Terrestrial paleoenvironmental reconstructions indicate transient peak warming during the early Eocene climatic optimum

Abstract

Major changes in climate and ecology occurred during the early Eocene climatic optimum, sometime between 52 and 50 Ma. Recent work suggests that the timing and duration of the event are characterized by different responses in the marine and terrestrial realms, and that traditional causal mechanisms may not adequately explain such differences. We applied high-resolution paleopedology, geochemical analysis, and phytolith biostratigraphy techniques to paleosol suites within the well-described Wind River Formation of western Wyoming, USA. This multiproxy record indicates a short (<1 m.y.) peak period of carbon isotopic enrichment (up to 2‰ higher) and elevated pCO2, high temperatures (up to 8 °C higher), increased precipitation (up to 500 mm yr –1 higher), and shifts in fl oral composition (up to 10%). Terrestrial climatic and ecological changes of this kind during the early Eocene climatic optimum are consistent with changes in contemporaneous records that have been ascribed to high atmospheric pCO2, but a transient peak interval suggests that the cause of high atmospheric pCO2 during the early Eocene was likely not increased volcanism or decreased silicate weathering, which operate on longer timescales. Instead, terrestrial records from across western North America agree that early Eocene climatic optimum changes may have been caused by other sources, such as a combination of increased ventilation of oceanic carbon and increased petroleum generation in sedimentary basins. The climatic and environmental changes exhibited by this and other North American terrestrial records also defi ne a pattern of regional response that is relevant for understanding the impacts of global climate change events.