LARGE-SCALE LATERAL ADVECTION OF SEAWATER THROUGH OCEANIC-CRUST IN THE CENTRAL EQUATORIAL PACIFIC

The existence of large-scale lateral advection of water through basaltic crust in the central equatorial Pacific Ocean is demonstrated by the calcium, magnesium, strontium, sulfate, and strontium isotopic compositions of pore waters from the overlying sediments. The advection is believed to extend throughout the region of the equatorial high-productivity sediment bulge from about 110 to 160-degrees-W and from about 5-degrees-S to 8-degrees-N. The corresponding crustal ages of this region vary from east to west from about 15 to about 70 Ma, respectively. This advection is responsible for the low-conductive heat flows previously observed throughout the region. The flow is recognized by the following characteristic pore water compositional variations. Calcium and magnesium concentrations remain nearly constant downhole from the sediment-water interface to basement. Species, such as strontium, sulfate, and strontium isotopes, which are more affected by diagenetic reactions in the sediment column, depart from seawater values with increasing depth in the sediments, but then they display concentration reversals near basement. At the sediment-basement interface, pore waters are chemically and isotopically nearly indistinguishable from present-day seawater. Fluid flow in basement is rapid, having a calculated average residence time in oceanic crust of about 20,000 years and an inferred pore fluid velocity between 1 and 10 m y-1. Because of the short reaction time between basement rocks and fluids, as well as the low temperature of this fluid, the chemistry of basement water remains similar to seawater. As a result, despite the important impact of this process on oceanic heat flow, the flow may have little effect on the long-term major element composition of seawater.