Oxygen isotopes from Chinese caves: records not of monsoon rainfall but of circulation regime

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 delta 18O variations conflict with independent palaeoclimate proxies (cave delta 13C, loess/palaeosol magnetic properties, delta 13C alkanes), which indicate no systematic decline in rainfall from the mid-Holocene, and no glacial rainfall maxima. Using mass balance calculations (which incorporate seasonality effects in both delta 18O concentration and amount of precipitation), we demonstrate that the cave delta 18O variations cannot be accounted for by summer rainfall changes, or rainfall seasonality or winter cooling, but instead reflect changes in moisture source. A possible driver of the delta 18O 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 delta 18O may have dominated at times of high boreal summer insolation, and 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. Copyright (c) 2012 John Wiley & Sons, Ltd.