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. chaisson@aphrodite.ucsc.edu
 |
 |