Microbial Activity in the Subseafloor Sediments of ODP Leg 201

Ocean Drilling Program Leg 201 was the first ocean drilling expedition dedicated to the study of life deep beneath the seafloor. Its equatorial Pacific and Peru Margin sites were selected to represent the general range of subsurface environments that exists in marine sediments throughout most of the world's oceans. In water depths as great as 5300 meters and as shallow as 150 meters, the expedition drilled up to 420 meters into oceanic sediments and the underlying basaltic crust. The sediments ranged in temperature from 1° C to 25° C and in age from 0 to almost 40 million years. Leg 201 scientists found biogeochemical evidence of metabolic activity throughout the sediment column at every site. This activity is supported at all sites by the diffusion of sulfate down from the overlying ocean, as well as by the dissolution of iron- and manganese-bearing minerals. At the open ocean sites, metabolic activity deep beneath the seafloor is also supported by the transport of sulfate, nitrate and oxygen from water circulating through the underlying basaltic crust. At both the open ocean sites and the Peru margin sites, multiple metabolic activities (sulfate reduction, iron reduction, manganese reduction and methanogenesis) co-occur. Chemical flux estimates suggest that sulfate is generally the principal terminal electron acceptor in these subseafloor sediments. However, at the open ocean sites with the lowest rates of microbial respiration, manganese may rival sulfate as a terminal electron acceptor. The recovered sediments and fluids are being studied further to document the controls on rates of subsurface activity, the influence of past oceanographic conditions on current activity in deeply buried sediments, and the effects of subseafloor biogeochemical processes on Earth's surface world.