Methane Hydrate Dissociation and Gas Seepage on Global Upper Continental Slopes Driven by Intermediate Ocean Warming

Abstract:

Evidence suggests that the short-term temporal pattern of seafloor gas emissions may be largely driven by tidal/barometric pressure fluctuations. However, thermal perturbations in the overlying intermediate waters are a key factor leading to gas hydrate dissociation that liberates methane to feed cold seeps near the updip limit of gas hydrate stability (GHS) on upper continental slopes. Over the past 5 years, studies have documented temperature-driven methane release at intraseasonal to century-long timescales on the West Spitsbergen margin. Our data on the US Arctic margin show that bottom water temperature (BWT) perturbations over ~2.5 months cause the theoretical updip GHS limit to migrate ~1.5 km upslope, a level of dynamism that may manifest as the observed elevated methane concentrations over the upper slope. Given the documented decades-long increase in oceanic heat content due to global warming, it is not surprising that evidence is emerging that upper slope hydrate dissociation is a global, not merely arctic, phenomenon. On the northern US Atlantic margin, most of the ~600 newly-discovered methane seeps mapped between Cape Hatteras and Georges Bank occur on the upper continental slope near the updip limit of GHS (505-575 m). A BWT database based on ~35,000 CTD casts reveals subtle along-margin variations in the theoretical updip GHS limit and first-order agreement with the observed onset of seepage. Other historical datasets provide evidence for short-term BWT variations of >1ºC on the upper slope. Hudson Canyon, the largest US Atlantic margin shelf-break canyon, has reversing current patterns that amplify short-term BWT variations, making this location an apt laboratory for study of possible gas hydrate degradation processes. Hudson Canyon surveys conducted in 2014 identified dozens more seeps than the ~50 described in Skarke et al. (Fall Meeting 2013), particularly at the updip limit of GHS At this depth, CTDs show that BWTs were initially within GHS conditions and had warmed outside the GHS limit in <2 days. Such rapid BWT fluctuations have important implications for the upper slope gas hydrate dynamics, variations in the amount of previously-sequestered methane available for release at seeps, and periodic clogging and re-opening of seep conduits as gas hydrate forms and breaks down.