| Summary: | The limited timespan (several decades) of dissolved oxygen (DO) instrumental records precludes a complete picture of how DO in seawater have changed in the past, and thus high-resolution paleo-DO reconstructions are essential for revealing DO variability on centennial to millennial timescales. A rapid and simultaneous bulk sediment elemental analysis method for determining multiple elemental concentrations was thus developed for future high-resolution paleoceanographic reconstructions in Chapter 2. To identify appropriate redox proxies minimizing post-depositional overprints, we applied Fe speciation to quantify reactive Fe towards sulfide. The results were cross-validated using magnetic analyses that characterize preserved Fe phases. We confirmed in Chapter 3 that instantaneous depositional events (e.g., flood and turbidite layers) could initiate post-depositional pyrite formation, and result in ‘false-positive’ sulfidic (no DO and HS- present) interpretations, compromising redox reconstructions. In contrast, redox-sensitive trace metals (Re, Mo, and U) seem to be unaffected by instantaneous depositional events. A post-Industrial oxygenation history was then constructed from redox-sensitive trace metal records from the Santa Barbara Basin (SBB), Southern California in Chapter 4. We demonstrated that the gradually intensified Southern California OMZ since the 20th century is associated with the anthropogenic warming. To further explore the natural variability on multi-centennial to millennial timescales, I explored a Common Era core (~2 – 5 year resolution) in SBB. Bulk sedimentary N isotopic composition was reconstructed in Chapter 5, which revealed competing tropical and subarctic waters that transport nitrate to Southern California. We confirmed more tropical water influences during the Medieval Climate Anomaly (MCA, 1000 – 1100 CE). However, stronger connections with the subarctic water transport during the Little Ice Age (LIA, 1670 – 1840 CE) coincided with the strongest Aleutian Low (AL), suggesting ...
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