The low-productivity South Pacific Gyre (SPG) is Earth's largest oceanic province. Its sediment accumulates extraordinarily slowly (0.1-1 m per million years). This sediment contains a living community that is characterized by very low biomass and very low metabolic activity. At every depth in cored SPG sediment, mean cell abundances are 3 to 4 orders of magnitude lower than at the same depths in all previously explored subseafloor communities. The net rate of respiration by the subseafloor sedimentary community at each SPG site is 1 to 3 orders of magnitude lower than the rates at previously explored sites. Because of the low respiration rates and the thinness of the sediment, interstitial waters are oxic throughout the sediment column in most of this region. Consequently, the sedimentary community of the SPG is predominantly aerobic, unlike previously explored subseafloor communities. Generation of H 2 by radiolysis of water is a significant electron-donor source for this community. The per-cell respiration rates of this community are about 2 orders of magnitude higher (in oxidation/reduction equivalents) than in previously explored anaerobic subseafloor communities. Respiration rates and cell concentrations in subseafloor sediment throughout almost half of the world ocean may approach those in SPG sediment.aerobic ͉ biomass ͉ oxic ͉ radiolysis ͉ respiration

Knowledge of salinity in the deep ocean is important for understanding past ocean circulation and climate. Based on sedimentary pore fluid chloride measurements of a single Pacific site, Adkins et al. (2002) suggested that, during the Last Glacial Maximum (LGM), the Pacific deep bottom water was saltier than expected based on lower sea level alone. Here we present high-resolution salinity profiles from five sites in the South, Equatorial, and North Pacific Ocean. Our study greatly constrains understanding of LGM salinity in the Pacific Ocean. Our results show that LGM chloride concentrations of deep Pacific bottom water were 4.09 ± 0.4% greater than today's values. Pacific Ocean bottom water salinity was also indistinguishable from being homogeneous across the wide range of latitudes studied here. These LGM salinity reconstructions are on average slightly higher (~1.4 to 1% higher) than expected from sea level of the time, which is generally inferred to have been~120 to~135 m lower than today.

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Commercial DNA extraction kits are widely used for cultivation-free surveys of marine sediment. However, the consequences of popular extraction-kit choices for sequence-based biological inferences about marine sedimentary communities have not previously been exhaustively assessed. To address this issue, we extracted DNA from multiple paired subsamples of marine sediment using two popular commercial extraction kits (MO BIO Laboratories PowerSoil V R DNA isolation kit and MP Biomedicals FastDNA TM Spin Kit for Soil). We report comparisons of (1) total DNA yield, (2) extract purity, (3) gene-targeted quantification, and (4) postsequencing ecological inferences in near-seafloor (< 1 meter below seafloor [mbsf]) and subsurface (> 1 mbsf) marine sediment. In near-seafloor sediment, the MP Biomedicals FastDNA TM Spin Kit for Soil exhibits higher extraction yields, higher 16S rRNA gene loads, higher taxonomic diversity, and lower contaminant loads. In subseafloor sediment, both kits yield similar values for all of these parameters. The MO BIO Laboratories PowerSoil V R DNA isolation kit generally co-extracts less protein with the DNA in both near-seafloor and subseafloor sediment. For samples from all depths, both kits exhibit similar depth-dependent community richness patterns, taxonomic composition, and ordination-based similarity trends. We conclude that, despite kitspecific differences in extract yields, purity and reagent contaminant loads, ecological inferences based on next-generation sequencing of DNA extracted using these popular commercial kits are robustly comparable, particularly for subseafloor sediment samples.

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The middle of the South Pacific Gyre is farther from continents and productive oceanic zones than any other site on Earth. To understand the nature of life in the most oxidized and food-limited subseafloor sediments, Cruise KNOX-02RR surveyed and cored sediment at ten sites throughout the South Pacific Gyre and one site at the southern margin of the gyre. Sediment at all of the South Pacific Gyre sites accumulated extraordinarily slowly for millions of years (0.1-1 m/m.y.).Shipboard scientific results indicate that South Pacific Gyre sediments contain a living community characterized by very low biomass and very low metabolic activity. At every depth in the cored South Pacific Gyre sediments, cell abundances are 2-6 orders of magnitude lower than at the same depths in all previously explored subseafloor communities. The net rate of respiration by the subseafloor sedimentary community at each gyre site is 1-3 orders of magnitude lower than the rates of subseafloor communities at previously explored sites.

Interstitial-water studies on samples collected during DSDP Leg 50 indicate that saline brines were advected through aquifers into the deeper sedimentary strata. Maxima in the magnesium concentration at about 500 meters at Site 415 and 600 meters at Site 416 support the hypothesis that high-magnesium silicates (palygorskite and sepiolite) are formed in situ in an environment conducive to silicate formation, i.e., in strata where silica recrystallization occurs.