Evidence of methane release from Blake Ridge ODP Hole 997A during the Plio-Pleistocene: benthic foraminifer fauna and total organic carbon

Abstract

Methane is a powerful greenhouse gas and may have played a significant role in global climate change in the geological past. Destabilization of gas hydrates, frozen methane stored within the ocean floor sediment and in permafrost, may have provided an important source of methane to the atmosphere. Ocean Drilling Program Hole 997A (water depth 2770 m), situated on the crest of the Blake Outer Ridge, is a potentially large reservoir of gas hydrate. Methane emissions from the Blake Outer Ridge have been reported previously, which has been suggested as a driver for global climate change. Methane at this site is of biogenic origin, produced by the bacterial decomposition of organic matter. We used benthic foraminifer faunal assemblages (>125 μm size fraction) and species diversity, combined with total organic carbon data from Hole 997A, to identify intervals of methane releases during the late Neogene (last 5.4 Ma). We identified a group of benthic foraminifera, which were taken to indicate methane fluxes based on previous work on seep-related benthic foraminifera. We then classified 'seep-related' benthic foraminifera, as well as high organic carbon taxa independent of deep-sea oxygenation. We recognized five intervals of increased abundance of the seep-related benthic foraminifera since last 3.6 Ma representing intervals of methane release, which coincide with intervals of lowered sea level. Changes in benthic foraminifera are more abrupt over the past 3.6 Ma when the northern hemisphere glaciation began to intensify and climate switched to a 41-kyr cycle world.