Methane transport from the active layer to lakes in the Arctic using Toolik Lake,Alaska, as a case study

Methane emissions in the Arctic are important, and may be contributing to global warming. While methane emission rates from Arctic lakes are well documented, methods are needed to quantify the relative contribution of active layer groundwater to the overall lake methane budget. Here we report measurements of natural tracers of soil/groundwater, radon, and radium, along with methane concentration in Toolik Lake, Alaska, to evaluate the role active layer water plays as an exogenous source for lake methane. Average concentrations of methane, radium, and radon were all elevated in the active layer compared with lake water (1.6 × 10⁴ nM, 61.6 dpm⋅m⁻³, and 4.5 × 10⁵ dpm⋅m⁻³ compared with 1.3 × 10² nM, 5.7 dpm⋅m⁻³, and 4.4 × 10³ dpm⋅m⁻³, respectively). Methane transport from the active layer to Toolik Lake based on the geochemical tracer radon (up to 2.9 g⋅m⁻²⋅y⁻¹) can account for a large fraction of methane emissions from this lake. Strong but spatially and temporally variable correlations between radon activity and methane concentrations (r² > 0.69) in lake water suggest that the parameters that control methane discharge from the active layer also vary. Warming in the Arctic may expand the active layer and increase the discharge, thereby increasing the methane flux to lakes and from lakes to the atmosphere, exacerbating global warming. More work is needed to quantify and elucidate the processes that control methane fluxes from the active layer to predict how this flux might change in the future and to evaluate the regional and global contribution of active layer water associated methane inputs.Methane is a powerful greenhouse gas estimated to be responsible for approximately one-fifth of man-made global warming, and its concentration has been increasing (1). The largest natural source of methane is wetlands (2), including a major component from northern high-latitude regions containing permafrost (3). Observations in the Siberian Arctic show high rates of methane release from both the coastal seabed and land-based permafrost soils (4, 5). Previous research has also documented extensive methane release from Arctic lakes, primarily via ebullition, with significant spatial variability within and between lakes (6, 7). Methane in Arctic lakes forms by microbial production (methanogenesis) in the water column and/or within anaerobic lake sediments (8). However, it is possible that some fraction of the observed methane is not produced within the lakes but is rather transported to the lakes from external, land-based sources through subterranean groundwater discharge (SGD) (9) (Fig. 1). Specifically, water in the active layer (the surficial layer of the soil system above the permafrost that thaws every summer) could be transported into lakes and rivers during the thaw season (May−August) (10). Groundwater in some temperate climate regions can be highly enriched in methane (11), and groundwater discharge contributions to the methane budget of the coastal ocean and lakes in temperate climate have been reported (12, 13). This conduit of methane transport may be particularly important in regions where organic-rich soils and anaerobic conditions promote methane production in the soil, such as in the Arctic (14). SGD occurs everywhere at the land−water interface in lakes, rivers, and the ocean, and evidence of water flow through the active layer in permafrost-dominated regions is found in springs, in open water reaches and base flow of rivers, in talik, and in the distribution of halophytic vegetation (15, 16). Methane fluxes sourced from land into lakes and coastal waters in the Arctic have not been directly measured. The organic-rich soils and anaerobic conditions in the active layer in the Arctic are prime conditions for methane production, and this methane could be transported to lakes and coastal waters.Fig. 1.(A) Schematic diagram of active layer water-associated methane flux to Arctic lakes and related climate feedbacks. Warming for more than a century has resulted in various degrees of permafrost thawing. As climate warms, permafrost stability is likely ...