Search for: All records

Plot-scale (660 square centimeters (cm2) measurements of methane (CH4) were made using a portable chamber system at North Star Yedoma (NSY), a grassland field in interior Alaska characterized by thermokarst (thaw) mounds forming due to degradation of ice-rich Yedoma, polygonal-ground permafrost soil and at 25 other extensive thermokarst-mound study sites in Alaskan tundra, boreal forest and grassland ecosystems. Measurements were made during summer, winter, and thaw seasons from March 2020 through March 2023. Soil temperature and moisture were measured in-situ with handheld probes on unfrozen soils. Thermokarst mounds are regularly spaced conical hills (≤15 meters (m) diameter, ≤5 m height) separated by trenches (≤3 m width) that form in degrading ice-rich Yedoma permafrost environments. Their formation and morphology are based on the melting of large syngenetic ice wedges in polygonal patterned ground, where the polygon margins (trenches) underlain by ice wedges subside faster and deeper than the less ice-rich polygon centers (mound tops), leaving behind distinct conical-mound features in regularly-spaced patterns. Thermokarst mounds are known to emit nitrous oxide [Marushchak et al. 2021, doi.org/10.1038/s41467-021-27386-2], but their carbon fluxes have until now remained largely uncharacterized. This data set characterizes thermokarst-mound methane fluxes in Alaska. 

Abstract Thermokarst lakes cause abrupt and sustained permafrost degradation and have the potential to release large quantities of ancient carbon to the atmosphere. Despite concerns about how lakes will affect the permafrost carbon feedback, the magnitude of carbon dioxide and methane emissions from deep permafrost soils remains poorly understood. Here we incubated a very deep sediment core (20 m) to constrain the potential productivity of thawed Yedoma and underlying Quaternary sand and gravel deposits. Through radiocarbon dating, sediment incubations and sediment facies classifications, we show that extensive permafrost thaw can occur beneath lakes on timescales of decades to centuries. Although it has been assumed that shallow, aerobic carbon dioxide production will dominate the climate impact of permafrost thaw, we found that anaerobic carbon dioxide and methane production from deep sediments was commensurate with aerobic production on a per gram carbon basis, and had double the global warming potential at warmer temperatures. Carbon release from deep Arctic sediments may thus have a more substantial impact on a changing climate than currently anticipated. These environments are presently overlooked in estimates of the permafrost carbon feedback. 

Large proglacial lakes could have been a significant methane source during the last deglaciation. Today, proglacial lakes are small and mostly limited in the northern hemisphere to the margins of ice sheets in Greenland, Alaska, and Canada, but much larger proglacial lakes collectively flooded millions of square kilometers in the northern hemisphere over the last deglacial period. We synthesize new and existing methane flux measurements from modern proglacial lakes in Alaska and Greenland and use these data together with reconstructed lake area and bathymetry, new paleorecords of sediment organic geochemistry, carbon accumulation, and other proxies to broadly constrain the possible deglacial methane dynamics of a single large North American proglacial lake, Lake Agassiz. While large influxes of glaciogenic material contributed to rapid organic carbon burial during initial lakes phases, limited bioavailability of this carbon is suggested by its likely subglacial origin and prior microbial processing. Water depths of >20 m across 37–90% of the lake area facilitating significant oxidation of methane within the water column further limited emissions. Later phases of lake lowering and subsequent re-expansion into shallow aquatic and subaerial environments provided the most significant opportunity for methane production according to our estimates. We found that Lake Agassiz was likely a small source [0.4–2.7 Tg yr−1 mean (0.1–9.9 Tg yr−1 95% CI)] of methane during the last deglaciation on par with emissions from modern wildfires. Although poor constraints of past global proglacial lake areas and morphologies currently prevent extrapolation of our results, we suggest that these systems were likely an additional source of methane during the last deglacial transition that require further study. 

This review examines recent developments in the application of stable isotope analyses (δ18O, δ13C, δ15N, δD) to lacustrine invertebrate remains. These remains are ubiquitous in lacustrine sediments and thus provide an opportunity to measure changes in stable isotope ratios across a range of timescales and environments and allow interpretive power beyond taxonomic studies. To date they have been relatively understudied in comparison to carbonate fossils and offer both opportunities and challenges and we explore both themes in this review. This review will explore improvements to analytical instrumentation and the opportunities that this presents, it will look at a range of new studies of the modern lacustrine environment and how these studies allow a more nuanced palaeoenvironmental approach. We review recent studies that have used these advancements in understanding to help to reveal new knowledge of past climates, environments and ecology. In addition, we explore new studies that help to elucidate the role of methane-derived carbon to lacustrine food webs and the drivers behind this, including new data to estimate the contribution of methane derived carbon to an arctic lake. We conclude that major progress is currently being made in invertebrate-isotope analyses, and we expect this to continue apace. 

Atmospheric methane (CH4) concentrations have gone through rapid changes since the last deglaciation; however, the reasons for abrupt increases around 14,700 and 11,600 years before present (yrs BP) are not fully understood. Concurrent with deglaciation, sea-level rise gradually inundated vast areas of the low-lying Beringian shelf. This transformation of what was once a terrestrial-permafrost tundra-steppe landscape, into coastal, and subsequently, marine environments led to new sources of CH4 from the region to the atmosphere. Here, we estimate, based on an extended geospatial analysis, the area of Beringian coastal wetlands in 1000-year intervals and their potential contribution to northern CH4 flux (based on present day CH4 fluxes from coastal wetland) during the past 20,000 years. At its maximum (∼14,000 yrs BP) we estimated CH4 fluxes from Beringia coastal wetlands to be 3.5 (+4.0/-1.9) Tg CH4 yr−1. This shifts the onset of CH4 fluxes from northern regions earlier, towards the Bølling-Allerød, preceding peak emissions from the formation of northern high latitude thermokarst lakes and wetlands. Emissions associated with the inundation of Beringian coastal wetlands better align with polar ice core reconstructions of northern hemisphere sources of atmospheric CH4 during the last deglaciation, suggesting a connection between rising sea level, coastal wetland expansion, and enhanced CH4 emissions.