Many cores raised from the Drake Passage are characterized by alternating zones of foraminiferal ooze and sandysilt. Cores raised from the East Pacific Rise are foraminiferal ooze or alternating siliceous and carbonate ooze. The uranium and thorium concentrations and isotopic ratios in foraminifers separated from these cores were measured by alpha-spectroscopy. 230Th in foraminiferal tests is found to be out of equilibrium with its parent (234U), enabling the “excess 230Th” method to be used in estimating sedimentation rates. The uranium concentration of the tests is constant, suggesting constant uranium concentrations in the sea and thus constant production of 230Th during periods of foraminiferal production. 232Th concentration is 2.1 ± 0.7 ppm in the Drake Passage and below the level of detection in tests from the East Pacific Rise. Within each core the 232Th is constant, varying closely with distance from the source.
Based on the degree of disequilibrium of 230Th, correlations between cores can be made. Age estimates of the foraminiferal zones indicate deposition occurred between 8 000 and 120 000 and between 190 000 and 280 000 years before present. Reported dates of high stands of the sea fall between the ages estimated for the foraminiferal zones, suggesting these zones were deposited during northern hemisphere glacial maxima.
