| Summary: | The goal of my dissertation has been to test hypotheses for the origins of high-latitude climatic warmth and the subsequent global cooling in the middle Miocene, aimed primarily at understanding the oceanographic evolution of the Pacific realm. I attempted to constrain the extent and magnitude of the middle Miocene climatic optimum (MMCO; ca. 14.5-17 Ma) using oxygen and carbon stable isotopes of planktic and benthic foraminifers and nannofossils from deep-sea sites in the high-latitude South and North Pacific (ODP Sites 883, 1170, and 1172) and mollusk fossils from an onshore section in Kodiak Island, Gulf of Alaska. At the subpolar North Pacific ODP 883, the d18O record suggests that the near-surface waters during the MMCO in the site were warmer than present by ~7-8ºC. Furthermore, the d18O data of fossil mollusks from Kodiak Island indicate that shallow-marine temperature of the northern Gulf of Alaska during the MMCO was higher than the modern sea-surface temperature by ~10ºC. These d18O-based paleotemperature estimates imply that a warm-temperate climatic regime prevailed in the subpolar North Pacific during the MMCO, in agreement with the previously reported paleoclimate reconstructions based on fossil assemblages of molluscan fauna. I speculate that the MMCO warmth in the high-latitude North Pacific was caused by increased poleward oceanic heat transport through northward redirection of tropical waters and intensified Kuroshio Current, primarily triggered by narrowing of the Indonesian Seaway in the western equatorial Pacific. In the southern high latitudes, paleotemperature reconstructions based on the d18O values of calcareous planktons at ODP Sites 1170 and 1172 off Tasmania suggest that near-surface waters in the subantarctic around the site paleo-locations were warmer than today by ~2ºC until about 14 Ma. The sea-surface temperatures were probably ameliorated by a warm current from the west that joined the circum Antarctic current. A decrease in the surface temperatures of 1~2ºC is recognized between ca. 14.5 and 12.5 Ma, offering direct evidence of the hypothesized middle Miocene cooling in the southern high latitudes in association with the intensified Antarctic circumpolar current. The benthic foraminiferal d13C record from Sites 883, 1170 and 1172, in conjunction with published records from other sites, suggest that northward advection of deepwater in the western Pacific persisted at least through the late early-middle Miocene.
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