New Insights into the Temporal Variability of Seafloor Methane Discharge on the Northern US Atlantic Margin

Abstract:

Multibeam echosounder water column backscatter data and remotely operated vehicle (ROV) video imagery collected from 2011 through 2013 by NOAA Ship Okeanos Explorer and ROV Deep Discoverer revealed methane discharge at over 570 seafloor gas seep locations on the US Atlantic margin between Cape Hatteras and Georges Bank. Subsequent water column surveys and video imaging conducted by Okeanos Explorer and ROV Deep Discoverer in 2014 as well as R/V Atlantis and DSV Alvin, as part of the SeepC program in 2015, re-imaged a majority of these gas seep locations providing an opportunity to make preliminary assessments of gas plume ephemerality and the variability of seafloor methane discharge as a function of time. Analysis of newly collected water column backscatter data indicates that some previously imaged gas plumes are no longer present and that a number of new gas plumes are present in locations where they were previously determined to be absent. Additionally, newly acquired video imagery demonstrates that seafloor gas emission at seep locations can vary between absence and abundant effusively on time scales as short as minutes and that this variability is not correlated with that of in situ temperature and pressure observations. These new results indicate that individual methane plume features can be ephemeral on intra-annual to much shorter time scales. Additionally, these results suggest that short-term variability in gas discharge at individual plumes, away from the upper limit of the gas hydrate stability zone, is not mediated by fluctuations in hydrostatic pressure or thermal perturbations. Despite evidence for high temporal variability in the discharge rates of individual gas plumes, video imagery of robust chemosynthetic communities and thick carbonate crusts suggests an overall persistence of methane emissions at seep sites on time scales of hundreds to thousands of years. Collected data are insufficient to fully constrain the temporality of gas discharge on the northern US Atlantic margin; however, presented results demonstrate the necessity of including a temporal variability term in any calculations of total methane flux and suggest how such a term may be parameterized.