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Climate and Anthropogenic Controls of Coastal Deoxygenation on Interannual to Centennial Timescales

Understanding dissolved oxygen variability in the ocean is limited by the short duration of direct measurements; however, sedimentary oxidation‐reduction reactions can provide context for modern observations. Here we use bulk sediment redox‐sensitive metal enrichment factors (MoEF, ReEF, and UEF) and scanning X‐ray fluorescence records to examine annual‐scale sedimentary oxygen concentrations in the Santa Barbara Basin from the Industrial Revolution (Common Era ~1850) to present. Enrichments are linked to measured bottom water oxygen concentrations after 1986. We reveal gradual intensification of the coastal oxygen minimum zone (OMZ) on the southern California margin coinciding with the twentieth century anthropogenic warming trend that leads to reduced oxygen solubility and greater stratification. High‐frequency interannual oscillations become more prominent over the last three decades. These are attributed to local “flushing events” triggered by the transition from El Niño to La Niña conditions, which further amplify changes in the extratropical southern Californian OMZ.