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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

Figure 1. Mean abyssal hill trends (thick solid lines) between fracture zones and trace of Tongareva triple junction (dotted lines) are superimposed on predicted topography from satellite altimetry (Sandwell et al., 1997). Lighter and darker topography depict shallower and deeper features. Thin solid lines indicate magnetic lineations from Cande et al. (1989), and white circle marks Deep Sea Drilling Project Site 317. Circle with cross locates origin of triple junction trace proposed by Winterer et al. (1974) and Joseph et al. (1993). In track chart (inset) solid lines indicate our multibeam surveys and dashed lines indicate previously collected regional multibeam tracks. FZ is fracture zone.

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[1] The hydrologic evolution of oceanic crust, from vigorous hydrothermal circulation in young, permeable volcanic crust to reduced circulation in old, cooler crust, causes a corresponding evolution of geophysical properties. Ocean Drilling Program (ODP) Hole 801C, which obtained the world's oldest section of in situ, normal oceanic crust, provides the opportunity to examine relationships among hydrologic properties (porosity, permeability, fluid flow), crustal alteration, and geophysical properties, at both core plug and downhole log scales. Within these upper crustal basalts, fluid flux in zones with high porosity and associated high permeability fosters alteration, particularly hydration. Consequently, porosity is correlated with both permeability and a variety of hydration indicators. Porosity-dependent alteration is also seen at the log scale: potassium enrichment is strongly proportional to porosity. We extend the crustal alteration patterns observed at Hole 801C to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of Deep Sea Drilling Project and ODP core physical properties and downhole logs. Increasing crustal age entails macroporosity reduction and large-scale velocity increase, despite intergranular velocity decrease with little microporosity change. The changes in macroporosity and velocity are significant for pillows but minor for flows. Matrix densities provide the strongest demonstration of systematic age-dependent alteration. On the basis of observed decreases in matrix density that are proportional to the logarithm of age, approximately half of all intergranular-scale crustal alteration occurs after the first 10-15 Myr. Apparently, crustal alteration continues, at a decreasing rate, throughout the lifetime of oceanic crust.