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 change over time using benthic
foraminiferal assemblages from three sediment cores, EW9504-09PC (Tanner
Basin, 1194 m water depth), EW9504-08PC (San Nicolas Basin, 1442 m), and
EW9504-05PC (San Clemente Basin, 1818 m) across the Southern California
Borderlands. We utilize indicator taxa, community ecology, and an
oxygenation transfer function to reconstruct past oxygenation, and
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. Early to
mid-Holocene (11.2-4.7 ka) oxygenation below 1400 m (San Clemente and
San Nicolas Basins) was relatively stable and reduced relative to
modern. San Nicolas Basin experienced a multi-centennial oxygenation
episode from 4.7-4.2 ka and oxygenation increased in Tanner Basin
gradually from 5.5-1.9 ka. Oxygenation of these offshore basins is
synchronous with an increase in oxygenation in Santa Barbara Basin at
~6 ka and may be due to increased oxygenation of North
Pacific Intermediate Water. 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.