Oxygen isotope variations in Chinese stalagmites have been widely interpreted as a record of the amount of East Asian summer monsoonal rainfall. This interpretation infers decreasing monsoonal rainfall from the mid-Holocene and large, dipolar rainfall oscillations within glaciations. However, the speleothem δ18O variations conflict with independent palaeoclimate proxies (cave δ13C, loess/palaeosol magnetic properties, n-alkanes), which indicate no systematic decline in rainfall from the mid-Holocene, and no glacial rainfall maxima.
Using mass balance calculations, we demonstrate that the cave δ18O variations cannot be accounted for by summer rainfall changes, nor rainfall seasonality nor winter cooling, but instead reflect changes in moisture source. A possible driver of the δ18O variations in Chinese stalagmites is precessional forcing of inter-hemispheric temperature gradients, and resultant shifts in the position and intensity of the subtropical pressure cells. Through such forcing, Indian monsoon-sourced δ18O may have dominated at times of high boreal summer insolation, local Pacific-sourced moisture at low insolation. Suppression of summer monsoonal rainfall during glacial stages may reflect diminished sea and land surface temperatures and the radiative impacts of increased regional dust fluxes.