On the timing of the East Asian summer monsoon maximum during the Holocene—Does the speleothem oxygen isotope record reflect monsoon rainfall variability?

The evolution of the East Asian summer monsoon (EASM) during the Holocene has long been of significant interest. Knowledge of past EASM variability not only increases our understanding of monsoon dynamics on a long timescale, but it also provides an environmental and climatic background for research into Chinese cultural development. However, the timing of the EASM maximum remains controversial. The popular concept of an “early Holocene maximum” is mainly based on speleothem δ18O (δ18Oc) records from caves in southern China; however, the interpretation of δ18Oc as a reliable proxy for EASM intensity is being increasingly challenged. The present paper is a critical review of the climatic significance of the δ18Oc record from China. Firstly, we suggest that precipitation in northern China is an appropriate index of EASM intensity, the variation of which clearly indicates a mid-Holocene monsoon maximum. Secondly, an interregional comparison demonstrates that the precipitation record in northern China is quite different from that in southern China on a range of timescales, and is inconsistent with the spatial similarity exhibited by speleothem oxygen isotope records. Furthermore, both modeling and observational data show that the δ18Oc records from southern China indeed reflect changes in precipitation δ18O (δ18Op) rather than precipitation amount, and therefore that their use as an EASM proxy is inappropriate. Finally, we address several significant monsoon-related issues—including the driving mechanism of the EASM on an orbital timescale, the climatic significance of speleothem oxygen isotopes, and the relationship between atmospheric circulation and precipitation in monsoonal regions.