Ocean and climate change in the Northeast Pacific through the Holocene
Anthropogenic climate change is altering global biogeochemical cycles and climate processes with critical implications for marine and terrestrial ecosystems and human communities. Investigation of climate records from the Holocene (11.75 ka to present) provides critical insights into contextualizing modern climate change and understanding biogeochemical and ecosystem responses to environmental change. Here, I investigated changes in marine oxygenation along the Southern California margin and explored Northeast Pacific marine-terrestrial interactions throughout the Holocene. Through investigation of marine sediment archives, specifically benthic foraminifera, I quantitatively refined oxygenation proxies, examined changes in the oxygen minimum zone (OMZ) in past 1.5 ka, and reconstructed oxygenation of waters below 1000 m throughout the Holocene. In the first chapter, I conducted paired spatial and temporal analysis of benthic foraminiferal assemblages across a modern oxygen gradient on the San Diego margin. Results show that modern oxygenation gradients structure foraminiferal species distributions, shell size, and diversity. Centennial-scale variability in the upper margin of the OMZ occurred in the past 1.5 ka, while the core of the OMZ and below the OMZ remained stable. A recent expansion of the OMZ and decline in oxygenation at 528 m in the last 400 years on the San Diego margin is synchronous with regional records of deoxygenation. In the second chapter, I analyzed oxygen and carbon stable isotope records from planktic and benthic foraminifera and benthic foraminiferal assemblages from three silled basins in the Southern California Borderlands through the Holocene to constrain timing and implications of oxygenation and ventilation change across depth. Reconstructed oxygenation at depth (from 1100-1800m) was consistently within a range of intermediate hypoxia (0.5-1.5 ml L-1 [O2]) through the Holocene and had reduced variability relative to shallower basins. To contextualize these marine changes within a linked marine-terrestrial system, in my third chapter I conducted a systematic review of Holocene climate and oceanography of Western North America and the Northeast Pacific. Findings reveal that the early Holocene is characterized by warming relative to pre-Holocene conditions, including warm sea surface conditions, a warm and dry Pacific Northwest, a warm and wet Southwest, and overall spatial and temporal stability. In the mid Holocene, these patterns reverse; this interval is characterized by cool sea surface temperatures, a cool and wet Pacific Northwest and warm and dry Southwest. The late Holocene is the most variable interval spatially and temporally, and a new spatial trend appears characterized by warmer coastal areas and cooler inland areas. This synthesis of climate and oceanographic processes and resultant impacts on ecosystems and human communities provides a window into understanding modern patterns of climate and environmental change in Western North America.