Ecostratigraphy of Plio-Pleistocene benthic foraminifers in ODP Hole 625B and four Eureka holes from the Gulf of Mexico

The benthic foraminiferal faunas from the Pliocene and Pleistocene sections of ODP Hole 625B (889m depth), drilled in the DeSoto Canyon area of the northeast Gulf of Mexico were examined from samples representing climatic maxima and minima according to the Joyce et al. (1990) 61 O record. The Pliocene and Pleistocene sections of four Eureka boreholes, two from the eastern Gulf of Mexico (E66-83A, 769m; E68-139, 1852m) and two from the western Gulf of Mexico (E66-40D, 776m; E67-116A, 1229m) were examined in less detail. The fossil faunas from these cores resemble closely those of the present-day Gulf of Mexico. The benthic faunas of the early Pliocene (about 5 Ma to 2.4 Ma) of all cores were very uniform. The respective fossil faunas are usually closer in composition to modem faunas that are deeper than the present coring sites. Faunal composition changed sharply at 2.4 Ma in the "interglacial" series of samples from Hole 625B, the time of the onset of Northern Hemisphere glaciations: several species that are indicative of very high biological productivity and/or moderate oxygen deficiency (e.g., Bulimina aculeata, Bulimina mexicana, Bulimina exilis) flooded the samples. This fauna occurs in three pulses during the late Pliocene and Pleistocene "interglacial" sample series: from 2.4 to l.9Ma, from 1.3 to 0.7 Ma, and from 0.4 Ma to the present. The intervening sections exhibit faunas that are essentially similar to the early Pliocene associations. An equivalent change in the faunas of the "glacial" series of samples from Hole 625B occurred 0.8 m.y. later, from about 1.6 Ma to 1.2 Ma in the early Pleistocene, persisting until about 0.4 Ma. The faunas of the Eureka boreholes closely follow the faunal evolution in Hole 625B. Discrepancies between faunal compositions of 625B and approximately coeval Eureka samples occur particularly in late Pliocene and Pleistocene intervals. These discrepancies originate from the chance recovery of Eureka samples they may represent intermediate climatic stages. Samples from Hole 625B were analyzed in three size fractions: 74 to 125pm, 125 to 150,um, and >150gm. The effect of different size fraction upon faunal composition is slight between the 125 to 150gm and the >150gm fraction. However, the contrast between the faunas of the fine fraction (74 to 125gm) and the coarse fraction (>125gm) is drastic. Exclusively small species belong primarily to the genera of Bolivina and Eponides, which are often indicators of very high biological productivity and/or oxygen deficiency. INTRODUCTION The microfaunas on the deep ocean floor live in an extremely stable environment, an environment that does not experience the drastic fluctuations that surface organisms have to endure. Deep sea benthic foraminifers show a remarkably slow rate of faunal turnover (Douglas and Woodruff 1981) that is inherent from the slow, random process of genetic mutation, which only occasionally receives direction from environmental change. One such environmentally forced event occurred during the late Eocene (Corliss et al. 1984), when the global deep ocean circulation switched from warm and salty low latitude source areas to cold high latitude areas. Boltovskoy and Boltovskoy (1988) argue that this was the one turnover during which the deep sea microfauna, that had its origin during the Mesozoic, became essentially moder. The difference between Oligocene and moder faunas, they argue, lies mostly in historically developed errors of taxonomic usage. However, many authors document another, perhaps lesser, faunal turnover during the late middle Miocene (e.g. Berggren 1972; Schnitker 1979b, 1986; Boersma 1986; Thomas 1986; Woodruff and Savin 1989; Miller et al. 1992). This Miocene faunal turnover is usually ascribed to the additional cooling of deep ocean water, coincident with the build-up of a permanent ice cap on Antarctica. However, Thomas and Vincent (1987) suggest that the faunal change predated the ice-cap formation and cooling. Very large climatic changes occurred during the last five million years that have also affected the deep ocean environment: the sudden beginning of intermittent ice sheet occurrences in the northern hemisphere at 2.4 Ma (Shackleton et al. 1984). It would be reasonable to expect significant faunal turnover at these climatic and oceanographic forcing points as well. Studies of late Quaternary piston cores (e.g., Streeter 1973; Schnitker 1974, 1979a) clearly demonstrated that the deep faunas rearranged themselves drastically in response to glacial/interglacial deep water changes. Several works on the PlioPleistocene sequences of DSDP and ODP holes (Schnitker 1984, 1986; Boersma 1985; Thomas 1986; Loubere and Banonis 1987; Boltovskoy and Boltovskoy 1988; Kurihara and Kennett 1988) also found a drastic change in the composition of the benthic faunas, but hardly any extinctions or occurrences of new species.