Using the deuterium isotope composition of permafrost meltwater to constrain thermokarst lake contributions to atmospheric CH₄ during the last deglaciation

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 δ D CH4 emissions from Siberian and North American lakes. δ D CH4 reflects regionally varying δ D 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 δ D CH4 was higher (−334 ± 17‰) than Siberian lake δ D CH4 (−381 ± 18‰). Weighted mean δ D CH4 values for Beringian lakes ranged from −385‰ to −382‰ over the deglacial period. Bottom‐up estimates suggest that Beringian thermokarst lakes contributed 15 ± 4 Tg CH 4 yr −1 to the atmosphere during the Younger Dryas and 25 ± 5 Tg CH 4 yr −1 during the Preboreal period. These estimates are supported by independent, top‐down isotope mass balance calculations based on ice core δ D CH4 and δ 13 C CH4 records. Both approaches suggest that thermokarst lakes and boreal wetlands together were important sources of deglacial CH 4 .