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doi:10.2204/iodp.pr.329.2011

Expedition synthesis

Sediment

Expedition 329 sites are located along two transects, hinged in the center of the South Pacific Gyre (Figs. F1, F2). 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).

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) (Figs. F2, F8). 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, although metalliferous zeolitic clay dominates the lowest portion of this sediment column.

The dominant lithology shifts from clay to carbonate ooze at depth in two of the sites (Fig. F8). At Site U1367, the transition from clay to carbonate at 6–7 mbsf marks the time that the site subsided beneath the CCD as the underlying basement cooled with age. At Site U1370, carbonate ooze is the dominant lithology for a short interval deposited during planktonic foraminiferal Zone P1. This foraminifer-bearing interval is most simply interpreted as resulting from the CCD diving to greater water depth than the water depth of this site during the early Paleocene interval of low planktonic carbonate production.

Sediment thickness is generally very low throughout the gyre (Fig. F8). When sites of broadly similar age are compared (Site U1366, 84–120 Ma; Site U1369, 58 Ma; Site U1370; 74–80 Ma; and Site U1371, 71.5–73 Ma), thickness of the sediment column generally increases with increasing distance from the gyre center (Figs. F1, F2, F8).

Sedimentary microbial communities and habitability

Throughout the South Pacific Gyre (Sites U1365–U1370), dissolved oxygen and dissolved nitrate are present throughout the entire sediment column (Fig. F9), indicating that microbial respiration is oxic throughout the column, as predicted by D’Hondt et al. (2009) and Fischer et al. (2009). The 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 (Fig. F1), detectable dissolved oxygen and dissolved nitrate are limited to just below the sediment/water interface and just above the sediment/basalt interface. Between these interfaces, the sediment is anoxic. Very high concentrations of dissolved (presumably reduced) manganese indicate that manganese reduction is a prominent electron-accepting process throughout most of this sediment column, with perhaps very short intervals of iron reduction suggested by minor peaks in dissolved iron concentration associated with local minima in dissolved manganese concentration. The rapid drop of dissolved oxygen and nitrate below their detection limits at the upper and lower edges of this sediment column and the relatively high concentrations of dissolved phosphate within this column suggest that the subseafloor rate of microbial respiration is much higher at this site than at the sites located in the gyre.

Geographic variation in subseafloor profiles of dissolved oxygen, dissolved nitrate, dissolved phosphate, dissolved inorganic carbon (DIC), total solid-phase organic carbon (TOC), and total solid-phase nitrogen (TN) are consistent with the magnitude of organic-fueled subseafloor respiration declining from outside the gyre (Site U1371) to gyre margins (Sites U1365 and U1370) to gyre center (Site U1368) (Figs. F1, F9, F10).

At all sites located within the gyre (Sites U1365–U1370), microbial cell counts are three or more orders of magnitude lower than at the same sediment depths in all sites previously cored by scientific ocean drilling (Fig. F11). Microbial cell counts are generally higher at Site U1371 than at the sites within the gyre (Sites U1365–U1370) but are lower than at all other sites previously drilled.

At all of the sites in the gyre, TOC and TN decline rapidly with depth in the upper sediment column and are generally constant at greater depth (Fig. F10). In contrast, at Site U1371 TOC and TN are generally much higher than at the other sites at all depths.

Countable cells disappear within the upper sediment column at every site in the gyre. Dissolved oxygen content, dissolved nitrate concentration, TOC, and TN stabilize as countable cells disappear. The downhole disappearance of countable cells and measurable oxygen reduction appears to result from the disappearance of organic electron donors.

Dissolved oxygen, dissolved nitrate, dissolved phosphate and DIC are present throughout the entire sediment column at all sites in the gyre (Sites U1365–U1370) (Fig. F9), indicating that microbial life is not limited by availability of electron acceptors or major nutrients (carbon, nitrogen, and phosphorus) in this sedimentary environment. At Site U1371, dissolved oxygen is absent from most of the sediment column, but the dissolved manganese concentration profile suggests that manganese reduction occurs through most of the sequence. Dissolved sulfate, dissolved nitrate, dissolved phosphate, and DIC are present throughout the entire sediment column at Site U1371, indicating that microbial life is not limited by availability of electron acceptors or major nutrients in this sedimentary environment either.

Dissolved hydrogen concentration is below detection in the upper sediment column of all sites within the gyre (Fig. F10). 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. In the deepest sediment at that site, the apparent coexistence of dissolved oxygen, dissolved manganese, and dissolved hydrogen suggests that microbial activity at that depth is insufficient to fully remove either oxygen or the electron donors, manganese and hydrogen.

The sulfate anomaly profile and the occurrence of disseminated pyrite in the upper 10 m of sediment at Site U1371 suggest that sulfate and manganese reduction co-occur within the column.

Paleoceanography

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

The uppermost basaltic basement at Site U1365 is composed of lava flows, whereas the uppermost basement at Sites U1367 and U1368 is primarily composed of pillow basalt.

