The relative abundance of all planktonic foraminifer species and the and 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 and 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 gradients were used as a proxy record for variation in thermocline strength and position, while changes in 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 gradients, and relatively large 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 species numbers were relatively low, mixed-layer gradients were relatively large, and mixed-layer 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 influence at Site 925 through the last 4 m.y.
1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
2Institute of Marine Sciences, University of California, Santa Cruz, CA 95064 U.S.A. email@example.com