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Changes in deep-water environments of the western Atlantic over the past six million years were investigated by studying benthic foraminiferal abundance changes in 335 samples from Sites 926 (3598 m), 928 (4010 m), and 929 (4358 m) on the Ceara Rise. The location of these sites on a transect at lower abyssal depths provides a unique opportunity to evaluate changes in water mass properties as well as in the supply of organic matter from the surface waters, especially because other Leg 154 studies provided a precise time scale for correlations between the sites. Relative abundances and ßuxes of the most abundant species were calculated, with emphasis on the four most common speciesÑNuttallides umbonifera, Globocassidulina subglobosa, Epistominella exigua, and Alabaminella weddellensis. There are two patterns in relative abundance of these species with time: the Þrst group of species (N. umbonifera and G. subglobosa) shows abundance variations that differ from site to site (i.e., with depth). The other group (E. exigua and A. weddellensis) shows variations in relative abundance that are similar at all three sites and thus independent of depth. In the Þrst group, G. subglobosa increased in relative abundance at the shallowest Site 926 from 2.4 to 1 Ma, but decreased at the two deeper sites. In contrast, N. umbonifera, a marker species for corrosive bottom waters and speciÞcally for Antarctic Bottom Water (AABW), shows large ßuctuations in relative abundance, but overall increased in relative abundance at the two deeper sites and decreased at the shallower site from 2.1 Ma to the present. Its relative abundance shows three peaks during the last 0.9 m.y., which were coeval at the two deeper sites. These large ßuctuations in abundance of N. umbonifera suggest that production of AABW ßuctuated in intensity, with increases during interglacial periods. AABW formation increased overall from about 3 Ma, and the amplitude of the ßuctuations increased from 0.9 Ma to present. The differences between sites are interpreted as showing that the deeper Sites 928 and 929 were commonly within AABW, whereas the shallower site was not. The species in the second group, E. exigua and A. weddellensis, occur in the recent oceans commonly in oceanic regions where spring blooms lead to seasonal deposition of phytodetrital material. At the Ceara Rise sites, they show variations in relative abundance that are similar at all three sites from 4.5 to 1.2 Ma, conÞrming that these species react to environmental factors that are not depth related, and thus possibly to surface productivity. The two species, however, do not covary exactly: at 2.6 Ma, for example, E. exigua decreased in relative abundance whereas A. weddellensis increased. INTRODUCTION Stratigraphic changes in deep-sea benthic foraminiferal faunas have been commonly associated with changes in North Atlantic deep-water circulation during the Pleistocene glacial-interglacial cycles (Streeter, 1973; Streeter and Shackleton, 1979; Schnitker, 1974, 1979, 1980; Peterson and Lohmann, 1982). Studies on recent benthic foraminiferal assemblages likewise revealed a relation between faunal composition and hydrographic parameters such as temperature, salinity, oxygen content, and carbonate saturation (Murray, 1973; Lohmann, 1978; Corliss, 1979; Corliss and Honjo, 1981; Peterson, 1984; Mead, 1985). Many different authors have attempted to correlate the distribution of deep-water benthic foraminifers with changes in water mass circulation patterns in the deep oceans, and benthic foraminiferal faunal changes have been linked to changes in stable isotope records (e.g., Woodruff and Savin, 1989; Boersma, 1990; Hermelin, 1991; Thomas, 1992). On the other hand, several authors reported that the abundance of several species of benthic foraminifers is correlated to surface primary productivity and thus the availability of food on the seafloor. Much data on faunas along continental margins suggest that faunas rich in genera such as Melonis, Uvigerina, and Bolivina reflect a high food supply rather than oxygen concentration in the bottom waters (e.g., Lutze and Coulbourn, 1984; Mackensen et al., 1985; Altenbach and 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). 22-5-1 Akebono-machi, Department of Geology, Kochi Univeristy, Kochi 780, Japan. yasuda@cc.kochi-u.ac.jp Table of C Previous Chapter Sarnthein, 1989; Schnitker, 1993; Rathburn and Corliss, 1994; Corliss, 1983; Miao and Thunell, 1993; Hermelin and Shimmield, 1990; Thomas et al., 1995). Other species of benthic foraminifers have been observed to respond opportunistically to a seasonal supply of phytodetritus under overall oligotrophic circumstances in the open ocean (Gooday, 1988, 1993; Gooday and Lambshead, 1989). High rates of phytodetritus deposition can not be linked in a simple way to high surface productivity, but reflect hydrographic conditions in the upper layers of the oceans (particularly, a deep layer of winter mixing), that lead to a strong phytoplankton bloom (Campbell and Aarup, 1992). The opportunistic, phytodetritus-exploiting species have been used to reconstruct the occurrence of episodes with common phytodetritus deposition in down-core investigations (Smart et al., 1994; Thomas et al., 1995; Thomas and Gooday, in press). Benthic foraminiferal faunas thus can play an important role in the investigation of deep-water circulation patterns as well as paleoproductivity (e.g., Mackensen et al., 1995). The purpose of this study is to examine the long-term changes in deep-water environments as based on benthic foraminiferal faunal and abundance changes during the past 6 m.y. MATERIAL AND METHODS Samples were taken from three sites drilled on Leg 154 on the Ceara Rise (equatorial Atlantic Ocean) on a depth transect between 3000 and 4300 m, at lower abyssal depths (Fig. 1). Hole 926A (3û43.146¢ N, 43û44.884¢W, 3598.4 m) is presently in the lower part of North Atlantic Deep Water (NADW). Hole 928A (5û27.320¢N, 395 ontents Next Chapter H. YASUDA 9 s r r d s f e w o t

Late Miocene–Holocene paleoceanography of the western equatorial Atlantic: evidence from deep-sea benthic foraminifers