Indian summer monsoon variability in southern India during the last deglaciation: Evidence from a high resolution stalagmite δ 18 O record
Abstract We present a high resolution record of Indian summer monsoon (ISM) variability during the last deglacial period. The record is based on high precision measurements of oxygen isotopes (δ18O) in 263 sub-samples from a ~ 18 cm long stalagmite (VSPM1) sample collected from the Valmiki cave in the southern India. The chronological framework is based on high precision U-Th dating in 8 sub-samples at regular intervals. The stalagmite exhibits a variable growth rate ranging from ~ 0.03 to 0.80 mm/yr with 9 years as an average sampling resolution for δ18O. X-ray diffraction analysis shows that the sample is composed of aragonite mineral. The δ18O record provides a strong evidence of abrupt changes in ISM activity between approximately 15,610 and 13,160 yr BP (before 1950 AD). Highly negative δ18O values at ~ 15,610 and 15,250 yr BP are suggestive of a wet climate followed by less negative δ18O at ~ 15,240 to 14,750 yr BP indicating relatively dry climate. These dry and arid conditions are punctuated by two wet events at 15,030 and 14,910 yr BP. A marked increase in δ18O between approximately 14,660 and 14,370 yr BP suggests weakening in ISM activity, followed by a gradual decrease in δ18O between 14,370 and 13,560 yr BP indicating strengthening of ISM. However we cannot rule out the possibility of some past flooding event influencing δ18O of VSPM1. Wavelet analysis of δ18O time series suggests strong influence of solar forcing on ISM activity. The amplitude variance of δ18O reveal decadal to centennial scale variability in monsoonal activity during the last deglaciation.