Annual to Millennial Scale Oxygen Minimum Zone Expansion on the Southern California Margin: Proxies and Drivers

Abstract

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 nitrogen cycling responses to atmospheric forcing. In Chapter 6, we generated the highest-resolution DO record (4 – 9 years) to date in the North Pacific using the same core, which revealed the lowest DO during the MCA corresponding to the warmest pre-Industrial interval in the Common Era. The natural OMZ deoxygenation rates were significantly faster than the post-Industrial reconstruction in Chapter 4, suggesting that the post-Industrial OMZ intensification is not unprecedented. Additionally, a low-oxygen interval during the cold LIA (1550 – 1750 CE) was inconsistent with either climate or productivity. We suggest that this low-oxygen interval was due to less ventilated North Pacific Intermediate Water (NPIW) that propagates to the east. A comparison of proxies (sea ice, oxygenation, and AL proxies) and the Last Millennium Reanalysis corroborates that a weaker/westward AL and weakened Siberian High resulted in weaker wind stress and prevailing easterlies over the Sea of Okhotsk, reducing the sea ice production and NPIW ventilation despite the relatively cold period. A similar oxygenation response to atmospheric teleconnections was indicated for the Holocene in Chapter 7, with much lower DO in the early- to mid-Holocene (~6 ka before present) than the post-Industrial. A state-of-the-art model corroborates basin-wide low DO responses due to NPIW oxygenation changes that are associated with reduced sea ice brine rejection tying to a weakened AL under mid-Holocene orbital forcing. These results demonstrate a nuanced mid-depth ventilation and OMZ response to climate change in the Holocene, and underline the role of atmospheric circulation in controlling natural OMZ variations. The dissertation thus calls for additional high-resolution reconstructions to constrain the natural OMZ variability, especially in the background of anthropogenic climate change.