The depth of oxygen penetration into marine sediments differs considerably from one region to another. In areas with high rates of microbial respiration, O2 penetrates only millimetres to centimetres into the sediments, but active anaerobic microbial communities are present in sediments hundreds of metres or more below the sea floor. In areas with low sedimentary respiration, O2 penetrates much deeper but the depth to which microbial communities persist was previously unknown. The sediments underlying the South Pacific Gyre exhibit extremely low areal rates of respiration. Here we show that, in this region, microbial cells and aerobic respiration persist through the entire sediment sequence to depths of at least 75 metres below sea floor. Based on the Redfield stoichiometry of dissolved O2 and nitrate, we suggest that net aerobic respiration in these sediments is coupled to oxidation of marine organic matter. We identify a relationship of O2 penetration depth to sedimentation rate and sediment thickness. Extrapolating this relationship, we suggest that oxygen and aerobic communities may occur throughout the entire sediment sequence in 15–44% of the Pacific and 9–37% of the global sea floor. Subduction of the sediment and basalt from these regions is a source of oxidized material to the mantle.
Integrated Ocean Drilling Program (IODP) 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 habitability within the igneous basement. A broad range of postexpedition studies will complete the expedition objectives. Throughout the South Pacific Gyre (SPG; Sites U1365–U1370), dissolved oxygen and nitrate are present throughout the entire sediment sequence, and sedimentary microbial cell counts are lower than at all previously drilled IODP/ Ocean Drilling Program (ODP)/Deep Sea Drilling Program (DSDP) sites. 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, manganese reduction is a prominent electron-accepting process, and cell concentrations are higher than at the same depths in the SPG sites throughout the sediment column. 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.
Chemical profiles in the sedimentary pore water and secondary mineral distributions in the basaltic basement indicate that basement alteration continues on the timescale of formation fluid replacement, even at the sites with the oldest basement (84–120 Ma at Sites U1365 and U1366).
The Integrated Ocean Drilling Program site U1365 drilled into the basement of the southwest Pacific crust formed from the mid-Cretaceous Osbourn Trough that rifted apart the Manihiki and Hikurangi Plateaus (the Greater Manihiki). The basalt geochemistry at this site is crucial for understanding the magmatic processes and mantle source of the mid-Cretaceous Osbourn Trough. The recovered fresh basalts were low-K tholeiitic normal (N) and depleted (D) mid-ocean ridge basalt (MORB). Their trace element and Sr–Nd isotope compositions indicate a Pacific-type mantle source rather than any significant influences from the nearby Louisville Seamount Chain or from the Greater Manihiki Plateau. Despite the presence of a plume head underneath the Osbourn Trough at its initial stage, the insignificance of a plume head could be explained by the long-distance (> 1000 km) southward migration of the Osbourn Trough. Lavas at site U1365 vary from low-MgO (< 6.9 wt.%) N-MORB at the bottom to high-MgO (8 wt.% to 9.5 wt.%) D-MORB and, then, to medium-MgO (7.3 wt.% to 8.2 wt.%) N-MORB according to their eruption sequences, which was accompanied by magma mixing in the magma reservoir. The D-MORB group lavas have higher melting degrees than those of N-MORB group based on their concentrations of TiO2, Na2O and CaO corrected for crystallization relative to MgO = 7.8 wt.%. The major element compositions of the high-MgO D-MORB lavas were consistent with partial melting in the spinel–peridotite zone over a pressure interval from ~ 3.1 GPa to 2 GPa in the mantle. The significant overlap of N-MORB and D-MORB in Sr–Nd isotopes suggests that chemical differences between the two groups were derived from the mantle melting processes. Based on comparison with lavas from the East Pacific Rise where a positive correlation of mantle melting degree vs. spreading rate is shown, we suggest that the Osbourn Trough might have a full spreading rate of ~ 140 mm/yr. Thus, the slow ridge-like axial morphology of the Osbourn Trough should be a character of an extinct fast ridge.
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).