[1] Thermokarst lakes are thought to have been an important source of methane (CH 4 ) during the last deglaciation when atmospheric CH 4 concentrations increased rapidly. Here we demonstrate that meltwater from permafrost ice serves as an H source to CH 4 production in thermokarst lakes, allowing for region-specific reconstructions of dD CH4 emissions from Siberian and North American lakes. dD CH4 reflects regionally varying dD values of precipitation incorporated into ground ice at the time of its formation. Late Pleistocene-aged permafrost ground ice was the dominant H source to CH 4 production in primary thermokarst lakes, whereas Holocene-aged permafrost ground ice contributed H to CH 4 production in later generation lakes. We found that Alaskan thermokarst lake dD CH4 was higher (À334 AE 17‰) than Siberian lake dD CH4 (À381 AE 18‰). Weighted mean dD CH4 values for Beringian lakes ranged from À385‰ to À382‰ over the deglacial period. Bottom-up estimates suggest that Beringian thermokarst lakes contributed 15 AE 4 Tg CH 4 yr À1 to the atmosphere during the Younger Dryas and 25 AE 5 Tg CH 4 yr À1 during the Preboreal period. These estimates are supported by independent, top-down isotope mass balance calculations based on ice core dD CH4 and d 13 C CH4 records. Both approaches suggest that thermokarst lakes and boreal wetlands together were important sources of deglacial CH 4 .Citation: Brosius, L. S., K. M. Walter Anthony, G. Grosse, J. P. Chanton, L. M. Farquharson, P. P. Overduin, and H. Meyer (2012), Using the deuterium isotope composition of permafrost meltwater to constrain thermokarst lake contributions to atmospheric CH 4 during the last deglaciation,
The magnitude and variability in methane (CH4) emissions from lakes are uncertain due to limitations in methods for quantifying the patchiness of ebullition (bubbling). We present a field method to estimate an important and highly uncertain source: ebullition from northern lakes. We defined four classes of CH4 bubble clusters trapped in lake ice representing distinct types of biogenic ebullition seeps that differed in flux rate. Mean annual ebullition determined through long‐term (up to 700 d) continuous flux measurements of 31 seeps in three Siberian and one Alaskan lake was (mean ± standard error, 4–10 seeps per class; g CH4 seep−1 y−1): A, 6 ± 4; B, 48 ± 11; C, 354 ± 52; Hotspot, 1167 ± 177. Discrete‐seep ebullition comprised up to 87% of total emissions from Siberian lakes when diffusive flux and background and seep ebullition were considered together. Including seep ebullition increased previous estimates of lake CH4 emissions based on traditional methods 5‐ to 8‐fold for Siberian and Alaskan lakes. Linking new ebullition estimates to an established biogeochemical model, the Terrestrial Ecosystem Model, increased previous estimates of regional terrestrial CH4 emissions 3‐ to 7‐fold in Siberia. Assessment of the method revealed that ebullition seeps are an important component of the terrestrial CH4 budget. They are identifiable by seep type by independent observers; they are consistent predictors of flux rate in both Siberia and Alaska; and they allow quantification of what was previously a large source of uncertainty in upscaling CH4 emissions from lakes to regions.
Thermokarst lakes accelerate deep permafrost thaw and the mobilization of previously frozen soil organic carbon. This leads to microbial decomposition and large releases of carbon dioxide (CO2) and methane (CH4) that enhance climate warming. However, the time scale of permafrost-carbon emissions following thaw is not well known but is important for understanding how abrupt permafrost thaw impacts climate feedback. We combined field measurements and radiocarbon dating of CH4 ebullition with (a) an assessment of lake area changes delineated from high-resolution (1–2.5 m) optical imagery and (b) geophysical measurements of thaw bulbs (taliks) to determine the spatiotemporal dynamics of hotspot-seep CH4 ebullition in interior Alaska thermokarst lakes. Hotspot seeps are characterized as point-sources of high ebullition that release 14C-depleted CH4 from deep (up to tens of meters) within lake thaw bulbs year-round. Thermokarst lakes, initiated by a variety of factors, doubled in number and increased 37.5% in area from 1949 to 2009 as climate warmed. Approximately 80% of contemporary CH4 hotspot seeps were associated with this recent thermokarst activity, occurring where 60 years of abrupt thaw took place as a result of new and expanded lake areas. Hotspot occurrence diminished with distance from thermokarst lake margins. We attribute older 14C ages of CH4 released from hotspot seeps in older, expanding thermokarst lakes (14CCH4 20 079 ± 1227 years BP, mean ± standard error (s.e.m.) years) to deeper taliks (thaw bulbs) compared to younger 14CCH4 in new lakes (14CCH4 8526 ± 741 years BP) with shallower taliks. We find that smaller, non-hotspot ebullition seeps have younger 14C ages (expanding lakes 7473 ± 1762 years; new lakes 4742 ± 803 years) and that their emissions span a larger historic range. These observations provide a first-order constraint on the magnitude and decadal-scale duration of CH4-hotspot seep emissions following formation of thermokarst lakes as climate warms.
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