BENTHIC FORAMINIFERAL I/CA RECORDING BOTTOM WATER OXYGENATION CONDITIONS IN THE LATE PALEOCENE-EARLY EOCENE OCEANS

Widespread oxygen depletion in the oceans’ upper and bottom waters is predicted to occur with future global warming, and may have occurred during past Greenhouse Earth times. We use a novel proxy, I/Ca in benthic foraminiferal tests, to reconstruct bottom and pore water redox conditions in the upper Paleocene-lower Eocene, and compare our data with other redox proxies (Mn speciation; Ce anomaly in fossil fish teeth), which we contrasted with the projections of Earth system models for this time period. The benthic foraminiferal I/Ca records across the PETM at open ocean sites in the SE Atlantic, Pacific and Indian Oceans suggest that benthic I/Ca values over a range of bathymetric and redox conditions can be used as a proxy for redox conditions in bottom waters (epifaunal species) and pore waters (infaunal species).

The deepest site studied (Site 1262, Walvis Ridge, SE Atlantic) has higher I/Ca than all other sites. All proxy data suggest that bottom water and shallow pore waters at that site were oxygenated over most of the time interval studied (potentially with an exception for the Paleocene Eocene Thermal Maximum interval over which no calcite has been preserved). In contrast, data from other locations could indicate widespread and long-term oxygen depletion. The bottom waters at intermediate depths at Walvis Ridge, the Southern Ocean and Southern Indian Ocean may be affected by the presence of an Oxygen Minimum Zone. The infaunal I/Ca data are consistent with overall less oxygenated bottom and pore waters. However, epi-benthic I/Ca data do not indicate the presence of anoxic waters at any site. Reconstruction of oxygen levels in bottom waters by proxy data will be compared with values reconstructed by climate models.

Average epifaunal I/Ca values in upper Paleocene-lower Eocene at all sites including those bathed in low O₂ waters are higher than the values for modern, well-oxygenated, open ocean locations, which suggests that total iodine concentrations in Paleocene-Eocene seawater were significantly higher than at present, even considering potential higher uptake efficiencies in N. truempyi. We conclude that the high seawater total iodine concentrations may have been caused by decreased global iodine burial or iodine release from allochthonous sources.