Ocean Drilling Program, United States

Bacterial populations and pore water acetate concentrations were quantified at two sites in the southeast Atlantic sector of the Southern Ocean during Ocean Drilling Program (ODP) Leg 177. Bacterial abundances in the carbonate-rich, low organic carbon Sites 1088 and 1093 were lower than the general trend for bacteria in deep marine sediments. Site 1088, which is approximately 10 degrees closer to the equator and only approximately half the water depth of Site 1093, had the lowest bacterial populations. Calcium carbonate was approximately 10 times more abundant in Site 1088 sediments (average = 88.2 wt%) compared to Site 1093 (average = 9.2 wt%). Thus, neither latitude nor water depth is as significant as organic carbon in controlling bacterial distributions. Including data for carbonate-rich sediments to a previous model, the global bacterial biomass in marine sediments extrapolates to approximately 10.2% of living carbon in the surface biosphere. Pore water acetate concentrations at both sites were generally low (0-15 mu M). At Site 1093 many acetate peaks (to 110 mu M) were present, probably because of the presence of localized diatom-rich laminae in the sediments. The low acetate concentrations are in marked contrast with data from gas hydrate sediments at Blake Ridge (ODP Leg 164), where concentrations exceeded 15,000 mu M by approximately 700 m below seafloor, suggesting that high acetate concentrations may be a characteristic of gas hydrate sediments and not the general situation in deep marine sediments.