Simultaneous quantification of methane and carbon dioxide fluxes reveals that a shallow arctic methane seep is a net sink for greenhouse gases

Abstract:

Warming of high-latitude continental-margin oceans has the potential to release large quantities of carbon from gas hydrate and other sedimentary reservoirs. To assess how carbon mobilized from the seafloor might amplify global warming or alter ocean chemistry, a robust analysis of the concentrations and isotopic content of methane and carbon dioxide (CO₂) in the water column and atmosphere is required. To this effect, a gas analysis system consisting of three cavity ring-down spectrometers was developed to obtain a real-time, three-dimensional characterization of the distribution and isotopic variability of methane and CO₂ at a shallow (<100 m water depth) bubbling methane seep offshore of western Svalbard. Surface water methane concentrations from the continuous-flow CRDS system agreed remarkably well with discrete samples analyzed by the GC-based headspace analysis technique and with a CRDS-based discrete sample analysis module. Reliable carbon isotope data were also obtained from the CRDSs once an isotopic calibration routine was applied.

The resulting data revealed that CO₂ uptake from the atmosphere within the surface water methane plume overlying the gas seep was elevated by 36-45% relative to surrounding waters. In comparison to the positive radiative forcing effect expected from the methane emissions, the negative radiative forcing potential from CO₂ uptake was 32-43 times greater. Lower water temperatures, elevated chlorophyll-fluorescence and ¹³C-enriched CO₂ within the surface methane plume suggest that bubble-driven upwelling of cold, nutrient-rich water stimulated CO₂ uptake by phytoplankton. The observation that a shallow methane seep has a net negative radiative forcing effect challenges the widely-held perception that methane seeps contribute to the global atmospheric greenhouse gas burden.