Monsoonal climate change during the Holocene
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Variations in δ18O and δ13C values, luminescence, gray color, and mineralogy along the growth direction of speleothems from Southwestern China, Northern India and the Southeastern USA provide high-resolution and absolute-dated records of Asian and Southeast USA monsoonal climate change through the Holocene. Stalagmite records from Southwestern China document a gradual weakening in the Asian summer monsoon from the mid-Holocene in response to declining Northern Hemisphere summer solar radiation. This brought wetter conditions during the mid-Holocene when the stronger Asian summer monsoon brought more intense and more total precipitation to the Yangtze River Valley. The Asian summer monsoon weakened during the late-Holocene and retreat of the monsoonal front to the further south resulted in less rainfall to the Shangdong Cave area. Stalagmite evidence from Northern India suggests a wetter Little Ice Age resulting in calcite deposition from AD 1480-1900 and aragonite deposited before and after these dates. Climate changes in Northern India from AD 1250-1800 were of low magnitude but after AD 1800 climate fluctuated significantly. A transition from calcite to aragonite deposition at 6 ka B.P. in a column collected from DeSoto Caverns in Alabama, USA, suggests a wetter early Holocene followed by a drier mid-Holocene. There is no clear trend in δ18O and δ13C values in records for the last 4400 years obtained from vertical stalagmite cores drilled from two stalagmites in DeSoto Caverns. Instead, the climate record is characterized by alternating wetter and drier periods, with wetter conditions centered at 4300, 3400, 2500, 1400, 770, 420, and 120 years B.P. and drier conditions at ~3900, 3000, 2700, 530, and 130 years ago. The data indicate a wetter Medieval Warm Period and a drier Little Ice Age in the Southeastern USA.