Paleogene monsoons across India and South China: Drivers of biotic change

摘要

Abstract Monsoonal climates at low latitudes (32°N) are an inevitable consequence of seasonal migrations of the Inter-tropical Convergence Zone (ITCZ), but the character of these monsoons depends on continental configuration, orographic expression and the strength of Hadley circulation. To explore the evolution of monsoon systems across southern Asia we compare climate signatures archived in ten Paleogene floras from northern India, Tibet and southern China, occupying low palaeolatitudes at a time of extreme global warmth and elevated CO2. Fossil leaf form reveals that under such 'hothouse' conditions megathermal early Eocene to earliest Miocene forests were exposed to strong monsoonal climates typical of those experienced today arising from annual migrations of the ITCZ, possibly enhanced by a lower equator-to-pole temperature gradient. Throughout the Paleogene an elevated Tibetan highland produced no discernable modification of this ITCZ monsoon, although rainfall seasonality similar to that of the modern South Asia Monsoon (SAM) is observed in northern India as early as the beginning of the Eocene, despite its near-equatorial palaeoposition. In South China rainfall seasonality increased progressively achieving modern monsoon-like wet season/dry season precipitation ratios by the early Oligocene. Despite evidencing weak rainfall seasonality overall, fossil leaves from South China have exhibited monsoon-adapted morphologies, comparable to those seen in today's Indonesia-Australia Monsoon, for at least 45millionyears. Together, the Indian and South China fossi