Response of the Patagonian floras to climatic cooling during Oligocene−Miocene transition and the expansion of Antarctic glaciation

Pulses of Antarctic ice sheet expansion and associated climatic deterioration are thought to have driven major shifts in the world's flora during the Cenozoic Era. However, the rarity of fossils from well-constrained sedimentary strata in high palaeolatitudes makes it difficult to explore to these vegetation shifts directly. In this study, we infer changing patterns of floristic richness through the Oligocene−Miocene cooling event (∼23 Ma), based on analysis of terrestrial palynological samples in the radiometrically-constrained sediments of the Río Guillermo Formation of southern Patagonia. Our fossil assemblages include plant families typical of modern Magellanic subpolar forests such as Nothofagaceae, Podocarpaceae, Araucariaceae, and Myrtaceae. Adjusted for sample completeness (or coverage), our floristic richness estimates are, on average, higher than those from modern subpolar forests yet considerably lower than any other Paleogene or Neogene palynological records published elsewhere from the continent. The transient Oligocene−Miocene cooling episode, whose effects were probably intensified in southern Patagonia due to its close proximity to the glaciated Antarctic Peninsula, may have contributed to the steep decline in floristic richness. We infer that most Gondwanan plant relicts that survived across the Oligocene−Miocene transition endured through subsequent Neogene climatic fluctuations and contribute to present-day Magellanic subpolar forests. Our study highlights how precisely-dated fossil assemblages and robust diversity methods can be used to track biodiversity shifts in response to past climate change.