UC Santa Cruz

The tropical Pacific is a significant component of the ocean's climate system and a source of considerable global climate variability, including El Niño. Tightly coupled sea surface and thermocline dynamics influence tropical climate on a variety of spatial and temporal scales. Records of past climate surface and subsurface temperature variability, on long and short times scales, may aid in identifying the radiative and dynamic forcing important to tropical climate during different global climate states. During the Pliocene warm period (3.0-4.3 Ma) and Last Glacial Maximum (LGM, ~0.020 Ma) the global mean state was substantially different and may have influenced the behavior and strength of various processes and mechanisms that determine tropical climate. In this dissertation, I use geochemical techniques, including the novel application of the magnesium-calcite ratio of individual planktonic foraminifera, to investigate the different processes that determine tropical change on different time scales and background climate states.

In the first project, I generated stable isotope and minor element records using a subsurface dwelling planktonic foraminifera from a transect of sites across the Pacific cold tongue. Using subsurface temperatures as a proxy for thermocline depth, my records suggest there was a gradual shoaling of the eastern equatorial Pacific thermocline from the early Pliocene to present day. In the second and third projects, I use the minor element ratio of individual surface and subsurface dwelling foraminifera to monitor the SST and subsurface temperature background state, high-resolution variability and radiative and dynamic processes that determine tropical climate. The results suggest the tropical western Pacific has responded primarily to radiative forcing on glacial-interglacial time scales since the early Pliocene. In contrast, the eastern Pacific has responded to changes in dynamic and radiative forcing during the last glacial period, which influenced the background spatial pattern of the cold tongue region. Additionally, changes in individual foraminifera variability suggest a decrease in El Niño Southern Oscillation during the most recent glacial period in comparison to the Holocene. These records show that different scales of variability (i.e. long-term, glacial-interglacial, and seasonal/interannual) may illuminate the processes and mechanisms important to paleoclimate interpretations on various temporal and spatial scales.