Active gas migration systems offshore northern Israel, first evidence from seafloor and subsurface data

Abstract

An active gas migration system across the eastern Mediterranean continental shelf, offshore northern Israel, is described for the first time. Seven vintages of industrial multi-channel seismic reflection and high-resolution single-channel data are integrated with data from a deep borehole, bathymetry and backscatter maps. Combined interpretation, aided by seismic attribute analysis, exhibits a variety of seismic gas signatures across the subsurface and seabed: acoustic blanking, bright spots, phase reversals, gas chimneys, reflector smearing, pockmarks and presumed gas release into the water column. Sub-seafloor gas stems from three main sources: (1) Mid-Pleistocene, (2) Last Glacial Maximum unconformities and (3) a series of incised channels in-between which are also related to glacial maxima. Close to the seafloor, gas accumulates across an area of 72km2 at depths ranging from 37 to 112m below sea-level, while ∼720 pockmarks pierce the seafloor mainly at the gas front periphery. Active bubble emission from one seep at the seafloor was recorded repeatedly over 3 years. We hypothesize that the well developed active gas system shown here for the first time is not unique to the continental shelf off northern Israel. Conversely, it represents the usually underestimated marine methane contribution from mid-latitude continental shelves into the marine environment and maybe into the atmosphere. This study hypothesizes that global warming and sea level rise induces a negative, restraining feedback for gas emission across mid-latitude continental margins and thus being inverse to high latitudes, where this feedback is most likely positive due to the stronger impact of bottom water warming.