River inflow dominates methane emissions in an Arctic coastal system

Abstract

Measurements of greenhouse gases in Arctic waters are strongly biased toward low-ice summer conditions, with few observations during periods of seasonal ice retreat. We present a year-round time series of dissolved methane (CH4) and nitrous oxide (N2O), along with targeted observations during ice melt of CH4 and carbon dioxide (CO2) in a river and estuary adjacent to Cambridge Bay, Nunavut, Canada. CH4 displayed dramatic seasonality, in contrast to limited seasonal changes in N2O. During the river freshet, CH4 concentrations in the river and ice-covered estuary were up to 240,000% saturation and 19,000% saturation, respectively, but quickly dropped by >100-fold following ice melt. Observations with a robotic kayak revealed that river-derived CH4 and CO2 were transported to the estuary and rapidly ventilated to the atmosphere once ice cover retreated. We estimate that river discharge accounts for >95% of annual CH4 sea-to-air emissions from the estuary.