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Abstract 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. 

Abstract. Microfossil assemblages provide valuable records to investigatevariability in continental margin biogeochemical cycles, including dynamicsof the oxygen minimum zone (OMZ). Analyses of modern assemblages acrossenvironmental gradients are necessary to understand relationships betweenassemblage characteristics and environmental factors. Five cores wereanalyzed from the San Diego margin (32∘42′00′′ N, 117∘30′00′′ W; 300–1175 m water depth) for core top benthic foraminiferalassemblages to understand relationships between community assemblages andspatial hydrographic gradients as well as for down-core benthic foraminiferalassemblages to identify changes in the OMZ through time. Comparisons ofbenthic foraminiferal assemblages from two size fractions (63–150 and>150 µm) exhibit similar trends across the spatial and environmental gradient or in some cases exhibit more pronouncedspatial trends in the >150 µm fraction. A range of speciesdiversity exists within the modern OMZ (1.910–2.586 H, Shannon index),suggesting that diversity is not driven by oxygenation alone. We identifytwo hypoxic-associated species (B. spissa and U. peregrina), one oxic-associated species (G. subglobosa) andone OMZ edge-associated species (B. argentea). Down-core analysis of indicator speciesreveals variability in the upper margin of the OMZ (528 m water depth) whilethe core of the OMZ (800 m) and below the OMZ (1175 m) remained stable inthe last 1.5 kyr. We document expansion of the upper margin of the OMZbeginning 400 BP on the San Diego margin that is synchronous with otherregional records of oxygenation.