An optimized chronology for a stalagmite using seasonal trace element cycles from Shihua Cave, Beijing, North China.

Stalagmites play an important role in paleoclimatic reconstructions from seasonal to orbital time scales as 230Th-dating can provide an accurate absolute age. Additionally, seasonal trace element and optical layers can provide a precise age. We analyzed the seasonal variability of multiple trace elements on a stalagmite (XMG) in Shihua Cave, Beijing and compared them with results from laminae counting. The results show that (1) the polished section of the topmost part of XMG has obvious bi-optical layers under a conventional transmission microscope, however, laminae are not observed using this method in the rest of the sample, and (2) The variations of P/Ca, Sr/Ca, Ba/Ca, U/Ca and Mg/Ca show seasonal cycles throughout the sample. The PC1 in the Principal Component Analysis (PCA) of five trace elements represents the annual cycle. This stalagmite was deposited over 150 ± 1 years through PC1 peak counting. This result corresponds well with the annual layers and U-Th dating. Trace element cyclicity of PC1 can increase the accuracy of stalagmite dating, especially in the absence of obvious laminae and are a powerful method to identify seasonal changes in a strongly contrasting wet-dry monsoon climate region.

The East Asian summer monsoon variability over the last 145 years inferred from the Shihua Cave record, North China.

The precipitation variability associated with the East Asian summer monsoon (EASM) has profound societal implications. Here, we use precisely dated and seasonally-resolved stalagmite oxygen isotope (δ18O) records from Shihua Cave, North China to reconstruct the EASM variability over the last 145 years. Our record shows a remarkable weakening of the EASM strength since the 1880s, which may be causally linked to the warming of the tropical Pacific and Indian Oceans. The δ18O record also exhibits a significant ~30-year periodicity, consistent with the instrumental, historical and proxy-based rainfall records from North China, plausibly driven by the Pacific Decadal Oscillation (PDO). Together, these observations imply that ~30-year periodicity is a persistent feature of the EASM, which remains significant with or without anthropogenic forcing. If indeed, the EASM rainfall in North China might decline significantly in the near future, which may affect millions of people in this region.