Changes in Southern Ocean bottom water environments associated with the Middle Eocene Climatic Optimum (MECO)

• We investigate benthic foraminiferal assemblages and Ce/Ce* in the Southern Ocean.
• Benthic foraminifera show a significant faunal turnover across the MECO warming.
• This shift is interpreted as an increase in productivity from 40.40 to 39.95 Ma.
• Our data suggest an interval of bottom-water oxygen depletion at peak MECO warming.
• Oxygen depletion is proposed to reflect influence of a North pacific water mass.

The Middle Eocene Climatic Optimum (MECO) was a ~ 650 kyr long interval of pronounced warmth superimposed on the long-term Eocene cooling trend. We investigate benthic foraminiferal assemblage and fossil fish tooth cerium anomaly variability from Ocean Drilling Program (ODP) Site 738 in the Indian sector of the Southern Ocean (Southern Kerguelen Plateau) to assess changes in bottom water chemistry and effects on benthic foraminiferal ecosystems. The studied section spans 41.9 to 38.4 Ma, encompassing the MECO as well as extensive Pre-MECO and Post-MECO intervals. We report a marked faunal turnover at 40.4 Ma, 200 kyrs after the onset of the MECO at ~ 40.6 Ma, from an assemblage dominated by epifaunal benthic foraminifera to an infaunal-dominated assemblage. The infaunal domination persisted until the termination of the MECO at 39.95 Ma. Thereafter, the Pre-MECO assemblage returned. This change is attributed to an increase in export productivity associated with the warming, possibly caused by an increase in continental runoff or the influence of a more eutrophic surface water mass. Superimposed on the change in tropic state the Ce/Ce* record shows a positive excursion during the 100 kyrs long MECO peak warming at around 40.05 Ma, indicative of a decrease in bottom-water oxygen content. Synchronously, infaunal benthic foraminifera drop briefly in abundance. Hence, we suggest that the peak warming is associated with bottom water oxygen depletion potentially caused by elevated productivity and/or a transient influence of an older, oxygen-depleted water mass.