UC Riverside

Accurately recreating how Oxygen Minimum Zones (OMZs) changed through time requires the combined use of classic and novel proxy approaches. Short-term proxy trends are often deemphasized in favor of the major patterns despite having the potential to capture environmental shifts in the OMZs. To address this problem decadal-to-millennial OMZ variations were reconstructed over the last 70 kyr BP in a major OMZ—the Eastern Tropical North Pacific (ETNP). Specifically, sedimentary material collected from basinal settings with diverse levels of restriction and nutrient availability in the Gulf of California were analyzed: Alfonso, La Paz, and Pescadero. This dissertation emphasizes a broad range of classic and novel paleoredox and paleoproductivity proxies (CaCO3, C-S-Fe relationships, Ba, Cd, Cu, Mo, Mn, Ni, V, Zn, as well as U concentrations and isotope ratios).

Global circulation effects on the availability of nutrients and deep-water oxygenation are the main controls for OMZ variability over glacial-interglacial time scales. Newly-generated data reveal that the ETNP OMZ achieved its peak strength during warm Marine Isotope Stages (MIS) accompanied by sea-level highstands (MIS3 and Holocene) exposed by high Mo, V, authigenic U (Uauth) and higher δ238U. Conversely, the cold sea-level lowstands (MIS4, LGM) often manifested as low Mo, V, and Fe/Al values indicating exposure of the marine basins to oxygenated waters. When it comes to submillennial variability, Heinrich events and cold Dansgaard-Oeschger phases enhanced the ventilation of the ETNP, as fingerprinted in low Mo, V, U, and lower δ238U values. The mechanisms that control decadal-to-centennial oxygen variability in the ETNP remain in the shadows but are likely influenced by solar forcing indirectly driving the intensity changes of the Pacific Walker Circulation (PWC). During the Little Ice Age, solar irradiance was at its lowest for the past millennium, which strengthened the PWC. The latter created more frequent La Niña-like conditions that enhanced upwelling of nutrient-rich waters of the west coast of North America, driving productivity and reducing bottom oxygen levels, as seen in the ETNP records.

This research brings new insights into the biogeochemistry of U in OMZs, revealing the sources and causes of its isotopic fractionation and enrichment mechanisms.