Planktonic foraminifera and upper water column variability in the South Atlantic: A multiple species approach to the deep sea sedimentary record of climate change.

摘要

This dissertation documents both glacial-interglacial and millennial-scale climatic oscillations from the δ18O and δ 13C composition of planktonic foraminifera preserved in Southern Ocean deep-sea sediments. Additionally, this work addresses the causes of such oscillations through isotopic measurements of multiple species of foraminifera that occupy different parts of the water column, an approach that allows the role of thermal, density, and nutrient stratification changes to be examined. The methods are constrained by way of modern observations (via plankton tow samples) of foraminiferal abundances and geochemical compositions.; The principal results of this work are threefold. Firstly, the modern observations of depth habitat and isotopic composition justify the application of the multi-species approach in the realm of paleoceanography. Globigerina bulloides and Neogloboquadrina pachyderma (s.) are the most abundant species in the upper part of the water column of the subantarctic region, but regional hydrographic differences between the northern and southern subantarctic suggest that these species cannot be used interchangeably in the fossil record. For example, G. bulloides δ 18O mimics that of predicted calcite over a more limited range than is the case for N. pachyderms δ18O. In the northern subantarctic where G. bulloides is unreliable, N. pachyderms appears more abundant at deeper depths and therefore less indicative of surface conditions. The δ18O gradient between these shallow dwelling species and the relatively deep dwelling Globorotalia truncatulinoides and Globorotalia inflata appears consistent with Δδ18O values predicted from water column properties, thus demonstrating the potential of the multi-species Δδ 18O approach for addressing gradients in surface oceans of the past.; Secondly, results from multiple high resolution δ18O records from surficial species at different parts of the subantarctic allow the Vostok ice core chronology to be fixed, and taken together these records suggest similar variations in the proxies for SST and atmospheric air temperature. Together these records suggest changes in ice accumulation rate, oceanic moisture source, orbital sensitivity, and water column structure on the millennial-scale over the past three climatic cycles.; Finally, the Δδ18O approach is used to explicitly address water column structure, and subantarctic glacial terminations are suggested to proceed from relatively less stratified to relatively more stratified.