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Paleodepth estimates obtained from empirical age-versus-subsidence curves of oceanic crust allow an independent determination of the paleobathymetric distributions of deep-sea benthic foraminifera. Such "backtracking" of DSDP sites together with studies of planktonic biostratigraphy, seismic stratigraphy, lithostratigraphy, and isotopic studies allows the placement of benthic foraminifera into a chronologic, paleobathymetric, and paleoceanographic framework. This approach has proven to be successful in recognizing several bathymetrically distinct deep-sea foraminiferal biofacies from the Paleogene of the Atlantic Ocean. Paleocene species have broad bathymetric ranges, but Eocene and Oligocene species tend to be bathymetrically more restricted.
Paleocene deep-water benthic foraminifera are predominantly relict Cretaceous taxa. Comparison of Paleocene deep-water benthic foraminiferal faunas with Cretaceous benthic faunas shows that, unlike planktonic organisms, there was no crisis in benthic foraminifera at the end of the Cretaceous. Most of the faunal variation in the Paleocene is attributable to the gradual bathymetric restriction of the shallower Gavelinella beccariiformis assemblage and the bathymetric expansion of the deeper Nuttallides truempyi assemblage. Such depth migrations, both expansions and restrictions, are prominent among the faunal changes noted in deep-sea benthic foraminifera studied to date. A major benthic faunal crisis occurred in the latest Paleocene (Zone P6a) with rapid massive extinctions at the gene ic and specific levels. Most of the extinctions occurred in the shallower G. beccariiformis assemblage containing predominantly Cretaceous relict species; the N. truempyi assemblage was characterized
more by appearances than extinctions.
Various lines of evidence (seismic, lithostratigraphic, isotopic) indicate that a major, rapid change in abyssal circulation occurred near the end of the Eocene. As modern benthic foraminifera distributions often correlate with modern water-mass distributions, benthic foraminifera may be expected to have responded to the circulation changes. However, the Eocene/Oligocene boundary was not catastrophic for deep-sea benthic foraminifera, and the late Eocene-Oligocene deep-sea fauna evolved in a series of events over several million years. The major faunal abundance change at all depths greater than ~0.5 km (1,600 ft) was the apparently synchronous decrease in abundance of Nuttallides truempyi just above the middle/late Eocene boundary (~38.5 to 40 Ma); this pre-dates by 2 m.y. the major SUP>18O enrichment and the change in abyssal circulation regime inferred from seismic stratigraphic studies. The record in deep abyssal locations (paleodepths > 3 km, 10,000 ft) shows the greatest changes, for here N. truempyi is associated with many endemic deep-water taxa (Abyssammina, Clinapertina, Aragonia, Alabamina dissonata, among others) that decrease in abundance and become extinct prior to the Oligocene. In shallower abyssal depths (2 to 3 km, 6,500 to 10,000 ft), a series of first and last appearances occurred in the late Eocene to earliest Oligocene. In lower bathyal depths (~0.5 to 1.5 km, 1,600 to 5,000 ft), a great number of first appearances occurred in the late Eocene through Oligocene.
Oligocene abyssal faunas mark a change from Paleocene (Cretaceous relict) and Eocene taxa (e.g. N. truempyi, Alabamina dissonata, Aragonia spp.) to abyssal assemblages that have many taxa in common with modern assemblages. The Oligocene abyssal fauna is dominated by stratigraphically long-ranging and bathymetrically wide-ranging taxa that survived the extinctions of the Eocene. During the middle Oligocene, Nuttallides umbonifera became important in deep abyssal locations in the North Atlantic and shallow and deep abyssal locations in the South Atlantic. Shallow abyssal Oligocene faunas throughout the Atlantic differ from Eocene faunas primarily by the absence of N. truempyi. Oligocene bathyal (0.5 to 1.5 km, 1,600 to 5,000 ft) assemblages are similar to the Eocene bathyal Lenticulina- ulimina-Osangularia assemblage, although many new taxa appeared in the late Eocene through Oligocene.
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