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

Abstract

Thermokarst lakes are thought to have been an important source of methane (CH4) during the last deglaciation when atmospheric CH4 concentrations increased rapidly. Here we demonstrate that meltwater from permafrost ice serves as an H source to CH4production in thermokarst lakes, allowing for region‐specific reconstructions ofδDCH4 emissions from Siberian and North American lakes. δDCH4 reflects regionally varying δD 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 CH4production in primary thermokarst lakes, whereas Holocene‐aged permafrost ground ice contributed H to CH4 production in later generation lakes. We found that Alaskan thermokarst lake δDCH4 was higher (−334 ± 17‰) than Siberian lake δDCH4 (−381 ± 18‰). Weighted mean δDCH4values for Beringian lakes ranged from −385‰ to −382‰ over the deglacial period. Bottom‐up estimates suggest that Beringian thermokarst lakes contributed 15 ± 4 Tg CH4 yr−1 to the atmosphere during the Younger Dryas and 25 ± 5 Tg CH4 yr−1during the Preboreal period. These estimates are supported by independent, top‐down isotope mass balance calculations based on ice coreδDCH4 and δ13CCH4 records. Both approaches suggest that thermokarst lakes and boreal wetlands together were important sources of deglacial CH4.