Abyssal record of the Middle Eocene Climatic Optimum in the Tasman Sea: Insights from benthic foraminiferal and clay mineral assemblages

The Middle Eocene Climatic Optimum (MECO), a ∼500 kyr global warm interval at ∼40 Ma, interrupted the gradual cooling trend of the mid-late Eocene. Unlike Eocene hyperthermal events with rapid onsets and slow recoveries, the MECO began gradually, ended swiftly, and lacked a global negative carbon isotope excursion, raising questions about warming‑carbon cycling links. Here we report for the first time the response of benthic foraminifera, which are excellent palaeoenvironmental proxies, across the MECO at abyssal depths, integrating our results with a mineralogical analysis of the sediment. International Ocean Discovery Program Site U1511 (Tasman Abyssal Plain, SW Pacific) was deposited below the carbonate compensation depth during the Eocene, and calcareous microfossils are absent. Agglutinated benthic foraminifera indicate a gradual onset of the environmental perturbations associated with the MECO, followed by rapid recovery. Changes in their assemblages, including the temporary disappearance of Lazarus taxa, the decreased abundance of suspension feeders and dominance of opportunistic detritivores, indicate weaker bottom-water currents during MECO and increased stratification of the water column. Mineralogical changes reinforce this interpretation, with increased smectite content indicating warm, humid conditions in the source-area, and possibly a change in the deep-water source. The reappearance of Lazarus taxa after the MECO indicates rapid recovery of deep-sea environmental conditions. The comparison of Site U1511 with available studies from other regions reveals the complex and regionally diverse nature of benthic foraminiferal response to the MECO, emphasizing the critical role of ocean circulation and palaeogeography during Eocene warm intervals.