Southern ocean temperatures and CO2 across the onset of the Middle Miocene Climatic Optimum

The Middle Miocene Climatic Optimum (MMCO, ∼16.9 - 14.7 Ma), is marked by lower δ¹⁸O values in bulk and benthic suggesting a warming and/or reduction in land ice. CO₂, due to its climatic influence, is suspected to be a key driver of this temperature shift. However, the climate and CO₂ during the MMCO was highly variable in some records. The limited number of high-resolution temperature and CO₂ reconstructions for this interval makes it difficult to characterize the temperature and CO₂ change across the MMCO onset. pCO₂ levels can be estimated using the fractionation factor (ε_p) of coccolithophores when they photosynthesize and produce alkenones as their biomass. We present a high-resolution record of sea surface temperature (SST) and ε_p values for 16 to 19.7 Ma, derived from alkenones at IODP Site 1168 in the Southern Ocean, providing insights into the onset of the MMCO. The data reveals an increase in temperature of approximately 4°C, from 23°C to 27°C, across the MMCO δ¹⁸O shift. There is a modest positive correlation observed between SST and bulk δ¹⁸O values. In addition, δ¹⁸O_sw trends, calculated from alkenone temperature and coccolith δ¹⁸O, indicate freshening between period 16.5 to 17.5 Ma, consistent with the expected retreat of the Antarctic ice sheet during the MMCO. Orbital scale temperature variations of 2 degrees can be observed. Further analysis of ε_p values will help reconstruct the contribution of CO₂ to this climatic transition.