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 (Mo-EF, Re-EF, and U-EF) and scanning X-ray fluorescence records to examine annual-scale sedimentary oxygen concentrations in the Santa Barbara Basin from the Industrial Revolution (Common Era similar to 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 Nino to La Nina conditions, which further amplify changes in the extratropical southern Californian OMZ. Plain Language Summary Because we have only been measuring the amount of oxygen in seawater for a few decades, we do not know whether humans have caused oxygen in the ocean to decrease. The chemistry of some metals in sediments records how much oxygen was present in the ocean in the past, as these metals form solids when oxygen is low. We use these metals to determine that the oxygen in the bottom water of the Santa Barbara Channel, California, has decreased since about 1850. This occurs because warmer water reduces the amount of oxygen that can dissolve with in it, while also creating a barrier that reduces contact with the oxygen-rich atmosphere. Over the last 35 years oxygen in the bottom water of the Santa Barbara Channel has briefly increased when El Nino weather conditions shift to La Nina conditions allowing cool, dense water into the depths of the channel.