Planktonic foraminiferal assemblages and paleoceanographic change in the trans-tropical Pacific Ocean : A comparison of west (Leg 130) an east (Leg 138), latest Miocene to Pleistocene

Cores from four Ocean Drilling Program (ODP) sites were examined for planktonic foraminifers. One sample per core (from core-catchers in Holes 806B and 807B and from Section 4 in Holes 847B and 852B) was examined through the interval representing the last 5.8 m.y. Sites 806 (0°19.1'N; 159°21.7'E) and 847 (012.1'N; 95°19.2'W) are beneath the equatorial divergence zone. Sites 807 (3°36.4'N; 156°37.5'E) and 852 (5°19.6'N; 110°4.6'W) are located north of the equator in the convergence zone created by the interaction of the westward-flowing South Equatorial Current (SEC) and the eastward-flowing North Equatorial Countercurrent (NECC). Specimens were identified to species and then grouped according to depth habitat and trophic level. Species richness and diversity were also calculated. Tropical neogloboquadrinids have been more abundant in the eastern than in the western equatorial Pacific Ocean throughout the last 5.8 m.y. During the mid-Pliocene (3.8-3.2 Ma), their abundance increased at all sites, while during the Pleistocene (after ~ 1.6 Ma), they expanded in the east and declined in the west. This suggests an increase in surface-water productivity across the Pacific Ocean during the closing of the Central American seaway and an exacerbation of the productivity asymmetry between the eastern and western equatorial regions during the Pleistocene. This faunal evidence agrees with eolian grain-size data (Hovan, this volume) and diatom flux data (Iwai, this volume), which suggest increases in tradewind strength in the eastern equatorial Pacific that centered around 3.5 and 0.5 Ma. The present longitudinal zonation of thermocline dwelling species, a response to the piling of warm surface water in the western equatorial region of the Pacific, seems to have developed after 2.4 Ma, not directly after the closing of the Panama seaway (3.2 Ma). Apparently, after 2.4 Ma, the piling warm water in the west overwhelmed the upwelling of nutrients into the photic zone in that region, creating the Oceanographic asymmetry that exists in the modern tropical Pacific and is reflected in the microfossil record. In the upper Miocene and lower Pliocene sediments, the ratio of thermocline-dwelling species to mixed-layer dwellers is 60%:40%. During the mid-Pliocene, the western sites became 40% thermocline and 60% mixed-layer dwellers. Subsequent to -2.4 Ma, the asymmetry increased to 20%: 80% in the west and the reverse in the east. This documents the gradual thickening of the warm-water layer piled up in the western tropical Pacific over the last 5.8 m.y. and reveals two "steps" in the biotic trend that can be associated with specific events in the physical environment. INTRODUCTION Sediments representing the last 5.8 m.y. were examined at four ODP sites on opposite ends of the equatorial Pacific Ocean circulation system (Fig. 1). Two cores in the western equatorial Pacific were drilled at Sites 806 and 807 during Leg 130 to the Ontong Java Plateau (Kroenke, Berger, Janecek, et al., 1991) and the two in the eastern equatorial Pacific were at Sites 847 and 852, drilled during Leg 138 (Mayer, Pisias, Janecek, et al., 1992). Planktonic foraminifers were counted and identified to species in one sample per core. The relative abundance of each species was calculated. The most common species and species grouped on the basis of depth habitat and trophic level are presented in time-series for all four sections. Temporal resolution in this study is relatively coarse. The interval between samples in Hole 806B is 273 k.y. , in Hole 807B, 316 k.y. , that in Hole 847B, 240 k.y. , and that in Hole 852, 750 k.y. The sites were selected for their Oceanographic positions (see Table 1). Though on opposite sides of the tropical Pacific, both Sites 807 (3°36.4'N; 156°37.5'E) and 852 (5°19.6'N; 110°4.6'W) have been drifting into the convergence zone between the westwardflowing SEC and the eastward-flowing NECC. Likewise, Sites 806 (0°19.1'N; 159°21.7'E) and 847 (0° 12.1 'N; 95°19.2'W) are separated by the entire expanse of the Pacific, but both are beneath the equatorial divergence zone created by the sign change of the Coriolis force 1 Pisias, N.G. , Mayer, L.A. , Janecek, T.R. , Palmer-Julson, A. , and van Andel, T.H. (Eds.), 1995. Proc. ODP, Sci. Results, 138: College Station, TX (Ocean Drilling Program). 2 Department of Geology and Geography, University of Massachusetts, Morrill Science Center, Amherst, MA 01003, U.S.A. as the southeast trade winds blow across the equator. Both of these sites have been beneath the divergence zone for the past 5.8 m.y. , but the Equatorial Undercurrent (EUC) has risen in the water column since the late Miocene (Kennett et al., 1985). The purpose of this study was to look for changes in the abundances of planktonic foraminifer species that occur in stratigraphic intervals associated with the closing of the Central American seaway between 3.8 and 3.2 Ma (Keigwin, 1978; 1982) and the intensification of Northern Hemisphere glaciation after 2.5 Ma. Northern Hemisphere glaciation began -3.1 Ma, when continental-sized ice sheets began to grow and shrink in North America and Eurasia (Raymo et al., 1989). After 2.5 Ma, δ 1 8 θ variations indicate the inception of large-scale glaciation in the Northern Hemisphere (Shackleton et al., 1984; Jansen et al., 1988; Jansen et al., 1993), with ice sheets, on average, at least half the size of those during the late Pleistocene (Raymo et al., 1989; see Fig. 2). The response of the faunal assemblages to the Oceanographic consequences of these events is dictated by the ecologies of individual species. In the eastern equatorial Pacific, where the thermocline has been lifted higher into the photic zone by upwelling, species that live in the thermocline are expected to form a larger portion of the faunal assemblage in response to the hydrographic change. In the western equatorial Pacific, as the mixed layer thickens because of the "piling-up" of warm surface water by the trade winds, the species of planktonic foraminifer that dwell in the mixed layer are expected to increase their relative abundance in the faunal assemblage consequent to that hydrographic phenomenon. Other proxy indicators of Oceanographic change during the Central American seaway closing and the Northern Hemisphere intensification of glaciation intervals suggest that these were times of cooler