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doi:10.2204/iodp.sp.329.2010

Introduction

The nature of life in the sediment beneath mid-ocean gyres is very poorly known. Almost all sites where subseafloor sedimentary life has been studied are on ocean margins (Ocean Drilling Program [ODP] Legs 112, 180, 201, and 204 and IODP Expeditions 301, 307, and 323) or in the equatorial ocean (ODP Legs 138 and 201). Despite those advances, the extent and character of subsurface life throughout most of the ocean remains unknown (Ocean Studies Board, 2003). This absence of knowledge is largely due to ignorance of subseafloor life in the major ocean gyres, which collectively cover most of the area of the open ocean.

The South Pacific Gyre is the ideal region for exploring the nature of subseafloor sedimentary communities and habitats in the low-activity heart of an open-ocean gyre. It is the largest of the ocean gyres. Its center is farther from continents than the center of any other gyre. Surface chlorophyll concentrations and primary productivity are lower in this gyre than in other regions of the world ocean (Behrenfeld and Falkowski, 1997) (Fig. F1). Its surface water is the clearest in the world (Morel et al., 2007). The sediment of this region has some of the lowest organic burial rates in the ocean (Jahnke, 1996). Our recent survey cruise demonstrates that shallow sediment of this region contains the lowest cell concentrations and lowest rates of microbial activity ever encountered in shallow marine sediment (Knox-02RR Shipboard Scientific Party, unpubl. data).

The region is also ideal for testing hypotheses of the factors that limit hydrothermal circulation and chemical habitability in aging oceanic crust (sedimentary overburden, basement permeability, and decreasing basal heat flux). It contains a continuous sweep of oceanic crust with thin (1–100 m) sedimentary cover spanning thousands of kilometers and >100 m.y. of seafloor age.

The South Pacific Gyre contains the largest portion of the seafloor that has never been explored with scientific ocean drilling. Consequently, the proposed drilling will advance scientific understanding across a broad front. It will help to constrain the nature of crustal inputs to the subduction factory. It will constrain the origin of the Cretaceous Normal Superchron and the tectonic history of a region as large as Australia. Recovery of sedimentary interstitial waters at several of the proposed sites will provide novel constraint on glacial–interglacial pCO₂ models.

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