Reconstructing tropical Atlantic climate from magnesium/calcium and oxygen isotopes of planktonic foraminifera

Recent proliferation of late Quaternary tropical high-resolution paleoclimate records clarifies the global reach of millennial-scale climate changes, prompting questions about their origins. What is the difference in mechanism between the Younger Dryas and Heinrich events? Do such shifts in climate originate in the tropical or polar regions of the planet? How can we better document past shifts in the organization of the ocean-atmosphere system?; This dissertation presents the results of two parallel research efforts aimed at answering these questions: reconstructing Holocene and last deglacial paleoclimate of the subtropical South Atlantic, and developing new methods of studying tropical Atlantic upper ocean temperature profiles. Chapter 1 outlines the motivations of these efforts, which include the need to understand millennial-scale climate changes such as the Younger Dryas and Heinrich events, and the utility of a more detailed proxy for thermocline depth and temperature for studying ocean-atmosphere interactions. A new set of Globigerinoides bulloides Mg/Ca and faunal abundance data from Ocean Drilling Program Hole 1084B in the Benguela upwelling region of the southeast Atlantic reveals shifts in sea surface temperature that corroborate the influence of the Younger Dryas on the southern hemisphere (Chapter 2). A mid-Holocene cooling and brief cooling centered on 17.5ka are consistent with previously suggested tropical triggers of Heinrich events. These new data are conspicuous for their lack of any evidence of climate shifts with a period of 1--2ky, at least during the early Holocene when sediment accumulation rates are high enough to expect a signal. Cooling during the last thousand years appears consistent with the timing of the Little Ice Age, however. These data alone can not distinguish between variability in upper ocean current advection and shifts in the intensity of upwelling, but comparison to several other Atlantic Basin paleoclimate records indicates coherent shifts in the ocean-atmosphere system.; One important clue to past shifts in ocean-atmosphere dynamics is changes in thermocline depth and temperature. Chapter 3 presents oxygen isotope data from several species of planktonic foraminifera in core-top sediments and a new multiple regression technique which together enable the high-resolution reconstruction of thermocline profiles. Chapter 4 investigates whether the use of Mg/Ca data can improve this technique, with mixed but promising results.