Studying West African Monsoon (WAM) dynamics during past warm time periods such as the Miocene Climatic Optimum (MCO; ~16.9 – 14.5 Ma) could greatly aid in better projecting monsoon intensity, in the context of future greenhouse warming. However, studies on regional MCO temperature change and its effect on the WAM system during this time period are lacking. Here, we reconstruct a ~1.5 °C rise in sea surface temperature (SST) at the onset of the MCO (16.9 Ma) in the eastern equatorial Atlantic Ocean, at Ocean Drilling Program (ODP) Site 959, based on TEX86 paleothermometry, accompanied by a transient increase in %total organic carbon (TOC). Prior to the MCO (~18.3 – 17.7 Ma), sediment composition, geochemical proxy data as well as dinoflagellate cyst assemblages imply a productive surface ocean at Site 959. Immediately following the MCO onset (~16.9 – 16.5 Ma), we record an intensification of the WAM characterized by higher amplitude variability in all proxy records on precession to obliquity timescales. We interpret increased orbital-scale SST variability to represent intensification of equatorial upwelling, forced by the WAM strength. Furthermore, higher SSTs during eccentricity maxima correlate to increased relative abundances of the warm and stratification-favoring dinocyst Polysphaeridium cpx., during periods of low WAM intensity. Finally, while long-term SSTs decline towards the middle Miocene, maximum SSTs and Polysphaeridium cpx. abundances occur during MCO peak warming at ~15.6 Ma.
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Wubben et al. (2023) studied this question.