Paleoceanography of the Pacific: studies of high latitude Pleistocene climate and low latitude sea surface temperatures since the late Miocene
This dissertation investigates the climate system response during periods of relative warmth over the last 10 million years (Myr) at different temporal scales: thousand-year (i.e., orbital) timescales during the warmer glacial cycles of the early Pleistocene and million-year timescales during the warm mean-state of the late Miocene. The relatively warmer and less intense glacial cycles of the early Pleistocene are an ideal period to investigate what drives high latitude climate during the ice ages, which remains not entirely understood. The warm mean-state of the late Miocene is a period not currently well characterized especially in the western equatorial Pacific (WEP) therefore, we investigate how WEP climate responds as the mean-state shifts from the warm late Miocene into the Pleistocene ice ages. This dissertation is presented in three parts. Chapter 2 provides an efficient method to characterize changes in high-resolution multi-sensor track data (i.e., Gamma Ray Attenuation (GRA) bulk density) as a function of sediment composition and/or grain size. This sedimentological study focuses on the diatom-rich sediments of the high latitude Bering Sea. An interpretation of the GRA bulk density data is developed based on laser particle size and smear slide analysis. Results show that down-core variability in GRA reflects changes in the sediment packing due to changings in diatom valve abundance and fragmentation. The method presented in Chapter 2 is adapted and applied in Chapter 3, which studies the cyclicity of the Bering Sea cryosphere to test theories of what causes ice ages. We focus on the warmer early Pleistocene glacial cycles because they are shorter (40 thousand years (kyr)), less intense, and are thought to be a more straightforward response to insolation forcing than the most recent ice ages. To measure local high latitude ice variability Chapter 3 presents an orbitally resolved Ice Rafted Debris (IRD) flux record that is interpreted with help from the sedimentological method of Chapter 2, modified to interpret Natural Gamma Radiation (NGR) instead of GRA. Results suggest the record dominantly represents transport of IRD by sea ice. While spectral analysis reveals there are no 40 kyr cycles, there is significant sub-orbital variability in the record, which suggests the Bering Sea cryosphere is sensitive to local seasonal forcing. Chapters 4 evaluates the long-term (~10 Myr) evolution of sea surface temperatures (SST) in the WEP warm pool since the late Miocene. There is little data from the WEP warm pool, but over this ~10 Myr period many other regions show a long-term cooling trend. We also assess whether the tropical Pacific El Padre mean-state, characteristic of the early Pliocene, is present in the late Miocene. The SST record presented in Chapters 4 use the Mg/Ca paleothermometer by measuring trace metal ratios of planktic foraminifer Trilobatus trilobus. Results show there is long-term stability in the SSTs of the WEP warm pool, and the El Padre mean-state is present during the late Miocene. Overall, this dissertation broadly investigates the climate system response during past periods of relative warmth. Chapters 2 and 3 focus on the high-latitude Bering Sea at orbital timescales and show that Bering Sea cryosphere is sensitive to local seasonal forcing during the early Pleistocene. Chapter 4 focuses on the low-latitude WEP over million-year timescales and finds long-term stability in the WEP warm pool SSTs since the late Miocene. Synthesized, this work finds strong regional responses, since the 40 kyr cycles were not found in Chapter 3, and the long-term cooling trend was not found in Chapter 4.