Role of climate and tectonism on the modernization of Patagonian floras: Evidence from the fossil record

Abstract

A major reorganization of Patagonian ecosystems occurred when permanent ice sheets appeared in Antarctica and the Andean Range uplifted in southern South America. Rich plant communities dominated by Gondwanan trees and other species with tropical modern distribution vanished and those with cool and arid distributions rose to prominence. Rather than a single event, this landscape-level floristic replacement may have occurred as pulses following the paleoclimatic dynamism of the Coolhouse state (~34–4 Ma). However, the associated shifts in diversity and in climatically-key sensitive plant species have never been studied in detail so far. Here, we estimated richness using robust (non-parametric) methods and selected climatically-sensitive species to trace back past floristic trends in the high southern latitudes during the Coolhouse state. We used a high resolution palynological record from well-constrained Patagonian sediments from the Early Oligocene to the Late Miocene (~34–10 Ma), spanning most of the Coolhouse state. Our fossil data reveal the presence of five major floristic phases, closely linked with global climate states and regional orographic events. We found that diversity rises during global warming periods (↑100% at the Late Oligocene Warming Event or LOWE and ↑50% at the Miocene Climatic Optimum or MCO) and drops during glaciation periods (↓60% at Oi-1 (earliest Oligocene), ↓50% at Mi-1 (earliest Miocene) and ↓35% at Mi-3 (Middle Miocene) glaciation events) or intense tectonism (↓60% after the Andean uplift), relative to background levels. The representation of warm-demanding taxa peaked during LOWE and MCO, while arid-adapted taxa rose briefly during the MCO and took over following the major surface uplift of the southern Andes by 15–14 Ma. Humid-demanding species (e.g., tree-ferns) showed a general decreasing trend in abundance from their climax at 34 Ma to their minimum occurrence at 10 Ma. Our study reveals the potential of using robust statistical diversity methods in unraveling the relationship between past floras and climatic-orogenic forcing during the Coolhouse state.