Methane flux from Beringian coastal wetlands for the past 20,000 years

Abstract

Atmospheric methane (CH₄) concentrations have gone through rapid changes since the last deglaciation; however, the reasons for abrupt increases around 14,700 and 11,600 years before present (yrs BP) are not fully understood. Concurrent with deglaciation, sea-level rise gradually inundated vast areas of the low-lying Beringian shelf. This transformation of what was once a terrestrial-permafrost tundra-steppe landscape, into coastal, and subsequently, marine environments led to new sources of CH₄ from the region to the atmosphere. Here, we estimate, based on an extended geospatial analysis, the area of Beringian coastal wetlands in 1000-year intervals and their potential contribution to northern CH₄ flux (based on present day CH₄ fluxes from coastal wetland) during the past 20,000 years. At its maximum (∼14,000 yrs BP) we estimated CH₄ fluxes from Beringia coastal wetlands to be 3.5 (+4.0/-1.9) Tg CH₄ yr⁻¹. This shifts the onset of CH₄ fluxes from northern regions earlier, towards the Bølling-Allerød, preceding peak emissions from the formation of northern high latitude thermokarst lakes and wetlands. Emissions associated with the inundation of Beringian coastal wetlands better align with polar ice core reconstructions of northern hemisphere sources of atmospheric CH₄ during the last deglaciation, suggesting a connection between rising sea level, coastal wetland expansion, and enhanced CH₄ emissions.