Palaeoceanographic significance of diatom seasonal laminations in the Upper Miocene Pisco formation (Ica Desert, Peru): Clues on past ‘El Padre’ climate mean state

We conducted a study of diatom seasonal laminations by means of scanning electron microscopy (SEM), to unravel the past ocean dynamics recorded by the diatomitic portion of the upper Miocene Pisco formation, at the site of Cerro Los Quesos (CLQ, Ica Desert, Peru). Field campaigns focused on CLQ due to the presence of datable tephra layers. Here, we collected a 25 cm long laminated diatomitic sample (CLQ20), its age being between 6.93±0.09 Ma and ≥6.71±0.02 Ma. The sea surface temperatures (SSTS) of this time frame have been analysed by other authors at IODP sites located in the Eastern Equatorial Pacific, pointing to an increase of SSTS during the deposition of the CLQ diatomites. These data surprised us: Diatomites are usually associated to cold waters. We solved this contrast when we observed the dominance of Coscinodiscus laminae in CLQ20. Coscinodiscus belongs to the ‘shade flora’, diatom taxa that thrive at the thermocline at low light conditions. Higher SST would have implied a position of the thermocline deeper-than-normal (thus resulting in the dominance of Coscinodiscus) and a smaller temperature gradient between the Eastern Equatorial and the West Pacific; nowadays these conditions happen during El Niño events. Although sample CLQ20 represents a small interval of the CLQ diatomites, the constant dominance of Coscinodiscus along them was found in previous studies. This implies that such climatic setting persisted during their entire deposition. Some authors recognized this mean state in the middle Pliocene warm period (MPWP) analysing geochemical proxies in core sediments and called it the ‘El Padre State’. Yet, the existence of such state lacked a confirmation by other proxies. The evidence raised by Coscinodiscus laminae, together with the scarcity of upwelling diatom genus, during a time frame as warm as the middle Pliocene, are the proof needed. They confirm that the ‘El Padre State’ may have characterized the MPWP and that it has played a role in the evolution of the Equatorial Pacific already in the late Miocene.