Oxygen penetration deep into the sediment of the South Pacific gyre

Abstract

Abstract. Benthic microbial oxygen consumption rates were investigated during an IODP site survey to the South Pacific Gyre. Primary production, particle fluxes and sedimentation rates are extraordinarily low in this most oligotrophic oceanic region on earth. We studied benthic microbial respiration rates from vertical oxygen profiles in sediments obtained on different spatial scales ex situ (in piston cores and multi cores), and in situ (using a benthic lander with a microelectrode profiler). Along a transect in the area 24 to 46° S and 165 to 117° W, cores at 10 of 11 sites were oxygenated for their entire lengths (as much as 8 m below seafloor), at concentrations >150 μmol L−1 O2. This represents the deepest oxygen penetration ever measured in marine sediments. Microprofiles from the top sediment layer revealed a diffusive oxygen flux to the sediment in the order of 0.2 mmol m−2 d−1. This is in the lower range of previously reported fluxes for oligotrophic sediments but corresponds well to the low surface water primary production. Because of the inert nature of the deeper sediment, oxygen that is not consumed in the surface layer diffuses downward to much greater depth. In deeper zones, a small O2 flux of ~0.1 μmol m−2 d−1 was therefore still present. This flux was constant with depth, indicating extremely low respiration rates. Modeling of the measured oxygen profiles suggests that the sediment is probably oxygenated down to the basalt, indicating an oxygen flux from the sediment into the basaltic basement.