Basalt alteration

Alteration in lava flow units, as evident at Site U1365, appears to be strongly controlled by lithologic structure, with most alteration focused at the flow boundaries (Fig. F14). In contrast, alteration in pillow lava units (the dominant lithologies at Sites U1367 and U1368) appears to be more evenly distributed.

At all sites, the presence of dissolved oxygen in the lowermost sediment at below–deep water concentrations (Fig. F9) 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 oldest basement, alteration of the basement basalt continues on the timescale of formation fluid replacement. Natural gamma radiation (NGR) core logs, downhole NGR logs (Site U1368), and chemical analyses of the rock demonstrate that potassium has been consistently taken up during basalt alteration at all three sites where the basaltic basement was drilled (Sites U1365, U1367, and U1368) (Fig. F13). At the sites with deepest sediment (Site U1365, where basalt was drilled, plus Sites U1370 and U1371, where basalt was not drilled), dissolved potassium concentrations are noticeably lower in the deepest sediment than in the shallow sediment (Fig. F14), indicating 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, respectively).

At all three sites where basement was rotary cored (Sites U1365, U1367, and U1368), secondary minerals provide evidence of both oxidative alteration (iron oxyhydroxide and celadonite) and oxygen-poor alteration (saponite and secondary sulfides). Some samples have undergone multiple stages of alteration. Late vein infills suggest that alteration may be continuous or at least occur intermittently throughout the life of the ocean crust. At Site U1365, the presence in the lowermost sediment of dissolved Mg at below–deep water concentrations and dissolved Ca at above–deep water concentrations indicates that basalt-water interaction in the form of Mg exchange for Ca has occurred since seawater migrated into the formation. This exchange may continue to drive late-stage calcite precipitation, despite the great age of basement at this site (80–120 Ma).

Habitability of basaltic basement

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 (Fig. F9), it is not limited by access to electron acceptors (oxygen and nitrate) or major nutrients (carbon, nitrogen, and phosphorus).

Past microbial activity?

Tubelike micro-scale weathering features occur in altered glass from Site U1365 (Fig. F12). They are arranged in discrete clusters or in masses adjacent to or near fractures and iron oxyhydroxide within the glass. Similar features have been observed in marine basaltic glass elsewhere and attributed to microbial origin (e.g., Fisk et al, 1998).

Technical advances

Expedition 329 used a wide range of instruments and techniques that have not often been used on scientific ocean drilling expeditions. Details of their Expedition 329 application are provided in the "Methods" chapter of the Expedition Report, particularly in the "Biogeochemistry," "Microbiology," and "Physical properties" sections.

Two technical approaches were used by the Expedition 329 Scientific Party on an experimental basis, with the intention of refining them heavily for future application. The first of these techniques was flow cytometric cell counting. The second was use of NGR core logging for shipboard quantification of absolute concentrations of ²³⁸U-series elements, ²³²Th-series elements, and potassium. Because these techniques were used on an experimental basis, they are described in dedicated chapters of the Expedition Report.

Preliminary scientific assessment

Expedition 329 had the following four fundamental objectives:

1. To document the habitats, metabolic activities, genetic composition and biomass of microbial communities in subseafloor sediment with very low total activity.

2. To test how oceanographic factors (such as surface ocean productivity, sedimentation rate, and distance from shore) control variation in sedimentary habitats, activities and communities from gyre center to gyre margin.

3. To quantify the extent to which subseafloor microbial communities of this region may be supplied with electron donors (food) by water radiolysis, a process independent of the surface photosynthetic world.

4. To determine how basement habitats, potential activities and, if measurable, microbial communities vary with crust age and hydrologic regime (from ridge crest to abyssal plain).

The expedition made major strides toward fulfilling all of these objectives.

Shipboard biogeochemistry, lithostratigraphy, microbiology, and physical property studies documented many fundamental aspects of subseafloor sedimentary habitats, metabolic activities, and biomass in this very low activity sedimentary ecosystem. Documentation of genetic composition and additional aspects of sedimentary habitability and biomass must await shore-based study.

Shipboard biogeochemical and cell-enumeration results have also significantly improved understanding of how oceanographic factors control variation in subseafloor sedimentary habitats, activities, and biomass from gyre center to gyre margin. Postexpedition studies are necessary to improve understanding of the underlying mechanisms and to test how oceanographic factors control variation in community composition.

Shipboard studies have quantified the availability of dissolved hydrogen throughout the sediment column and taken the samples necessary to quantify in situ rates of radiolytic hydrogen production. Postexpedition analyses of these samples are required to quantify these rates and their distribution throughout the sediment column of each site.

Shipboard biogeochemical, petrological, and physical property data document first-order patterns of basement habitability and potential microbial activities. A broad range of postexpedition studies will be necessary to further constrain habitability and to test how microbial community structure varies with basement age, water-rock interactions, and hydrologic regime.

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