Integrated Ocean Drilling Program Expedition 329 made major strides toward fulfilling its objectives. Shipboard studies documented (1) fundamental aspects of habitability and life in this very low activity subseafloor sedimentary ecosystem and (2) first-order patterns of basement habitability. A broad range of postexpedition studies will complete the expedition objectives.
Throughout the South Pacific Gyre (Sites U1365–U1370), dissolved oxygen and nitrate are present throughout the entire sediment sequence. Concentration profiles of oxygen and nitrate indicate that subseafloor heterotrophic respiration is oxic and proceeds very slowly. In contrast, at Site U1371 in the upwelling zone just south of the gyre, detectable oxygen and nitrate are limited to the top and bottom of the sediment column and manganese reduction is a prominent electron-accepting process.
Geographic variation in subseafloor profiles of dissolved and solid-phase chemicals are consistent with the magnitude of organic-fueled subseafloor respiration declining from outside the gyre to the gyre center.
Microbial cell counts are lower than at all sites previously drilled. Countable cells disappear with increasing depth in the sediment at every site in the South Pacific Gyre (Sites U1365–U1370). Concentrations of dissolved oxygen and nitrate, total organic carbon, and total nitrogen stabilize as countable cells fall below the minimum detection limit. The downhole disappearance of cells and measurable organic oxidation appears to result from the disappearance of organic electron donors.
At the South Pacific Gyre sites, dissolved hydrogen concentration is low but often above detection in deep sediment. At Site U1371, where most of the sediment is anoxic, dissolved hydrogen concentration is above detection through much of the column.
High-resolution chemical and physical measurements provide the opportunity for reconstructing glacial seawater characteristics through the South Pacific Gyre. Such reconstruction will greatly contribute to understanding the global ocean-climate system.
Dissolved chemical profiles and igneous petrology indicate that basement alteration continues on the timescale of formation fluid replacement, even at the sites with oldest basement (84–120 Ma at Sites U1365 and U1366). Profiles of dissolved chemicals indicate that microbial habitability of the entire sediment sequence and the uppermost basalt is not limited by access to electron acceptors (oxygen and nitrate) or major nutrients (carbon, nitrogen, and phosphorus).