25. DOWNHOLE FLOW OF BOTTOM WATER: IMPLICATIONS FROM TEMPERATURE

Temperature logging at Hole 462A, which has remained undisturbed for 4'/i years after drilling, reveals a very small temperature increase from 1.3°C at the seafloor to 3.9°C at 400 m sub-bottom depth, followed by a steep increase to 24 °C at 500 m. This temperature gradient strongly indicates downhole flow of bottom water into the sediments between 400 and 500 m below the seafloor at the time of Leg 89. The downhole flow of bottom water is probably caused by the underpressure in the 70-Ma sediments of the Nauru Basin; this is in contrast to other DSDP holes with similar down-hole flow phenomena, such as 395A and 504B, where the bottom water permeates into the underpressured young basal-tic basements by hydrothermal circulation. By assuming a constant flow rate of bottom water and a linear temperature gradient of 0.045°C/m for undisturbed sediments, the volumetric downhole flow rate is calculated to be about 1700 L/ hr (or 6m/hr. linear rate). For 4!4 years, at least 6 × I07 kg of seawater has been drawn down through the drilled hole into the sediments. The heat-flow value of 48 mW/m2 for undisturbed sediments at Site 462 is concordant with the in-traplate volcanism about 100 Ma in the Pacific Plate