Ecological and Environmental Stability in Offshore Southern California Marine Basins Through the Holocene. Issue 8 (10th August 2022) - Palmer, Hannah M.
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. 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‐centennial oxygenation episode from 4, 700 to 4, 300 years before present and oxygenation increased in Tanner Basin gradually from 2, 000 to 800 years before present. When compared to modern, the range of values of reconstructed oxygen through the entire time studied (thousands of years) is similar to the range of values of modern oxygen at the same depths, indicating that the changes in the last 10 thousand years were similar to the amount of change occurring on annual and decadal timescales in the modern ocean. Key Points: In the Southern California Borderlands, oxygenation below 1, 400 m was stable and reduced relative to modern from 11.0 to 4.7 ka San Nicolas Basin experienced an oxygenation episode from 4.7 to 4.3 ka and oxygenation in Tanner Basin increased at 1.7 ka relative to 5.4–1.7 ka Variance in reconstructed Holocene dissolved oxygen concentration is similar to decadal scale variance in modern dissolved oxygen