adult female Pacific walruses was similar between the NBS and Chukchi Sea (3.2 and 3.5, respectively), supporting similar diets (i.e. clams). Given the higher quality of organic carbon from ice algae, the retreat of seasonal sea ice in recent decades may create an additional vulnerability for female and juvenile Pacific walruses and should be considered in management of the species. 

Permafrost sediments contain one of the largest reservoirs of organic carbon on Earth that is relatively stable when it remains frozen. As air temperatures increase, the shallow permafrost thaws which allows this organic matter to be converted into potent greenhouse gases such as methane (CH4) and carbon dioxide (CO2) through microbial processes. Along the Beaufort Sea coast in the vicinity of the Tuktoyaktuk Peninsula, Northwest Territories, Canada, warming air temperatures are causing the active layer above permafrost to deepen, and a number of active periglacial processes are causing rapid erosion of previously frozen permafrost. In this paper, we consider the biogeochemical consequences of these processes on the permafrost sediments found at Tuktoyaktuk Island. Our goals were to document the in situ carbon characteristics which can support microbial activity, and then consider rates of such activity if the permafrost material were to warm even further. Samples were collected from a 12mpermafrost core positioned on the top of the island adjacent to an eroding coastal bluff. Downcore CH4, total organic carbon and dissolved organic carbon (DOC) concentrations and stable carbon isotopes revealed variable in situ CH4 concentrations down core with a sub-surface peak just below the current active layer. The highest DOC concentrations were observed in the active layer. Controlled incubations of sediment from various depths were carried out from several depths anaerobically under thawed (5°C and 15°C) and under frozen (−20°C and −5°C) conditions. These incubations resulted in gross production rates of CH4 and CO2 that increased upon thawing, as expected, but also showed appreciable production rates under frozen conditions. This dataset presents the potential for sediments below the active layer to produce potent greenhouse gases, even under frozen conditions, which could be an important atmospheric source in the actively eroding coastal zone even prior to thawing.