Oligocene and Lower Miocene Calcareous Nannofossil Biostratigraphy and Paleoceanography of the Ross Sea

University Of Nebraska-Lincoln, Lincoln NE

Investigators

Abstract

0126170 Watkins This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, provides funds for a study of calcareous nannofossils in cores from the Ross Sea. Much of the direct evidence of the early history of Cenozoic glaciation on Antarctica comes from cores that were drilled in the waters of the Ross Sea. One set of these cores was drilled by rigs mounted on the fast ice that rings the southern part of the Ross Sea, just to the north of the Ross Ice Shelf. These include the CIROS-1 hole and Cape Roberts Project (CRP) holes 2 and 3, which cored thick sections of Oligocene and lower Miocene sedimentary rocks deposited within about 15 km offshore of the glaciated coast of Victoria Land. These clastic rocks include sandstones, siltstones, mudstones, conglomerates and diamictite that indicate glaciers reached sea level by the Oligocene and were very close (or actually on top of) the site of deposition for all or part of the Oligocene and early Miocene. A second set of cores, drilled by the Deep Sea Drilling Project (DSDP) drilling vessel Glomar Challenger includes Site 270, located at the same latitude (77 degrees) as CIROS and CRP, but more than 300 km away from the nearest source of glacial sediment. The Oligocene - lower Miocene sedimentary sequence at this site is thinner and more fine-grained, but contains abundant evidence of nearby ice in the form of ice rafted debris (IRD). A second DSDP Site (274), located some 8 degrees latitude north, contains a Oligocene and lower Miocene dominated by diatomaceous sediment including mud and ooze, reflecting its largely open oceanic position. These four drill-holes form a latitudinal and paleoenvironmental transect to investigate the surface water history of the Ross Sea using calcareous nannofossils. Calcareous nannofossils, the fossil remains of coccolithophorid algae that secrete calcite skeletal plates, will be used in two ways to examine the Ross Sea: 1) Paleoceanographic reconstruction - The distribution of the coccolithophorid algae is strongly controlled by the thermal structure of the surface water mass, making their fossils sensitive indicators of past water mass temperature. This is especially useful in an area that is dominated by sediments with too little calcite to use isotopic methods of estimating water temperatures. Analysis of the calcareous nannofossils by statistical paleoecologic methods will provide a temperature history of the Ross Sea surface waters for a period of geological time characterized by fluctuating ocean temperatures and partial or ephemeral continental ice sheets. In addition, because of the sensitivity these phytoplankton are known to have to abnormal salinity, the analysis of the assemblages from deep ocean to the mouths of the Victoria Land glaciers may provide a new way to estimate the meltwater outflow of the evolving East Antarctic Ice Sheet. 2) Biostratigraphy - The rapid rate of evolution of the coccolithophorid algae through the mid-Cenozoic, coupled with their widespread distribution in the oceans, provides the means of age dating the sediments that contain their fossils. The numerical ages of extinction and speciation events have been well documented, providing a means to accurately and precisely date the sedimentary sequences in the Ross Sea and, thus, the events responsible for the deposition. Because of the excellent calibration of calcareous nannofossil biostratigraphy to the time scale, these fossils offer the possibility of constraining the paleomagnetic stratigraphy and other fossil biostratigraphies (such as the largely endemic Antarctic shelf diatoms) that occur in these Oligocene and lower Miocene sections. If successful, this work will have important, and wide reaching ramifications to paleoceanographic research that seeks to understand the role of the Antarctic ice sheet in global systems.