Unraveling the palaeoecology of Paralia sulcata-dominated assemblages from Late Miocene marine deposits of Patagonia (South Atlantic, Argentina)

The study of fossil diatom assemblages constitutes an effective tool for inferring past water conditions. Several studies from around the world suggest the Miocene was a key period in the evolutionary history of diatoms. However, Miocene diatom assemblages from the southwestern Atlantic Ocean are nearly unknown. This study aims to provide a palaeoenvironmental interpretation of the analysed succession. Additionally, to highlight temporal and regional changes, the assemblage is compared with known modern and fossil assemblages from Patagonia and other regions of the world. Finally, we provide a detailed description of the extinct diatom taxa recognised throughout the sedimentary succession. A total of 36 diatom taxa were recognised in the 25 analysed samples. Despite the evidence from sedimentology and macro-palaeontology indicating temporal changes from inner shelf to estuarine environments, the diatom assemblages are dominated by the coastal marine Paralia sulcata sensu lato throughout the succession. An important freshwater input into the coastal system is interpreted from our results reflecting more humid climatic conditions when compared with modern diatom assemblages from the same location. In addition, four extinct species were recognised: Thalassiosira cf. yabei, Pseudopodosira westii, Rhaphoneis diamantella, and Lancineis rectilatus. The presence of R. diamantella and L. rectilatus in the analysed deposits would imply an expansion of their geological range. •First diatom analysis for the Late Miocene Puerto Madryn Formation.•P. sulcata (s.l.) is the dominant taxon.•Diatom assemblages suggest freshwater supply to the inner shelf.•R. diamantella and L. rectilatus imply an expansion of their age ranges.