What Can Measurements Near the Sea Surface Tell Us About Subsea Permafrost Methane Emissions?

Seafloor seeps of methane (CH₄) gas are found globally, but the existence of subsea permafrost beneath some Arctic seas complicates understanding of CH₄ emissions in these regions. Subsea permafrost may act as both a cap on deeper CH₄ sources, and the frozen sediments themselves can be a source of CH₄ after thawing. Ultimately, much of the interest in subsea permafrost is on potential effects on the modern carbon cycle—in particular related to CH₄ 's high global warming potential on sub-century timescales. Whether any released CH₄ from subsea permafrost areas reaches the atmosphere is a critical question to address, and one that must be examined against the backdrop of a multitude of Arctic CH₄ sources to the atmosphere, many of which are presently growing.

During the 2014, 2015, 2016, and 2018 Arctic summers we made measurements of near-surface seawater and atmospheric CH₄, and CH₄ isotopologues, as well as CH₄ sea-air fluxes, from the Swedish icebreaker Oden. These measurements were obtained across broad regions of the Arctic Ocean, above regions of suspected subsea permafrost, and regions where subsea permafrost was not expected. Large but localized positive sea-air fluxes, likely attributable to CH₄ bubbles, were observed in small areas of the Laptev and East Siberian Seas, reaching 170 mg m^-2 d^-1 and 600 mg m^-2 d^-1, respectively. However, the area-average emissions on a larger scale were much smaller, < 5 and < 2 mg m^-2 d^-1, respectively. In regions of the Chukchi Sea not believed to include subsea permafrost, fluxes were < 0.2 mg m^-2 d^-1. Distinguishing subsea permafrost emissions from other oceanic and land-based sources of CH₄ based on observed isotopologues in the water and air is presently frustrated by lack of suitable methane source signature data. Subsea permafrost slowly thaws once submerged; the effect of submerged duration on potential CH₄ emission remains to be understood. Combining sediment porewater studies with sea surface and tropospheric boundary layer studies may help understand the stability and/or efficiency of the transfer of CH₄ from old carbon stores to the atmosphere.