Tree-ring stable isotopes suggest an increase in Asian monsoon rainfall at 4.2 ka BP

A synthesis of stable oxygen isotope records from tree rings and speleothems does not support a significant hydroclimate transition in our study region around 4.2 ka, nor the notion that this rapid climate deterioration should be regarded as generalized climatic transition from the mid to late Holocene. 

Methods
Comparison of normalized speleothem δ18O records (thin line) from different parts of southern China spanning 5–3 ka.

Each record is interpolated annually, and then their linear trends are removed before normalization. A 100-yr low-pass filter is applied to each series to highlight centennial fluctuations. The grey bar covers the period 3.97–4.26 ka. A regional composite and 95% CIs (yellow shading) for southern China is shown, which is derived by averaging the six higher-resolution normalized speleothem δ18O series from Dongge Cave, Xianglong Cave, Wuya Cave, Sanbao Cave 43, Heshang Cave and Shennong Cave using Z-score method. Each of the six speleothem records has a temporal resolution better than 20 years, at least 5 U-Th ages and dating precision higher than 60-yr average age error (2σ) in the 5–3 ka interval. A 100-point low-pass filter (heavy line) is applied in each normalized series to highlight low-frequency variability.