Evidence for large-scale methane venting due to rapid drawdown of sea level during the Messinian Salinity Crisis

Abstract

We report the discovery, from three-dimensional seismic mapping, of a field of pockmarks buried at present depths of ∼4 km in the Levant Basin (southeastern Mediterranean). The pockmarks cover an area of ∼1000 km 2 , have diameters as great as 2 km, and erode as much as 200 m into their substrate of deep-water clastic sediments, which immediately predate the Messinian Salinity Crisis (MSC, ca. 6.2–5.5 Ma). These craters are filled by the basal units of the Messinian evaporites, thus implying they formed at or close to the time of the earliest major drawdown of sea level. All the pockmarks are developed at a single, regionally correlatable surface, Horizon N, which is observed throughout the Mediterranean to coincide with the onset of the MSC. In the study area, this surface formed by erosion and drawdown with a magnitude of 900–1000 m. We propose that this rapid basinal drawdown led to a dramatic increase of the shallow subsurface pore-fluid overpressure regime in the mostly fine-grained pre-Messinian sediments. Subsequent pressure release then resulted in high flux fluid venting, sediment remobilization, and pockmark formation. The spatial association of the craters with an underlying canyon suggests that their formation is linked to biogenic methane generation. The model of drawdown-induced overpressuring and remobilization may be applicable to many other evaporitic basins, and represents a novel mechanism for inducing large-scale sediment remobilization of shallow buried depositional systems at the earliest stages of salt basin development.