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The relative abundance of all planktonic foraminifer species and the d18O and d13C 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 d18O and d13C 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 d18O gradients were used as a proxy record for variation in thermocline strength and position, while changes in d13C 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 d18O gradients, and relatively large d13C 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 d18O gradients were relatively large, and mixed-layer d13C 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. INTRODUCTION AND BACKGROUND Depth Ecology of Planktonic Foraminifers A striking and useful aspect of planktonic foraminifer ecology is the vertical stratification of population maxima in the water column. The character of planktonic foraminifer assemblages is determined by the thermal structure and nutrient distribution in the upper water column. Depth stratification of species in the water column has been verified by numerous workers with respect to both modern thermal structure and nutrient distribution (e.g., Emiliani, 1954, 1971; B, 1960; Lidz et al., 1968; B et al., 1971; Shackleton and Vincent, 1978; Williams et al., 1979; Durazzi, 1981; Deuser et al., 1981; Fairbanks and Wiebe, 1980; Fairbanks et al., 1980, 1982; Deuser, 1987; Kroon and Ganssen, 1988; Deuser and Ross, 1989) and paleo-conditions (Shackleton, 1967; Douglas and Savin, 1978; Keller, 1985; Gasperi and Kennett, 1992, 1993; Norris et al., 1993, 1994; Hodell and Vayavananda, 1993). When the thermocline shoals in the photic zone, thermocline-dwelling foraminifers become a relatively larger percentage of the assemblage (Leckie, 1989). Different thermoclinedwelling species favor various levels of productivity (Fairbanks and Wiebe, 1980). The details of depth ecology are far from completely understood. The depth habitat of some species definitely changes during their ontogeny (e.g., Lohmann and Schweitzer, 1990), while the depth ecologies of some extinct species are still undetermined. Vertical stratification is probably an adaptive strategy for partitioning resources. The partitioning of resources for maximum utilization by the largest possible number of organisms has often been observed in ecological communities (MacArthur et al., 1966; Ricklefs, 1979). The physiological mechanism in planktonic foraminifers that maintains this stratification is not known. However, collections made 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 Table of C Previous Chapter in several locations around the world have consistently documented similar relative positions in the water column for several species, and d18O measurements generally corroborate tow-net and sediment-trap observations. A short coming of many of the older studies is that the depth rankings were not made with respect to thermal structure, but rather were made with respect to absolute depth. The absolute depth of a particular species habitat may vary greatly from one location to another depending on position of the seasonal thermocline (mean state or seasonal variation). For example, a species that lives at thermocline depths, such as Menardella (=Globorotalia) menardii, may be listed as a shallow or surface dweller in locations where the thermocline is near the surface and as an ÒintermediateÓ dweller in locations were the thermocline is deep. In this study the depth habitats of all species are described with respect to thermal structure. Many extinct Pliocene species are the immediate ancestors of modern species. Where their ranges overlap in the fossil record, they often show covarying abundances, suggesting similar ecological niche boundaries. Isotopic measurements in some cases confirm that extinct species occupied depth habitats similar to those of closely related modern species (e.g., Neogloboquadrina acostaensis and N. dutertrei in Keller, 1985). Given these similarities, extant and extinct species have been combined in faunal groups to produce continuous faunal records (Dowsett and Poore, 1990). Climatic and Oceanographic History The West Antarctic ice sheet was established in the late Miocene (Shackleton and Kennett, 1975; Ciesielski et al., 1982; Haq, 1984). During the latest Miocene, the Mediterranean was isolated (Òthe Messinian salinity crisisÓ) by falling sea levels, a consequence of the Antarctic ice volume increase (Shackleton and Kennett, 1975; McKenzie and Oberhnsli, 1985; Hodell et al., 1986). Except for an episode of strong glaciation between 4.6 and 4.3 Ma (Jansen et al., 1993), the period from 5.0 to 3.2 Ma was one of climatic warmth and general stability in the size of Antarctic ice sheets (Shackleton and Opdyke, 1977; Keigwin, 1979, 1982; Jansen et al., 1993). High-lati255 ontents Next Chapter W.P. CHAISSON, A.C. RAVELO tude surface water seems to have warmed, while low-latitude water seems to have become relatively cool through this interval (Kennett and von der Borch, 1986). It was while the globe was in this climatic state that the tectonic uplift made the Isthmus of Panama subaerial, cutting off the low-latitude interoceanic connection between the Pacific and Atlantic basins (Berggren and Hollister, 1977; Thunell and Belyea, 1982). The final closing of the Central American seaway may have taken place in the mid-Pliocene (Kaneps, 1979; Keigwin, 1978, 1982; Duque-Caro, 1990), but Coates et al. (1992), using near-shore marine records, have placed the final closing at 3.8−3.7 Ma, which is older than the 3.4−3.2 Ma estimate given by Keigwin (1978, 1982) based on pelagic records. Farrell et al. (1995) estimated, on the basis of carbonate records from several (Leg 138) sites in the eastern equatorial Pacific, that the seaway may have been effectively closed as early as 4.5 Ma. The intensification of Northern Hemisphere cooling seems to have followed the closing of the seaway. Shackleton and Opdyke (1977) ascribed a large positive shift in the oxygen isotopic record on the Ontong Java Plateau at 3.2 Ma to the beginning of ice-sheet growth in the Northern Hemisphere, which agreed with an initial 3.0Ma age estimate for the first appearance of ice-rafted debris in North Atlantic sediments (Berggren, 1972; Poore, 1981). Re-evaluation of biostratigraphy at Deep Sea Drilling Project (DSDP) Sites 111 and 116 caused Backman (1979; Shackleton et al., 1984) to date the first appearance of ice-rafted debris in North Atlantic sediments at about 2.5 Ma. However, δ18O data from Site 806 in the western equatorial Pacific (Jansen et al., 1993) suggest that ice sheet growth was not a steady expansion from the 3.2 Ma enrichment, but rather a more complex enrichment pattern beginning after 3.7 Ma. 256 Other studies of North Atlantic records suggest that the interval from 3.1 to 2.45 Ma showed a trend toward progressively cooler seasurface temperatures with increasing amplitude changes (Dowsett and Poore, 1990), and that cold episodes after 2.95 Ma began to be colder than the modern climate, but the warm intervals were also still significantly warmer (Raymo et al., 1992). After 2.5 Ma, a permanent enrichment of oxygen isotope values in North Atlantic DSDP cores marked what is probably the beginning of Northern Hemisphere glaciation (Thunell and Williams, 1983; Raymo et al., 1989). In this paper we will interpret changes in tropical planktonic foraminifer assemblages at Ocean Drilling Program (ODP) Site 925 as responses to changes in the position of the thermocline in the photic zone and consequent changes in the productivity of the upper water column from the late Miocene to the Pleistocene.

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