Expansion of the Southern California oxygen minimum zone during the early-to mid-Holocene due to reduced ventilation of the Northeast Pacific

Abstract Oceanic deoxygenation observed since the 1950s has drawn increasing concern as dissolved oxygen (DO) regulates marine biodiversity and biogeochemical cycles. Yet the DO time series from direct instrumental measurements is insufficiently long (several decades) to constrain the centennial to millennial variability. Here we present redox-sensitive metal (Re, U, and Mo) records from Santa Barbara Basin (SBB), California to reconstruct subsurface DO through the Holocene. Significantly higher authigenic metal fluxes and coherent benthic foraminifera faunal responses along a published depth transect reveal vertical expansion and intensification of the Southern California oxygen minimum zone (OMZ) prior to ∼6 ka. Comparison with post-Industrial trace metal records suggests low-oxygen conditions were more extreme during early to mid-Holocene (MH), highlighting the magnitude of OMZ variability in the absence of anthropogenic influences. Inconsistencies between authigenic metal fluxes and local oxygen control indicate a remote weakening of the North Pacific Intermediate Water (NPIW) formation prior to the MH. Reduced ventilation of the mid-depth North Pacific is supported by state-of-the-art Earth system model simulations with realistic MH (6 ka) forcing, highlighting the importance of brine rejection during sea ice formation in changing the buoyancy of NPIW and its basin-wide influences on North Pacific OMZ. Evidence from Holocene marine/lake records and model simulations further suggests a southward Intertropical Convergence Zone (ITCZ) migration and a consequent deepening of the Aleutian Low (AL) towards late Holocene, which favors more sea ice brine rejection. Thus, mid-depth ventilation in the North Pacific was closely associated with atmospheric circulation during the Holocene.