Congratulations to Yi Wang who defended her dissertation on Friday, March 13, 2020
Advisor: Ingrid Hendy
The limited timespan (several decades) of modern dissolved oxygen (DO) instrumental records precludes identification of potential extremes or full magnitude of historical oxygenation change, and thus high-resolution paleo-DO reconstructions are essential for revealing DO variability on centennial to millennial timescales. A rapid bulk sediment elemental analysis method was thus developed in Chapter 2 for simultaneous determination of major, minor, and trace element concentrations, which can be applied to future high-resolution paleoceanographic reconstructions. To identify appropriate redox proxies that minimize post-depositional overprints, I applied Fe speciation that quantifies Fe in terms of reactivity toward sulfide (e.g., pyrite that is representative of low O2 and high sulfide [HS-] conditions). The results were cross-validated using magnetic analyses that characterize Fe minerals preserved. 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 oxygen with HS- presence) water columns that compromise redox reconstructions. In contrast redox-sensitive trace metals seem to be unaffected by instantaneous depositional events. After validation of redox proxy responses in a modern environment, redox-sensitive trace metals are then used in the paleo-environments to reveal past oxygenation changes and forcing mechanisms. A post-Industrial Revolution oxygenation history was constructed in Chapter 4, using a trace metal record from the Santa Barbara Basin (SBB) in Southern California. We demonstrated that decreased oxygen concentrations since the 20th century is associated with the anthropogenic warming trend.
Intrigued by longer time scale variability (multicentennial to millennial), 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 water masses that transport nitrate to Southern California. We confirmed more tropical water influences during the Medieval Climate Anomaly (MCA, 1000 – 1100 CE) yet stronger connections with the subarctic water transport during the Little Ice Age (LIA, 1670 – 1840 CE) that coincided with the strongest Aleutian Low (AL), suggesting responses of oceanic nitrogen cycling to remote atmospheric forcing. In Chapter 6, we generated the highest-solution DO record (4 – 9 year resolution) to date in the North Pacific using the same core. We revealed the lowest DO during the MCA, corresponding to the warmest interval in the Common Era. The deoxygenation rates were suggested to be significantly faster than the post-Industrial reconstruction in Chapter 4. However, a low-oxygen interval during the LIA (1550-1750 CE) was inconsistent with either a colder climate or productivity. We suggest that this low oxygen interval was due to less ventilated North Pacific Intermediate Water (NPIW) that forms in the northwest Pacific and propagates to the east. A comparison of proxies and the Last Millennium Reanalysis corroborates that a weaker/westward AL and weakened Siberian High resulted in weaker wind stress over the Sea of Okhotsk, reducing the sea ice production and NPIW ventilation.
A similar oxygenation response to atmospheric teleconnections was indicated for Holocene in Chapter 7, which indicates much lower DO in the early to mid Holocene (~6 ka before present) than post-Industrial. A state-of-the-art model corroborates basin-wide low DO responses due to NPIW oxygenation changes, which is associated with reduced sea ice brine rejection tying to a weakened AL under mid-Holocene orbital forcing. We thus suggest a coupled oceanic response in North Pacific (e.g., mid-depth ventilation) to atmospheric circulation changes in Holocene.