First geological survey and characterisation of a giant depression in carbonate strata at the Rio Grande Rise (southwestern Atlantic)

Pockmarks are submarine depressions often considered the result of fluid (gas and oil) expulsion from the seafloor. Recent geophysical surveys at the Rio Grande Rise (RGR - SW Atlantic) revealed extensive presence of seafloor depressions, generically interpreted as pockmarks. To date, direct observations or sampling from these depressions are lacking. Moreover, hydrocarbons have never been detected at the site, questioning the possibility of pockmarks formation by fluid expulsion. Here we present the first visual inspection and rock characterisation of a giant circular depression located inside the RGR central graben. The depression was discovered during multibeam prospection and surveyed with a Remotely Operated Vehicle, with rock sampling along a vertical transect. The depression is conical (350 m wide and 80 m deep) with walls of stratified, sub-horizontal, white fossiliferous wackestone, with incipient gravitational collapse. Lithological, mineralogical, and geochemical characterisation of the wackestone rule out methane in the formation of the depression. Based on geochemical data and on the finding of anhydrite III (forming at 110–300 °C), we propose a novel perspective on the formation of this kind of depressions, invoking hypogenic karstification and collapse, due to hydrothermal fluids along deep faults. Our geochemical approach complements previous geophysical observations of circular depressions at the RGR and contribute to the general understanding of the formation mechanisms of these intriguing seafloor features.