Investigating Bering Sea oceanographic response to the Milankovitch orbital cycle climatic shift during the middle Pleistocene

The transition of Earth’s glacial-interglacial cycles from 41 kyr to 100 kyr periodicity during the middle Pleistocene (the Mid-Pleistocene Transition (MPT); ~1.2–0.6 Ma) marks one of the largest climate events of the Cenozoic, but the causes of this cooling transition remain unclear, as the emergence of the 100 kyr Milankovitch orbital ‘eccentricity’ in climate records occurred without a long term change in external orbital forcing. Hypotheses for this transition have so far remained largely untested due to a lack of detailed, high resolution climate proxy information from critical regions on the planet. Major hypotheses infer changes to North American Ice Sheet dynamics, an early expansion of subpolar sea ice, and decreasing atmospheric CO2. Using sediment geochemistry and palaeontological proxies, this thesis assesses how the variability in sea ice, nutrient upwelling and primary productivity in the Northern Bering Sea impacted regional and global climate through the MPT, via their impact on North Pacific Intermediate Water expansion, regional carbon cycling and the subpolar biological pump. Through calculation of a semi-quantitative nutrient upwelling index (based on nitrogen isotopes and opal accumulation), key findings of this thesis indicate that sea ice played a dominant role on orbital scale variability in nutrient upwelling at the Bering slope, following global changes in atmospheric pCO2, continental ice sheet accumulation and sea level fluctuations. This is supported by fossil diatom assemblages which distinguish how sea ice dynamics evolved through the MPT, including high resolution variability in response to atmospheric teleconnections in the early Pleistocene. Principally, results support the notion that enhanced glacial formation of NPIW since the 900 kyr event (0.9 Ma) acted to cause region-wide suppression of deep water CO2 ventilation in the subarctic Pacific Ocean. Preliminary assessment of diatom silicon isotopes also suggests that sea ice delivery of iron, in combination with changes to nutrient cycling, may have additionally contributed to lowering glacial pCO2 which promoted increased duration of post-MPT glacials. Overall, this thesis calls for increased attention to subarctic Pacific palaeoceanography in Quaternary climate studies