ALBERT

Description: Benthic foraminiferal biofacies may vary independently of water depth and water mass; however, calibration of biofacies and stratigraphic ranges with independent paleodepth estimates allows reconstruction of age-depth patterns applicable throughout the deep Atlantic (Tjalsma and Lohmann, 1983). We have attempted to test these faunal calibrations in a continental margin setting, reconstructing Eocene benthic foraminiferal distributions along a dip section afforded by the New Jersey Transect (DSDP Sites 612, 108, 613). The following independent estimates of Eocene depths for the transect were obtained by "backtracking," "backstripping," and by assuming increasing depth downdip ("paleoslope"): Site 612, near the middle/lower bathyal boundary (about 1000 m); Site 108, in the middle bathyal zone (about 1600 m); and Site 613, near the lower bathyal/upper abyssal boundary (about 2000 m). Within uncertainties of backtracking (hundreds of meters), these estimates agree with estimates of paleodepth based on comparison of the New Jersey margin biofacies with other backtracked faunas. The stratigraphic ranges of many benthic taxa correspond to those found at other Atlantic DSDP sites. The major biofacies patterns show: (1) a depth dichotomy between an early to middle Eocene Nuttallides truempyidominated biofacies (greater than 2000 m) and a Lenticulina-Osangularia-Alabamina cf. dissonata biofacies (1000- 2000 m); and (2) a difference between a middle and a late Eocene biofacies at Site 612. The faunal boundary at about 2000 m, between bathyal and abyssal zones, occurs not only on the margin, but also throughout the deep Atlantic. The faunal change between the middle and late Eocene at Site 612 was due to a decrease of Lenticulina spp., the local disappearance of N. truempyi, and establishment of a Bulimina alazanensis-Gyroidinoides spp. biofacies. Although this change could be attributed to local paleoceanographic or water-depth changes, we argue that it is the bathyal expression of a global deep-sea benthic foraminiferal change which occurred across the middle/late Eocene boundary.

Description: Quantitative study of benthic foraminifers from the upper Miocene to lower Pliocene section at Site 612 (1404 m present water depth) and the Pliocene section at Site 613 (2323 m present water depth) shows no evidence of widespread downslope transport of shallow-water biofacies or reworking of older material in the greater than 150 µm size fraction. In contrast, upper Miocene sediments from Site 604 (2364 m present water depth) show extensive reworking and downslope transport. At Site 612, benthic foraminifers show a succession from an upper Miocene Bolivina alata-Nonionella sp. biofacies, to an uppermost Miocene Bulimina alazanensis biofacies, to a lower Pliocene Cassidulina reflexa biofacies, to an upper Pliocene Melonis barleeanum-Islandiella laevigata biofacies. Evidence suggests that the Pliocene biofacies are in situ, although they could have been transported downslope from the upper-middle bathyal zone. At Site 613, Uvigerina peregrina dominated the "middle" Pliocene, while Globocassidulina subglobosa was dominant in the early and late Pliocene. High abundances of U. peregrina at Site 613 are associated with high values of sedimentary organic carbon.

Description: ODP Leg 114 recovered sections at four sites east of the Falkland Plateau that cover a wide range of paleodepths and provide the opportunity to evaluate the response of benthic foraminifers to late Paleocene and Eocene Oceanographic changes. Early Paleogene paleodepth estimates were obtained by "backtracking" assuming simple thermal subsidence (Site 698, -900 m; Site 702, -2000 m; Site 700, -2400 m; and Site 699, -2800 m). These estimates agree with paleodepths determined by comparing our quantitative benthic foraminiferal assemblages to previously published assemblages associated with known paleodepths. Previous studies document that a major benthic foraminiferal crisis occurred in the latest Paleocene in the Atlantic, Caribbean, and Pacific; a similar faunal turnover occurred in the latest Paleocene throughout the Atlantic sector of the Southern Ocean. At the Leg 114 sites, Stensioina beccariiformis-dominated assemblages were replaced by Nuttallides tryempyi-dominated assemblages just prior to the Paleocene/Eocene boundary. A preponderance of benthic foraminiferal taxa last appeared immediately prior to the Paleocene/Eocene boundary, as recognized at these high latitudes by the last appearance of the calcareous nannofossil F?sciculithus spp. and the first appearance of the planktonic foraminifer Pseudohastigerina spp. Recovery and biostratigraphic control at the Leg 114 sites is insufficient to constrain precisely the timing of the extinction event, although studies of material from the Maud Rise (Weddell Sea) suggest that it occurred in the latest Paleocene. The benthic foraminiferal crisis may have been caused by deep-water warming, a drop in food supply, or changing deep-water source regions. Oxygen isotope data show that there is no clear correlation between d18O changes and extinctions. Similarly, most of the extinctions occurred well after the start of the drop in global d13C values, which may, in part, reflect a decrease in productivity. Interbasinal carbon isotope comparisons suggest that the Southern Ocean was supplied "young" (high O2, low nutrient, and high d13C) deep water in the latest Paleocene (approximately 60-58 Ma) and early Eocene (approximately 57-52 Ma). Oxygen isotope evidence indicates that the Southern Ocean was filled with cooler water than that in the Pacific beginning at approximately 60 Ma, supporting our contention that the deep-water source was antarctic. However, near the Paleocene/Eocene boundary (approximately 58-57 Ma), the supply of Southern Ocean "young" deep water was reduced or eliminated. We speculate that elimination of this inferred antarctic source between 58 and 57 Ma triggered the benthic foraminiferal turnover.