AbstractIntegrated Ocean Drilling Program (IODP) Expedition 336 successfully initiated subseafloor observatory science at a young mid-ocean-ridge flank setting. All of the drilled sites are located in the North Pond region of the Atlantic Ocean (22°45′N, 46°05′W) in 4414–4483 m water depth. This area is known from previous ocean drilling and site survey investigations as a site of particularly vigorous circulation of seawater in permeable 8 Ma basaltic basement underlying a <300 m thick sedimentary pile. Understanding how this seawater circulation affects microbial and geochemical processes in the uppermost basement was the primary science objective of Expedition 336. Basement was cored and wireline-logged in Holes U1382A and U1383C. Upper oceanic crust in Hole U1382A, which is only 50 m west of Deep Sea Drilling Project (DSDP) Hole 395A, recovered 32 m of core between 110 and 210 meters below seafloor (mbsf). Core recovery in basement was 32%, yielding a number of volcanic flow units with distinct geochemical and petrographic characteristics. A unit of sedimentary breccia containing clasts of basalt, gabbroic rocks, and mantle peridotite was found intercalated between two volcanic flow units and was interpreted as a rock slide deposit. From Hole U1383C we recovered 50.3 m of core between 69.5 and 331.5 mbsf (19%). The basalts are aphyric to highly plagioclase-olivine-phyric tholeiites that fall on a liquid line of descent controlled by olivine fractionation. They are fresh to moderately altered, with clay minerals (saponite, nontronite, and celadonite), Fe oxyhydroxide, carbonate, and zeolite as secondary phases replacing glass and olivine to variable extents. In addition to traditional downhole logs, we also used a new logging tool for detecting in situ microbial life in ocean floor boreholes—the Deep Exploration Biosphere Investigative tool (DEBI-t). Sediment thickness was ~90 m at Sites U1382 and U1384 and varied between 38 and 53 m at Site U1383. The sediments are predominantly nannofossil ooze with layers of coarse foraminiferal sand and occasional pebble-size clasts of basalt, serpentinite, gabbroic rocks, and bivalve debris. The bottommost meters of sections cored with the advanced piston corer feature brown clay. Extended core barrel coring at the sediment/basement interface recovered <1 m of brecciated basalt with micritic limestone. Sediments were intensely sampled for geochemical pore water analyses and microbiological work. In addition, high-resolution measurements of dissolved oxygen concentration were performed on the whole-round sediment cores. Major strides in ridge-flank studies have been made with subseafloor borehole observatories (CORKs) because they facilitate combined hydrological, geochemical, and microbiological studies and controlled experimentation in the subseafloor. During Expedition 336, two fully functional observatories were installed in two newly drilled holes (U1382A and U1383C) and an instrument and sampling string were placed in an existing hole (395A). Although the CORK wellhead in Hole 395A broke off and Hole U1383B was abandoned after a bit failure, these holes and installations are intended for future observatory science targets. The CORK observatory in Hole U1382A has a packer seal in the bottom of the casing and monitors/samples a single zone in uppermost oceanic crust extending from 90 to 210 mbsf. Hole U1383C was equipped with a three-level CORK observatory that spans a zone of thin basalt flows with intercalated limestone (~70–146 mbsf), a zone of glassy, thin basaltic flows and hyaloclastites (146–200 mbsf), and a lowermost zone (~200–331.5 mbsf) of more massive pillow flows with occasional hyaloclastites in the upper part.
AbstractIntegrated 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 lifeThe 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.