Abyssal record of Eocene warming in the Tasman Sea

The Middle Eocene Climatic Optimum (MECO) was a global warming period that interrupted the gradual cooling trend of the middle-late Eocene at around 40 Ma. It is characterized by high temperatures, increased pCO₂ levels in the atmosphere, and marine carbonate dissolution. Its gradual onset, rapid termination, long duration (500 kyr), and the lack of a global negative carbon isotope excursion clearly differentiate the MECO from other Eocene hyperthermals, making its study of utmost interest to understand the effects of warming on the carbon cycle. Herein, we present the first record of the benthic foraminiferal response to the MECO at abyssal depths, and we reconstruct the paleoenvironmental impact. The Eocene sediment samples from International Ocean Discovery Program Site U1511 in the Tasman Abyssal Plain (Southwest Pacific Ocean) were deposited below the CCD and contain well-preserved agglutinated benthic foraminifera. The foraminiferal assemblages show low diversity values across the whole study interval. The MECO is characterized by the dominance of the opportunistic species Spiroplectammina spectabilis, which points to environmental instability and changes in food availability, likely linked to water column stratification associated with warming. Mineralogical analyses show an increase in smectite content during the MECO, suggesting changes in deep-water sources. These findings are consistent with previously documented changes in ocean circulation and nutrient dynamics in the region during the MECO.