Evidence for methane seepage in South Atlantic from the occurrence of authigenic gypsum and framboidal pyrite in deep-sea sediments

Abstract

An association of authigenic pyrite and gypsum has been found in deep-sea sediments from a core retrieved on the continental slope of the Santos Basin, South Atlantic. Pyrite occur mainly as tubular concretions formed by aggregates of framboids while gypsum show a rosette and acicular microcrystals habit. The concomitant presence of these minerals indicates the occurrence of biogeochemical reactions in marine environments that can be associated with methane seepage environments. Sulphur isotopic composition of authigenic minerals, magnetic susceptibility, and redox sensitive trace elements concentrations in bulk sediments are also used to characterize redox conditions and delimit a sulphate-methane transition zone (SMTZ) in this site. In this region, a pockmark field with methane release activity has already been recognized in sediments deposited during the Last Glacial Maximum. The conditions for the formation of these minerals, as well as their crystal habits, indicate the effect of biogeochemical processes consistent with methane seep environments. Therefore, our data provides new evidence about the occurrence of a past methane activity in this area.