Precession-band variance missing from East Asian monsoon runoff

Abstract

Speleothem CaCO₃ δ¹⁸O is a commonly employed paleomonsoon proxy. However, inferring local rainfall amount from speleothem δ¹⁸O can be complicated due to changing source water δ¹⁸O, temperature effects, and rainout over the moisture transport path. These complications are addressed using δ¹⁸O of planktonic foraminiferal CaCO₃, offshore from the Yangtze River Valley (YRV). The advantage is that the effects of global seawater δ¹⁸O and local temperature changes can be quantitatively removed, yielding a record of local seawater δ¹⁸O, a proxy that responds primarily to dilution by local precipitation and runoff. Whereas YRV speleothem δ¹⁸O is dominated by precession-band (23 ky) cyclicity, local seawater δ¹⁸O is dominated by eccentricity (100 ky) and obliquity (41 ky) cycles, with almost no precession-scale variance. These results, consistent with records outside the YRV, suggest that East Asian monsoon rainfall is more sensitive to greenhouse gas and high-latitude ice sheet forcing than to direct insolation forcing.