Palynological evidence for floristic turnover and rising diversity in the early Burdigalian of south-western Patagonia (Argentina)

During the Early Miocene (ca. 23–16 Ma), Patagonia underwent pronounced climatic reorganisations that influenced distribution patterns and biodiversity across terrestrial and marine ecosystems. Here, we explore the response of the floras from south-western Patagonia to the climatic shifts that occurred during the early Burdigalian (ca. 20 Ma–18 Ma), just before the onset of the Miocene Climatic Optimum (MCO), which took place between 17 Ma and 15 Ma. Using fossil spores and pollen grains recovered from the El Chacay Formation (south-western Patagonia), we found a ~ 50% increase in diversity from ~ 20 Ma to ~ 18 Ma. The major difference between the two ages lies in the number of rare species (singletons and doubletons) according to our non-asymptotic standardisation analysis. The increasing trend in richness mirrors the re-appearance of many plant species with predominantly tropical and subtropical current distributions (e.g. Cupania, Ilex, Arecaceae) that had severely declined from the high latitudes during the cooling trend of the Oligocene-Miocene Transition (~ 23 Ma). Overall, the recovered assemblages point to the dominance of closed-canopy forests although the record of open vegetation elements — that would later expand — became evident. Our finding reveals that plant richness increased prior to the onset of the MCO, though not reaching the peak levels observed during this climatic optimum.