| Abstract: | Paleotemperature proxies enable scientists to reconstruct past ocean temperatures from sediment recovered in deep-sea cores. This dissertation takes an in-depth look at two proxies, Mg/Ca ratios of planktonic foraminifera and the TEX₈₆ index, and applies the Mg/Ca paleothermometer in a novel way to assess past changes in tropical Pacific climate. Chapter II is a reinvestigation of a proposed large salinity effect on Mg/Ca ratios in the foraminifera Globigerinoides ruber based on a recent Atlantic core-top meridional transect study. Shell weight analyses and scanning electron microscopy are used to assess the preservation of similar G. ruber shells used in the core-top study. Shells from the equatorial Atlantic are highly dissolved compared to those from the subtropical gyres, impacting their Mg/Ca-temperatures. When Mg/Ca-temperature calibrations that account for dissolution differences are used, there is no longer an apparent large salinity effect, suggesting that regional differences in preservation, rather than salinity, significantly affect Mg/Ca-temperatures. Chapter III investigates the TEX₈₆ temperature proxy in the eastern Pacific by utilizing Mg/Ca paleothermometry on multiple species of planktonic foraminifera across the Holocene and Last Glacial Maximum (LGM) to determine the recording depth of the TEX₈₆ proxy. Holocene TEX₈₆ temperatures match sea surface temperatures, but during the LGM, there is a cold bias in TEX₈₆ temperatures that are more representative of the upper thermocline. The best explanation for the offset is a decrease in LGM nutrient availability. Therefore, caution should be applied when interpreting TEX₈₆ records based solely on the relationship between core-top/Holocene TEX₈₆ temperatures and modern observational temperatures. Chapter IV reconstructs past changes in the tropical Pacific mean state across Marine Isotope Stage 3 by utilizing Mg/Ca paleothermometry on the thermocline dwelling foraminifera Neogloboquadrina dutertrei from an eastern equatorial Pacific sediment core. The thermocline temperature record reveals interstadials are characterized by a more El Niño-like mean state, with increases in thermocline temperatures up to 6°C. Thermocline warming events are more pronounced from 64-44 kyr when global climate was in a warmer state. From 44-32 kyr, the record shows cooler thermocline temperatures, suggesting a shift to a more La Niña-like mean state, as climate began transitioning into the LGM. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/155637 |
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