21. ELEMENTAL AND STABLE-ISOTOPE COMPOSITION OF PORE WATERS AND CARBONATE SEDIMENTS FROM DEEP SEA DRILLING PROJECT SITES 501/504 AND 5051

Two sites on the southern flank of the Costa Rica Rift were drilled on DSDP Legs 68 and 69, one on crust 3.9 m.y. old and the other on crust 5.9 m.y. old. The basement of the younger site is effectively cooled by the circulation of sea-water. The basement of the older site has been sealed by sediment, and an interval in the uppermost 560 meters of base-ment recently reheated to temperatures of 60 to 120°C. Although the thickness of the sediments at the two sites is similar (150-240 m versus 270 m), the much rougher base-ment topography at the younger Site 505 produces occasional basement outcrops, through which 80 to 90 % of the total heat loss apparently occurs by advection of warm seawater. This seawater has been heated only slightly, however; the temperature at the base of the sediments is only 9°C. Changes in its composition due to reaction with the basement basalts are negligible, as indicated by profiles of sediment pore water chemistry. Bacterial sulfate reduction in the sediments produces a decrease in SO4 (and Ca) and an increase in alkalinity (and Sr and NH3) as depth increases to an intermediate level, but at deeper levels these trends reverse, and all of these species plus Mg, K, Na, and chlorinity ap-proach seawater values near basement. Si, however, is higher, and Li may be lower. At the older site, Site 501/504, where heat loss is entirely by conduction, the temperature at the sediment/basement contact is 59°C. Sediment pore water chemistry is heavily affected by reaction with the basaltic basement, as indicate