Globorotalia truncatulinoides as an indicator of change in the permanent thermocline of the glacial Arabian Sea

The permanent thermocline is the area of contact between the surface and the deep ocean and is important to the global transport of heat. However, current methods used in paleoceanographic studies have limits when trying to reconstruct this section of the water column. Constraints are found when proxies such as planktonic foraminifers respond to more than one environmental variable. Other approaches that focus on the intersection of the thermocline with the sea floor are limited in their geographic extent. The thermocline depths of the modern Arabian Sea are characterized by one of the strongest oxygen minimum zones (OMZ) in the world ocean. Ocean response to changes in climate would affect the processes that create this OMZ. A planktonic foraminifer, G. truncatulinoides, is present in the glacial sediments of the Arabian Sea for a brief amount of time. As a deep dwelling species, its presence suggests that environmental conditions in the OMZ did, indeed, change.;This dissertation seeks to extend existing techniques and to develop new approaches to study the permanent thermocline using G. trunc. from modern sediments of the Indian Ocean. The development of a growth model for the species provides a basis for a more detailed understanding of the paleoceanographic proxies recorded in the shell. Two major areas of concentration are the development of a model for carbon isotopic fractionation based principally on [CO3=] and the calibration of Mg/Ca in the shell calcite to ambient water temperature during calcification. These results are then applied to the problem of the glacial Arabian Sea.;The coordinated use of Mg/Ca and delta 18O demonstrates that during the last glacial stage, G. trunc. (R) calcified in water that was wanner and more saline than would have been determined from delta 18O alone. The calculated glacial delta13CDIC at depth in the water column is equal to modern surface values. Circulation changes that include increased flow from the southern Indian Ocean and more vigorous winter convective mixing would support the environmental conditions recorded in the properties of the shells of G. trunc. (R).