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, 1194 m water depth), San Nicolas Basin (EW9504-08PC, 1442 m), and San Clemente Basin (EW9504-05PC ,1818 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 1400 m (San Clemente and San Nicolas) was relatively stable and reduced relative to modern. San Nicolas Basin experienced a multi-centennial oxygenation episode from 4.7-4.3 ka and oxygenation increased in Tanner Basin gradually from 1.7-0.8 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.

The prevailing hypothesis to explain pCO2 rise at the last glacial termination calls upon enhanced ventilation of excess respired carbon that accumulated in the deep sea during the glacial. Recent studies argue lower [O2] in the glacial ocean is indicative of increased carbon respiration. The magnitude of [O2] depletion was 100-140µmol/kg at the glacial maximum. Because respiration is coupled to d13C of dissolved inorganic carbon (DIC), [O2] depletion of 100-140µmol/kg from carbon respiration would lower deep water d13CDIC by ~ 1‰ relative to surface water. Prolonged sequestration of respired carbon would also lower the amount of 14C in the deep sea. We show that Pacific Deep Water d13CDIC did not decrease relative to the surface ocean and D14C was only ~50‰ lower during the late glacial. Model simulations of the hypothesized ventilation change during deglaciation lead to large increases in d13CDIC, D14C and ε14C that are not recorded in observations.