Occurrence of methane in the near-surface waters of the western subtropical North-Atlantic

Abstract

Abstract Methane observations from the western subtropical North Atlantic show maxima at about 100 m in a zone of sharp density contrast immediately above the regional salinity maximum. Open ocean surface waters have methane concentrations 48 to 67% higher than predicted from equilibrium with the atmosphere, while concentrations at the maxima reach 1.9 to 2.5 times the equilibrium solubility. A physical model shows that transport processes cannot supply sufficient methane from nearshore reducing environments to account for the observed mixed layer excesses in offshore waters. Instead, several observations suggest that in situ biological production in the oxygenated water column is important in determining the methane distribution. All known methane bacteria are obligate and strict anaerobes; thus some new mode of methane production must be postulated.