Methane emissions show exponential inverse relationship with electrical resistivity from discontinuous permafrost wetlands in Alaska
Abstract
Terrestrial wetland emissions are the largest single source of greenhouse gas methane (CH4). Since soil resistivity (Ω-m) and anaerobic CH4 production are dependent on temperature and water table position (i.e. the ion-mobility), we hypothesize that resistivity and methanogenesis are correlated. We used geophysical surveys to demarcate both the presence or absence of permafrost, and couple CH4 emissions atop thermokarst (TK) talik geometry. We report a CH4 flux inventory (n=610) from a watershed near Fairbanks AK and Council Seward Peninsula AK from 2019-2022. We computed and plotted CH4 flux emissions atop soil resistivity. CH4 trends were found to be spatially related to talik geometry along boundaries of permafrost-TK transitions. As soil resistivity decreases and becomes supersaturated, CH4 flux increases exponentially. In thawed soils (<200 Ω-m), we found that for roughly every 45 Ω-m decrease, CH4 flux increased by an order of magnitude. The CH4 emissions followed exponential growth and was inversely related to electrical resistivity at sites Fairbanks (y = 1901.3e-0.016x, R2 = 0.87) and Council (y= 51002.0e-0.05x R2 = 0.9). Analysis revealed that hotspots of CH4 (>500 mg m2 d-1) dominated both study area budgets, representing 90% of the flux inventory but sourced from only 15% of total observations. CH4 Hotspots ranged 506 - 23,447 mg m2 d-1 (N=79) and were most frequent along TK escarpments between low-wetlands and dry-uplands. Geophysical surveys (VLF,ERT) revealed that hotspots of CH4 were associated with the lowest resistivity (e.g. 5-20 Ω) showing the influence of water table position. CH4 hotspots, which comprised 90% of total emissions, persisted in both cold and growing season. Radiocarbon dating of a winter CH4 hotspot revealed a 14C age of 35,360 YBP (δ13C -73.8 ‰) corresponding to 14 m of thaw. The winter mean flux of 212 ± 15 mg CH4 m2 d-1 (~40% of annual emissions), resembled summer emissions from other ecosystems. The mean summer flux 1,151 ± 18 mg CH4 m2 d-1 is much higher than 105 prior investigations in other permafrost ecosystems. This study finds the key drivers for CH4 hotspots to be: (1) topographic relief boundaries of TK, (2) water table position, (3) subsurface hydraulic connectivity, and (4) wetland geomorphology. This alludes to the important influence of hydrology on soil biogeochemistry.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.B15E..06H