Expanded oxygen minimum zones during the late Paleocene-early Eocene: hints from multi-proxy comparison and ocean modelling

Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between de-oxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore-water redox conditions in the South Atlantic and Southern Indian Oceans, and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5-2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low oxygen sites, than at well-oxygenated modern sites, indicate higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (CCSM3), but the simulated absolute dissolved O2 values are low (<~35 µmol/kg), while higher O2 values (~60-100 µmol/kg) were obtained in an Earth system model (cGENIE). Multi-proxy data together with improvements in boundary conditions and model parameterization are necessary if the details of past oceanographic oxygenation are to be resolved.