17. CHANGES IN UPPER WATER-COLUMN STRUCTURE AT SITE 925, LATE MIOCENEÐ PLEISTOCENE: PLANKTONIC FORAMINIFER ASSEMBLAGE AND ISOTOPIC EVIDENCE 1

Abstract

The relative abundance of all planktonic foraminifer species and the δ 18 O and δ 13 C values of three species ( Globigerinoides sacculifer, a mixed-layer dweller; Neogloboquadrina dutertrei, a thermocline dweller; and Truncorotalia crassaformis, a deep dweller) was determined in 25 samples at ~250 k.y. intervals through the last 6 m.y. at Ocean Drilling Program Site 925. Combined, the data indicate that mixed-layer depth was a minimum at the end of the early Pliocene (4.0 Ma) and gradually increased toward the Pleistocene. The δ 18 O and δ 13 C gradients in the mixed layer and the thermocline were derived by subtracting isotopic values of Globigerinoides sacculifer from Neogloboquadrina dutertrei and values of N. dutertrei from Truncorotalia crassaformis. Changes in