Integrated Ocean Drilling Program Expedition 329 made major strides toward fulfilling its objectives. Shipboard studies (1) documented many fundamental aspects of subseafloor sedimentary habitats, metabolic activities, and biomass in this very low-activity sedimentary ecosystem; (2) significantly improved understanding of how oceanographic factors control variation in subseafloor sedimentary habitats, activities, and biomass from gyre center to gyre margin; (3) quantified the availability of dissolved hydrogen throughout the sediment column; and (4) document first-order patterns of basement habitability and potential microbial activities. A broad range of postexpedition studies will complete the expedition objectives.
Expedition 329 sites are located along two transects, hinged in the center of the South Pacific Gyre. The first transect progresses from the western edge of the gyre (Site U1365) to the gyre center (Site U1368). The second transect goes from the gyre center (Site U1368) through the southern gyre edge (Site U1370) to the northern edge of the upwelling region south of the gyre (Site U1371).
Subseafloor sedimentary habitability and life
The dominant lithology is zeolitic metalliferous clay at the deeper water sites on older basement (58 to ≤120 Ma) within the gyre (Sites U1365, U1366, U1369, and U1370). Manganese nodules occur at the seafloor and intermittently within the upper sediment column at these sites. Chert and porcellanite layers are pronounced in the lower half of the sediment column at Sites U1365 and U1366. The dominant lithology is carbonate ooze at Site U1368, the site on youngest basement (13.5 Ma) and, consequently, in the shallowest water. At Site U1371, which lies on relatively old basaltic basement (71.5–73 Ma) just south of the gyre, the dominant lithology is siliceous ooze, with metalliferous zeolitic clay at the base of the sediment column.
Throughout the South Pacific Gyre (Sites U1365–U1370), dissolved oxygen and dissolved nitrate are present throughout the entire sediment column. Concentration profiles of oxygen and nitrate demonstrate subseafloor O2 loss and NO3– production and indicate that the subseafloor rate of microbial respiration is generally extremely low. In contrast, at Site U1371 in the upwelling zone just south of the gyre, detectable dissolved oxygen and dissolved nitrate are limited to just below the sediment/water interface and just above the sediment/basalt interface. Manganese reduction is a prominent electron-accepting process throughout most of this sediment column.
Geographic variation in subseafloor profiles of dissolved oxygen, dissolved nitrate, dissolved phosphate, dissolved inorganic carbon (DIC), solid-phase total organic carbon (TOC), and solid-phase total nitrogen (TN) are consistent with the magnitude of organic-fueled subseafloor respiration declining from outside the gyre to the gyre center.
At all sites located within the gyre, microbial cell counts are three or more orders of magnitude lower than at the same sediment depths at all sites previously cored by scientific ocean drilling. Microbial cell counts are generally higher at the site outside the gyre (Site U1371) than at the sites within the gyre but are lower than at all other sites previously drilled. Countable cells disappear within the upper sediment column at every site in the gyre (Sites U1365–U1370). Dissolved oxygen content, dissolved nitrate concentration, TOC, and TN also decrease with depth and then stabilize as countable cells disappear. The downhole disappearance of countable cells and measurable organic oxidation appears to result from the disappearance of organic electron donors.
Dissolved electron acceptors (oxygen at Sites U1365–U1370 and sulfate at Site U1371), dissolved nitrate, dissolved phosphate, and DIC are present throughout the entire sediment column at all sites in the gyre, indicating that microbial life is not limited by availability of electron acceptors or major nutrients (carbon, nitrogen, and phosphorus) in these sedimentary environments.
Dissolved hydrogen concentration is below detection in the upper sediment column of all sites within the gyre. At most sites, it rises above detection with increasing depth. Because dissolved H2 is continually produced by in situ water radiolysis, the presence of dissolved H2 in many samples suggests that hydrogen-utilizing microbial activity is impaired or absent at sample depths where H2 concentration is detectable and oxygen is present. At Site U1371, which is anoxic throughout most of the sediment column, dissolved hydrogen concentration is low but above detection through much of the column, with slightly higher values at the base of the column.
High-resolution measurements of dissolved chloride and nitrate concentrations, as well as formation factor, provide the opportunity for reconstruction of glacial seawater characteristics through the South Pacific Gyre. Given the importance of this region in terms of ocean circulation, such reconstruction will greatly contribute to understanding of the global ocean-climate system.
Basalt alteration and habitability
At the sites with oldest basement, alteration of the basement basalt continues on the timescale of formation fluid replacement. At all sites, the presence of dissolved oxygen in the lowermost sediment at below-deepwater concentrations suggests that either (1) basement oxidation has occurred since seawater migrated into the formation or (2) oxygen has been lost to the overlying sediment along the flow path. At the sites with deepest sediment (Sites U1365, U1370, and U1371), dissolved potassium profiles indicate that (1) dissolved potassium fluxes into the underlying basalt and (2) basalt alteration continues despite the great age of basement at all three sites (84–120, 74–79.5, and 71.5–73 Ma, respectively). Profiles of dissolved oxygen, DIC, dissolved nitrate, and dissolved phosphate in the lowermost sediment at each site indicate that if microbial life is present in the uppermost basalt, it is not limited by access to electron acceptors (oxygen and nitrate) or major nutrients (carbon, nitrogen, and phosphorus).