Flat meridional temperature gradient in the early Eocene in the subsurface rather than surface ocean

The early Eocene (49-55 million years ago) is a time interval characterized by elevated surface temperatures and atmospheric CO2 (refs 1,2), and a flatter-than-present latitudinal surface temperature gradient(3,4). The multi-proxy-derived flat temperature gradient has been a challenge to reproduce in model simulations(5-7), especially the subtropical warmth at the high-latitude surface oceans(4,8), inferred from the archaeal lipid-based palaeothermometry, TEX86H. Here we revisit the TEX86H interpretation by analysing a global collection of multiproxy temperature estimates from sediment cores spanning millennia to millions of years. Comparing the variability between proxy types, we demonstrate that the present TEX86H interpretation(9) overestimates the magnitude of past climate changes on all timescales. We attribute this to an inappropriate calibration, which reflects subsurface ocean but is calibrated to the sea surface, where the latitudinal temperature gradient is steeper. Recalibrating the proxy to the temperatures of subsurface ocean, where the signal is probably formed, yields colder TEX86H -temperatures and latitudinal gradient consistent with standard climate model simulations of the Eocene climate(10), invalidating the apparent, extremely warm polar sea surface temperatures. We conclude that there is a need to reinterpret TEX86H -inferred marine temperature records in the literature, especially for reconstructions of past warm climates that rely heavily on this proxy as reflecting subsurface ocean.