Methane dynamics along the Southeast U.S. Atlantic Margin

Seas along continental margins boast high biodiversity and productivity. We investigated methane dynamics across cold seep and canyon habitats along the Southeast U.S. (SEUS) Atlantic coast off Virginia, North Carolina, and South Carolina. The SEUS is a complex natural laboratory with numerous hotspots of methane cycling. Hundreds of active methane seeps are known in this region and at depths below 500m, methane hydrate is metastable and vulnerable to destabilization. We assessed methane dynamics in sediments and in the water column as a function of environmental biogeochemical signatures, physical dynamics, and microbial communities. Cold seeps exhibited high concentrations of methane in pore fluids but other habitats exhibited low to modest methane concentrations. Similarly, rates of anaerobic methane oxidation were highest in cold seep sediments but concentration profiles show that methane escapes the sediment microbial biofilter and reaches the water column. Water column methane profiles show substantial enrichment in deepwaters and rapid attenuation of concentrations that could suggest efficient consumption. However, measured pelagic oxidation rates show a turnover time of hundreds of days, underscoring inefficient methane consumption. We suggest instead that strong physical stratification of the water column efficient traps methane in deepwaters, concentrating it and transporting it southward, and that it is oxidized along the way over attenuated temporal scales. As such, physical stratification is the main controller of the methane distribution and changes in the hydrographic properties in the region that result in more efficient vertical mixing could potentially increase methane exchange from the sediments to the upper water column.