Cool tropics in the Middle Eocene: Evidence from the Changchang Flora, Hainan Island, China

• Middle Eocene mega- and microfloral remains from Hainan Island, China, are described.
• Deposited at 22°N they reflect low latitude humid climate in a ‘hothouse’ world.
• Temperatures were surprisingly cool even when corrected for possible palaeoelevation.
• Similar results were derived from clumped isotope and organic geochemical proxies.
• Significant heat transport away from the tropics is inferred for ‘hothouse’ climates.

The middle Eocene (Lutetian–Bartonian, 48.6–37.2 Ma) near-equatorial megafossil flora from swamp and lacustrine facies of the lower Changchang Formation, Hainan Island, South China (19.631463°N, 110.445049°E) is highly diverse (> 200 taxa) dominated by an unusual mixture of angiosperms typical of modern temperate, subtropical and tropical evergreen and deciduous forms. It is also rich in palms. Multivariate analysis of the architecture of minimally transported woody dicot leaves reveals a mean annual air temperature (MAT) of ~ 22 ± 4.7 °C with a marked thermal seasonality range of ~ 21 °C. The year-round humid climate lacked any monsoonal signature. The overall climate signal is compatible with the growth characteristics exhibited by fossil wood, but is warmer than the climate signal derived from pollen and spores using Co-existence Analysis. Corrections for possible palaeoelevation of the basin bring the megafossil-derived MAT estimate in line with 54–52 Ma sea surface and soil temperatures obtained from the Gulf Coast, USA, (palaeolatitude ~ 30°N) using multiple geochemical proxies and supports the claim that the low latitude Eocene climate was not uniformly warm. This challenges previous conclusions based on ∂18O analysis of unaltered calcareous microfossils. Our air temperature data also adds to the challenge of understanding heat transport away from the equator to higher latitudes during ‘hothouse’ climate regimes.