| Summary: | Oceanic circulation remains one of the poorly understood elements of the global climate system, despite its importance to planetary heat redistribution and carbon cycling. The nature of deep-water formation and circulation in ancient oceans are even more poorly constrained. In order to understand climate dynamics of past and future climates we must have a better understanding of the role of deep-ocean circulation. In this dissertation I investigated changes in intermediate- to deep-water circulation in three different ocean basins during two different geologic eras. The first study focused on the late Pleistocene (~25 ? 60 ka) California margin to investigate the role of intermediate water circulation in abrupt climate fluctuations. The other two studies investigated deep-water circulation during the Late Cretaceous (~70 ? 100 Ma) greenhouse interval, to determine if deep waters formed in the southern Indian or Atlantic basins. The above studies employed neodymium isotopes preserved in biogenic apatite (fish teeth and bones) and foraminiferal calcite to reconstruct the provenance of intermediate- to deep-water masses. Here I present data from two sites located at intermediate depths on the late Pleistocene California margin as well as seven Deep Sea Drilling Project and Ocean Drilling Program Cretaceous aged sites; four in the South Atlantic Ocean, and three in the Indian Ocean. The new Pleistocene data rule out changes in the source of intermediate waters to the California margin, thus the recorded changes in seafloor oxygenation were caused by changes in sea surface productivity. In the Cretaceous, the spread of deep waters formed in the high-latitude South Atlantic was hindered by tectonic barriers until the mid Campanian when the subduction of Rio Grande Rise allowed for the continuous flow of deep waters from the Southern Ocean into the North Atlantic. The deep Cretaceous Indian Ocean was filled with deep waters formed in the high-latitude Indian Ocean, until being replaced with waters sourced in the Pacific from the late Cenomanian to early Campanian before a return to southern Indian-sourced waters for the remainder of the study interval.
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