Ocean Drilling Program Leg 201 was the first ocean drilling expedition dedicated to the study of life deep beneath the seafloor. Its seven sites were selected to represent the general range of subsurface environments that exist in marine sediments throughout most of the world's oceans. In water depths as great as 5300 m and as shallow as 150 m, the expedition drilled as deep as 420 m into oceanic sediments and the underlying rocky crust. The sediments ranged in temperature from 1° to 25°C and in age from 0 to almost 40 Ma.
To document metabolic interactions in deeply buried marine sediments and the differences between those interactions in ocean-margin sediments and in open-ocean sediments, shipboard scientists measured an unprecedented array of metabolic reactants and products in the interstitial waters of Leg 201 sediments. To document subsurface communities, shipboard scientists initiated an unprecedented number and range of microbial experiments. To document the nature of subsurface environments, shipboard scientists documented an exhaustive range of core and downhole geophysical and sedimentological properties.
Active prokaryotic respiration occurs throughout the sediment column at every site. Subseafloor respiration is supported at all sites by the diffusion of sulfate from the overlying ocean, as well as by the dissolution of iron- and manganese-bearing minerals. At the open Pacific sites, respiration deep beneath the seafloor is also supported by the transport of sulfate, nitrate, and dissolved oxygen from water circulating through the underlying basaltic crust. At both the open Pacific sites and the Peru margin sites, methanogenesis consistently co-occurs with electron-accepting processes that are often assumed to outcompete it, including sulfate reduction, iron reduction, and manganese reduction. In all of the sedimentary environments sampled during Leg 201, sedimentary properties and, by inference, past oceanographic conditions affect current rates of prokaryotic activities.
Prokaryotic abundances are much higher in sediments buried on the continental shelf of Peru than in sediments of the open Pacific Ocean. The open Pacific sites contained some of the lowest average prokaryote concentrations ever observed in deep-sea sediments. In contrast, some of the sediments recovered on the Peru shelf contained the highest concentrations of prokaryotes ever observed beneath the seafloor. At the Peru shelf sites, the concentration of sedimentary prokaryotes was highest in a narrowly focused zone of anaerobic methanotrophy tens of meters beneath the seafloor.
The recovered sediments and fluids will be studied further to document the composition of subseafloor prokaryotic communities, the principal 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.
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
Ms 201IR-101