Plant Proxy Evidence for High Rainfall and Productivity in the Eocene of Australia

During the early to middle Eocene, a mid‐to‐high latitudinal position and enhanced hydrological cycle in Australia would have contributed to a wetter and “greener” Australian continent where today arid to semi‐arid climates dominate. Here, we revisit 12 southern Australian plant megafossil sites from the early to middle Eocene to generate temperature, precipitation, and seasonality paleoclimate estimates, net primary productivity (NPP), and vegetation type, based on paleobotanical proxies and compare them to early Eocene global climate models. Temperature reconstructions are uniformly subtropical (mean annual, summer, and winter mean temperatures 19–21°C, 25–27°C, and 14–16°C, respectively), indicating that southern Australia was ∼5°C warmer than today, despite a >20° poleward shift from its modern geographic location. Precipitation was less homogeneous than temperature, with mean annual precipitation of ∼60 cm over inland sites and >100 cm over coastal sites. Precipitation may have been seasonal with the driest month receiving 2–7× less than the mean monthly precipitation. Proxy‐model comparison is favorable with a 1,680 ppm CO2 concentration. However, individual proxy reconstructions can disagree with models as well as with each other. In particular, seasonality reconstructions have systemic offsets. NPP estimates were higher than modern, implying a more homogenously “green” southern Australia in the early to middle Eocene when this part of Australia was at 48–64°S and larger carbon fluxes to and from the Australian biosphere. The most similar modern vegetation type is modern‐day eastern Australian subtropical forest, although the distance from coast and latitude may have led to vegetation heterogeneity. Plain Language Summary Australia today is dominated by arid environments, with sparse shrubland or grassland vegetation, and forest biomes limited to areas with abundant moisture supply. These arid environments have low primary productivity and store relatively little carbon. Examination of 12 early to middle Eocene (55–40 million years old) fossil floras shows that southern Australia at that time was much “greener.” In a globally warmer world and poleward positioning of southern Australia, southward of mid‐latitude high‐pressure systems, rainfall in now arid environments was much higher. What is now almost barren landscapes would have supported a much more luxurious forested biome. As a result, primary productivity was enhanced and on‐land carbon storag