8th International Nannoplankton Association Conference
Giuliana Villa, David K. Watkins, Sherwood W. Wise, CRP Science Team:
The Cape Roberts Project is an international co-operation of the Antarctic Programs of six countries intended to document the glacial history of the E Antarctic ice sheet, and the history of the W Antarctic Rift System (i.e. when this region began to rift to form the Ross Sea and the Transantarctic Mountains).
Cape Roberts cores were drilled from a rig established on fast sea-ice, in three locations at 70oS, some 150km NW of McMurdo Station, Antarctica. The three drill-holes cored over 1500m of strata, representing Quaternary to Oligocene (possibly Eocene) sediments.
During three austral-summer drilling seasons (1997-98-99), a team of some 60 scientists studied various aspects of the core; we examined the calcareous nannofossils. The assemblages differ significantly from coeval ones from the temperate and tropical regions of the world, showing ecological exclusion of tropical and many mid-latitude forms (e.g. discoasters, sphenoliths). These palaeobiogeographical differences necessitated the use of the Southern Ocean biostratigraphical zonation of Wei & Wise to subdivide the Oligocene of our cores. However, experience from the last two drilling seasons indicates that several key taxa used in that biozonation are routinely absent in our samples. As a result, it has been necessary to use other taxa as additional biostratigraphical indicators for the Cape Roberts cores.In general, the nannofossils are rare, but preservation is fairly good. Reticulofenestrids and Dictyococcites dominate the assemblages, and we plan to focus our attention on the taxonomy and distribution of these forms. The main results of the three-year project are: (1) CRP-1 recovered only 148m of Quaternary-Miocene sediments. We documented the consistent occurrence of Thoracosphaera spp. in bioclastic Quaternary sediments. This seemingly trivial discovery opened an interesting investigation into whether nannofossils are present in other Quaternary sediments of high latitudes of the Antarctic. Consequentially, if coccoliths, partly responsible for primary palaeoproductivity in this region, have a response to glacial-interglacial cycles; (2) CRP-2 recovered 624m of Pliocene to Lower Oligocene sediment. Within the latter sequence are two defined biostratigraphic horizons: the last appearance datum (LAD) of Dictyococcites bisectus and LAD of Chiasmolithus altus; these datums constrained the age of the section. Influxes of nannofossils, indicated by increases in abundance and species richness, may indicate either warm-water events or reworking. Each occurrence must be interpreted according to its sedimentologic context and association with other fossils; (3) CRP-3 recovered 939m, spanning the Lower Oligocene, possibly reaching the uppermost Eocene, terminating at an unconformity with the Devonian Beacon Sandstone. Although Cenozoic sediments were recovered down to 750m, nannofossils have been detected so far only above 270m. A Lower Oligocene assemblage is delimited by the last occurrences of Transversopontis pulcheroides and Chiasmolithus oamaruensis, the latter an accepted marker for the Upper Eocene-Lower Oligocene high-latitude sediments. Influxes of nannofossils, indicated by increases in abundance and species richness, indicate warm-water events. As to the primary goals of the project, stated above: a) the nannofossil data, combined with that from other members of the research team, delineates individual advance-retreat cycles of the ice-sheet margin. Cycles 20 million years ago had a frequency of 100ky, which indicates that the Antarctic ice-sheet millions of years ago responded to orbital cycles in a manner similar to that of Northern Hemisphere ice-sheets during the Quaternary. This is an important observation on the study of ice-sheet behaviour; b) the age of the oldest Cenozoic sediments overlying the Devonian Beacon Sandstones was a surprise, as these had been estimated by geophysical surveys to be between 50 to 100my old. Instead, the biostratigraphy shows it to be around 34my old, thereby providing new insights into the rifting history of this basin.
Copyright © 2000, most recent revision July 28, 2000Tania Hildebrand-Habel (email@example.com)