Increased effective moisture in northern Vietnam during the Little Ice Age

We present a 650-yr record of hydroclimate, inferred from δ18O values of lacustrine carbonate from Ao Tiên (Fairy Pond), in northern Vietnam. Ao Tiên occupies a collapsed karst basin that formed during the worst drought of the last millennium (i.e., the Angkor I drought), which marks a dry transitional period between the Medieval Climate Anomaly and the Little Ice Age. A multi-centennial trend of increasing effective moisture spans the Little Ice Age, ending with aridity in the late 18th century that accompanied great societal turmoil across Southeast Asia. After ~1850 CE, the Ao Tiên record exhibits a drying trend that continues to the present. Over the last 650 years, the δ18O-inferred hydroclimate is significantly correlated with the reconstructed Southern Oscillation Index of precipitation (SOIpr) suggesting that a prolonged La Niña-like state existed during the Little Ice Age generating overall wetter conditions in Vietnam. Decadal-scale droughts and pluvials from the Ao Tiên record agree with a robust tree-ring hydroclimate reconstruction from southern Vietnam that identified the Angkor I and II droughts of the late 13th/early 14th centuries, and significant droughts during the early and late 17th century and latter half of the 18th century. The decrease in effective moisture during the last 30 years is also evident in the tree-rings and is consistent with a documented weakening of the Pacific Walker circulation. Major pluvials are indicated for the period between the two Angkor Droughts, and the wettest period of the record is during the early 18th century. Comparison of the Ao Tiên data with records from Indonesia to China indicates regionally coherent climate during the Little Ice Age; however, the overall wet conditions in easterly sites are difficult to ascribe to shifts in the ITCZ. Rather, the pattern suggests that Pacific Walker circulation may have been the overriding control on precipitation.