Ecological and Environmental Stability in Offshore Southern California Marine Basins Through the Holocene

In the face of ongoing marine deoxygenation, understanding timescales and drivers of past oxygenation change is of critical importance. Marine sediment cores from tiered silled basins provide a natural laboratory to constrain timing and implications of oxygenation changes across multiple depths. Here, we reconstruct oxygenation and environmental change over time using benthic foraminiferal assemblages from sediment cores from three basins across the Southern California Borderlands: Tanner Basin (EW9504‐09PC, 1,194 m water depth), San Nicolas Basin (EW9504‐08PC, 1,442 m), and San Clemente Basin (EW9504‐05PC,1,818 m). We utilize indicator taxa, community ecology, and an oxygenation transfer function to reconstruct past oxygenation, and we directly compare reconstructed dissolved oxygen to modern measured dissolved oxygen. We generate new, higher resolution carbon and oxygen isotope records from planktic (Globigerina bulloides) and benthic foraminifera (Cibicides mckannai) from Tanner Basin. Geochemical and assemblage data indicate limited ecological and environmental change through time in each basin across the intervals studied. Early to mid‐Holocene (11.0–4.7 ka) oxygenation below 1,400 m (San Clemente and San Nicolas) was relatively stable and reduced relative to modern. San Nicolas Basin experienced a multi‐centennial oxygenation episode from 4.7 to 4.3 ka and oxygenation increased in Tanner Basin gradually from 1.7 to 0.8 ka. Yet across all three depths and time intervals studied, dissolved oxygen is consistently within a range of intermediate hypoxia (0.5–1.5 ml L−1 [O2]). Variance in reconstructed dissolved oxygen was similar to decadal variance in modern dissolved oxygen and reduced relative to Holocene‐scale changes in shallower basins. Plain Language Summary Globally, marine oxygenation is declining with detrimental impacts to ecosystems and economies. To better understand the drivers and consequences of ocean oxygen change, we can examine the fossil record to identify how oxygenation changed in the past. Specifically, we use the relative abundance and chemistry of microfossils (i.e., foraminifera) to reconstruct past oxygenation. Here, we examined microfossils from three sediment cores in three basins (Tanner, San Nicolas, San Clemente) off the coast of Southern California. Marine dissolved oxygen (below 1,400 m water depth) was relatively stable and lower than modern from 11,000 to 4,700 years before present. San Nicolas Basin experienced a multi