The coherent evolution of Miocene global temperature, ecosystems, and greenhouse gas levels

Miocene climates exhibited large swings in global ice volume and temperature. The mid-Miocene (late Burdigalian through Langhian) saw peak deglaciation of the Antarctic during Neogene time. Subsequent late Miocene cooling, which intensified in the time interval 8-6.5 Ma, set the earth on its path to the Plio-Pleistocene ice ages. Along with cooling, terrestrial ecosystem changes testify to increasing aridity and seasonality of rainfall in the subtropics in the late Miocene. But what underlying forcing explains these large transformations? Here, we present new sea surface temperature records that confirm that the mid-Miocene time was a local maximum in ocean temperatures (warmer than the early Miocene), consistent with direct and indirect inferences on the reduced cryosphere. A somewhat cooler early Miocene to latest Oligocene permitted episodic southern hemisphere ice ages. To explain the climate evolution of the last ~23 Myr, we present evidence for an underlying tectonic forcing by conducting a global, high-resolution synthesis of mid-ocean ridge spreading rates, which we take as a proxy for mantle degassing. Spreading reached its Neogene global maximum precisely during the middle Miocene, and decelerated strongly in the late Miocene to approximately modern rates. We use this spreading rate history to force a simple carbon cycle model that couples degassing, silicate weathering, global temperature and the marine carbon cycle. The model generates predictions of CO₂ levels in the range of 4-5 X pre-industrial for the middle Miocene, with a strong decline in late Miocene time in parallel with the expansion of C4 ecosystems on land. The inferred high levels of greenhouse forcing are in conflict with most proxy estimates for mid-Miocene CO₂levels, but concordant with important and robust constraints on temperature and ice volume at this time.