Subglacial geology and thermal conditions regulate methane emissions from Svalbard glaciers.

Melt rivers from valley glaciers and ice sheets release methane by transporting microbial or geologic methane from beneath glaciers, making it available for atmospheric emission. However, the mechanisms and magnitude of such methane release remain poorly understood due to limited studies. Here, we show how subglacial geology and glacier thermal conditions control methane supersaturation in meltwaters from 19 valley glaciers across central Svalbard. We find that all sampled meltwaters are supersaturated with methane, with peak concentrations up to 425-times atmospheric equilibrium. The carbon isotopic compositions of the methane and the presence of ethane and propane indicate that the methane is largely of thermogenic origin. Ground-penetrating radar surveys show that the extent of temperate ice at the glacier bed plays a critical role in facilitating methane acquisition, particularly in regions underlain by organic-rich, shale-dominated geological formations. This mechanistic insight is relevant for glaciated regions worldwide with organic-rich geology and suggests that accelerated glacier melt may amplify methane mobilization from beneath glaciers.