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The evolution of North Pacific climate since the late Miocene


A lack of paleo-proxy data has made it difficult to determine the mechanisms responsible for the warm climates of the late Miocene and early Pliocene. Though modeling work suggests that the global distribution of sea surface temperatures (SSTs) and the depth of the global thermocline may have been important to maintaining past warm climates, paleoceanographic data is needed to test these ideas. This dissertation uses geochemical proxy reconstructions of SST, thermocline depth, and precipitation from North Pacific sediments to test the role that the oceans played in shaping late Miocene and Pliocene climates. Chapter 2 uses alkenone paleothermometry and d18O of planktonic foraminifera to show that the SSTs of the mid-latitude North Pacific ~13-5 Ma were decoupled from atmospheric pCO2, possibly due to a deeper-than-modern global thermocline. The data indicates that thermocline shoaling after ~5 Ma, possibly related to changes in ocean basin shape, led to a stronger coupling between SSTs and atmospheric pCO2 in more recent epochs. Chapter 3 uses new alkenone and Mg/Ca SSTs with published SST estimates to test the importance of SST gradients in the warm late Miocene and early Pliocene. Kriging of SST data was used to construct maps of late Miocene, Pliocene, and Pleistocene time-slices. The results, which show that basin-wide SST gradients were weak (relative to modern) in the early Pliocene but even weaker during the late Miocene, support the hypothesis that weak SST gradients contributed to warm global temperatures. Chapter 4 assesses the utility of the dDn-alkanes in marine sediments as a proxy of precipitation on the continents. Ground-truthing of California Margin core tops indicated that dDn-alkanes from southern California Margin sediments are suitable for down-core reconstructions of past precipitation. Additionally, a low-resolution dDn-alkanes record generated for this study showed that the establishment of the modern mediterranean climate in southern California might have resulted from the SST changes described in Chapters 2 and 3. Overall, this dissertation shows that reconstructions of past oceanographic and climate conditions of the North Pacific support the hypothesis that SST gradients played a major role in shaping the warm climates of the early Pliocene and late Miocene.

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