ALBERT

Beschreibung: The distribution of calcareous nannofossils at Ocean Drilling Program Site 1139 on Skiff Bank off the western edge of the Northern Kerguelen Plateau has been used to delineate Quaternary, middle Miocene-middle Oligocene, and lower Oligocene sediments in an expanded 385-m rotary-cored section. Sedimentation rates through most of the Tertiary are high (average = 23 m/m.y.) as a result of clastic input and dilution by high siliceous productivity; chert, however, was encountered in only a single sample. These characteristics, plus the presence of multiple, reasonably well preserved siliceous and calcareous microfossil groups deposited well above the calcite compensation depth, make this site a potential candidate for more detailed coring for paleoceanographic objectives in the future. A scanning electron microscope study revealed that diagenetic etching, apparently of gephyrocapsids, caused some of these placoliths to superficially resemble Emiliania huxleyi at levels in the Quaternary well below the evolutionary first occurrence of this taxon. Other larger specimens of etched gephyrocapsids and/or reticulofenestrids resembled Pseudoemiliania lacunosa. These observations suggest that caution should be exercised when age-dating Quaternary sediments not necessarily rich in carbonate. An unusual Braarudosphaera bloom recorded in upper Oligocene sediments on Skiff Bank is similar to that reported previously at Site 737 on the Northern Kerguelen Plateau and may correlate with other such occurrences of this age in the Atlantic and Indian Oceans. The base of the pelagic section is dated as earliest Oligocene by nannofossils and foraminifers (32.8-34.3 Ma; Subchron CP16a/b and basal AP13).

Beschreibung: The evolution of the Southern Ocean climate during the late Eocene-late Oligocene interval is examined through highresolution, quantitative calcareous nannofossil analyses on samples from the Southern Ocean sections on Maud Rise and Kerguelen Plateau. We determined the abundance patterns of the counted species to clarify the biostratigraphy, which we correlated with high-resolution magnetostratigraphy [Roberts, A.P., Bicknell, S.J., Byatt, J., Bohaty, S.M., Florindo, F., Harwood, D.M., 2003a. Magnetostratigraphic calibration of Southern Ocean diatom datums from the Eocene-Oligocene of Kerguelen Plateau (Ocean Drilling Program Sites 744 and 748). In: Florindo, F., Cooper, A.K., O'Brien, P.A. (Eds.), Antarctic Cenozoic Palaeoenvironments: Geologic Record and Models. Palaeogeogr., Palaeoclimatol., Palaeoecol. 198 145–168; Florindo, F., Roberts, A.P., in press. Eocene-Oligocene magnetobiochronology of ODP Sites 689 and 690, Maud Rise, Weddell Sea, Antarctica. Geol. Soc. Am. Bull.], and used this data to interpret paleoceanographic changes through the late Eocene to late Oligocene. Percentage plots of the individual species, compared with R-mode principal component and cluster analysis results, allowed us to divide the assemblages into three groups: temperate-water taxa, cool-water taxa, and no temperature-affinity taxa. We attempt correlations between these paleoecological groups and the major sea-surface temperature (SST) variations with tectonic and paleoceanographic changes in the Southern Ocean. During the late Eocene, the nannofossil assemblage data reveal that there were several minor SST decreases (coolings) from 36 to 34 Ma, before the Eocene/Oligocene (E/O) boundary. A sharp cooling event, dated at 33.54 Ma (earliest Oligocene), occurred about 160 kyr after the E/O boundary, which is dated at 33.7 Ma. Relatively stable, cool conditions are interpreted to persist until the latest Oligocene, when an increase in abundance of temperate-water taxa, which corresponds to an antithetical decrease in abundance of cool-water indicators, is recorded. On the basis of our dating, the opening of the Drake Passage, allowing shallow-water circulation, began by 33.54 Ma at the latest, while the establishment of deep-water connections through the Tasmanian Gateway occurred at 33 Ma, as suggested by Exon et al. [Proc. ODP, Init. Rep. 189 (2001) 1].