University of Aarhus, Denmark

Sediment oxygen concentration profiles and benthic microbial oxygen consumption rates were investigated during an IODP site survey in the South Pacific Gyre. Primary production, particle fluxes and sedimentation rates are extremely low in this ultra-oligotrophic oceanic region. We derived O (sub 2) consumption 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 equipped with a microelectrode profiler). Along a transect in the area 24 to 46 degrees S and 165 to 117 degrees W, cores from 10 out of 11 sites were oxygenated over their entire length (as much as 8 m below seafloor), with deep O (sub 2) concentrations >150 mu mol L (super -1) . This represents the deepest oxygen penetration ever measured in marine sediments. High-resolution microprofiles from the surface sediment layer revealed a diffusive oxygen uptake between 0.1 and 1.3 mmol m (super -2) d (super -1) , equal to a carbon mineralization rate of approximately 0.4-4.5 gC m (super -2) yr (super -1) . This is in the lower range of previously reported fluxes for oligotrophic sediments but corresponds well to the low surface water primary production. Half of the pool of reactive organic matter was consumed in the top 1.5-6 mm of the sediment. Because of the inert nature of the deeper sediment, oxygen that is not consumed within the top centimeters diffuses downward to much greater depth. In deeper zones, a small O (sub 2) flux between 0.05 and 0.3 mu mol m (super -2) d (super -1) was still present. This flux was nearly constant with depth, indicating extremely low O (sub 2) consumption rates. Modeling of the oxygen profiles suggests that the sediment is probably oxygenated down to the basalt, suggesting an oxygen flux from the sediment into the basaltic basement.