Changes in upper water-column structure at Site 925, late Miocene–Pleistocene: planktonic foraminifer assemblage and isotopic evidence

Abstract

The relative abundance of all planktonic foraminifer species and the d 18 O and d 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. Com-bined, 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 d 18 O and d 13 C gradients in the mixed layer and the thermocline were derived by subtracting isotopic values of Globi-gerinoides sacculifer from Neogloboquadrina dutertrei and values of N. dutertrei from Truncorotalia crassaformis. Changes in d 18 O gradients were used as a proxy record for variation in thermocline strength and position, while changes in d 13 C gradients served a like purpose for productivity variation. The early Pliocene was characterized by relatively high numbers of thermocline-dwelling taxa, relatively small mixed-layer d 18 O gradients, and relatively large d 13 C gradients. These indicators suggest that downwelling was at a minimum for the last 6 m.y. during this period at Site 925, the temperature difference between G. sacculifer and N. dutertrei habitats was small, and surface productivity was relatively great. After 2.5 Ma thermocline-dwelling speciesÕ numbers were relatively low, mixed-layer d 18 O gradients were relatively large, and mixed-layer d 13 C were relatively small. The increased pole-to-equator temperature gradient that accompanied the growth of Northern Hemisphere ice sheets caused the intertropical convergence zone to move toward the equator and exert a growing inßuence at Site 925 through the last 4 m.y.