Megaflora and palaeoclimate of a Late Oligocene tropical delta, Makum Coalfield, Assam: Evidence for the early development of the South Asia Monsoon

The fossil megaflora, from the late Oligocene deltaic sediments exposed in the Tirap coal mine, Assam, India (27° 17′ 20″ N, 95° 46′ 15″ E), laid down at a palaeolatitude of 10–15° N, records the composition and climate of tropical vegetation during the last episode of pronounced global warmth before the progressive overall cooling to present day conditions. Using a new calibration of the Climate Leaf Analysis Multivariate Program (CLAMP) that includes 18 sites from modern vegetation in India, the analysis of 80 fossil leaf morphotypes from the Tirap mine indicates a mean annual temperature (MAT) of 26.1 ± 2.7 °C, a warm month mean temperature (WMMT) of 27.9 ± 3.3 °C and a CMMT of 20.1 ± 4.3 °C. With adjustments for evapotranspirational cooling and underestimation of the WMMT at high temperatures these can be revised upwards to a minimum MAT of 28.3 ± 3.7 °C, a CMMT of 23 ± 5.5 °C and a WMMT of 33.6 ± 5.2 °C. Maximum estimates based on evaporative cooling seen in a modern analogue of the Tirap environment yield an MAT of 32.9 ± 3.8 °C, a CMMT of 26.8 ± 6 °C and a WMMT of 39 ± 7.1 °C. All uncertainties are 2 sigma. Precipitation estimates reveal a marked annual variation in rainfall showing a wet season with 20 times the rainfall of the dry season. This is a similar regime to that seen in the Sunderbans in the modern Ganges/Brahmaputra/Meghna delta. We interpret this to suggest that the South Asian Monsoon was already established by late Oligocene times at an intensity similar to that of today.

► Late Oligocene plant megafossils from the Tirap mine, Assam, India, are described. ► Most, but not all, modern Indian megathermal plant families are present. ► New CLAMP calibrations are introduced using monsoon-influenced forests from India. ► The palaeoclimate of the Tirap mine is determined quantitatively. ► The South Asia Monsoon, similar to today, existed in Late Oligocene times.