Effects of intraseasonal variation of summer monsoon rainfall on stable isotope and growth rate of a stalagmite from northwestern Thailand

Abstract

An annually laminated stalagmite from Namjang cave (194030N, 981212E), northwestern Thailand, has been analyzed to investigate the annual resolution climate signal persevered within its oxygen isotopic composition (δ¹⁸O) and growth rate parameters. The cave site is under the influence of Asian monsoon, and local rainfall shows noticeable variations through the monsoon season. Both δ¹⁸O and growth rate records, covering the last 105 years, exhibit persistent decadal-scale variability and can be compared with local instrumental data. Low δ¹⁸O values coincide with high growth rates in the specimen and correspond to higher relative amounts of rainfall in later monsoon season (August-October; hereinafter referred to as ASO rainfall) versus rainfall in early monsoon season (May-July; hereinafter referred to as MJJ rainfall). The strong correlation between the δ¹⁸O value and the 5 year averaged ratio of ASO to MJJ rainfall (r = -0.50, p < 0.001) indicates a significant imprint of intraseasonal variation of monsoonal rainfall on stalagmite δ ¹⁸O. The close resemblance between the speleothem δ¹⁸O record and Western Pacific Warm Pool (WPWP) sea surface temperature (SST) implies that the WPWP may play an important role on the decadal variability of later monsoon rainfall in this region. Unique in its annual chronology, high-resolution δ¹⁸O, and direct comparison with instrumental data, our record shows for the first time that the climate in northwestern Thailand has undergone decadal-scale variability and speleothem δ¹⁸O is a robust proxy for regional monsoon intensity. Copyright © 2010 by the American Geophysical Union.