High Resolution δ18O and δ13C Records of AMS 14C Dated Stalagmites From Jinlun and Yilingyan Caves in Guangxi, China: Climate Variability and Controlling Factors in the Monsoonal Region During the Past 2300 Years

Abstract:

Jinlun Cave in Mashan County and Yilingyan Cave in Wuming County are ~120km and ~60km north of Nanning in Guangxi Province under influence of both Indian Monsoon and North Western Pacific Monsoon. Several stalagmites have been dated by AMS ¹⁴C dating method since ²³⁰Th/U is not applicable due to very low U contents. Twenty (20) AMS ¹⁴C dates on Stalagmite JL20131005-10 (10-cm long) show “Bomb carbon curve”, spanning the past 60 years. Lamination counting further confirms the chronology. Thirty nine (39) AMS ¹⁴C dates on Stalagmite JL20131005-12 (33-cm long) reveal 2300-year continuous growth. Stalagmite YLY20130727-12 (10-cm long) from Yilingyan Cave covers a continuous record of past 2300 years. All studied stalagmites in the caves contain low dead carbon fractions. The annual resolution δ¹⁸O and δ¹³C records obtained from the stalagmites allow us to compare the stalagmite δ¹⁸O records with the instrumental rainfall and temperature records, Pacific Decadal Oscillation (PDO), Southern Oscillation Index (SOI), and Sunspot variation, etc. The δ¹⁸O and δ¹³C records exhibit relatively good correlation throughout the time, indicating climatic control on vegetation change. Based on the high-resolution δ¹⁸O and δ¹³C records, we interpret that dry climatic conditions and poor vegetation coverage during periods of AD1880~1850, 1700~1600, 1460~1320, 1210~1280, 860~750, 540~420, 300~220, and AD100~0 shown by increased δ¹⁸O and δ¹³C. The δ¹⁸O and δ¹³C were strongly depleted during the Medieval Warm Period (MWP between AD900 and AD1100) and Current Warm Period (CWP, since AD1900), reflecting strongly increased East Asian Summer Monsoon. After AD1900, the δ¹³C decreased about 6‰, perhaps indicating human impact on surface vegetation. The δ¹⁸O records from the study area are comparable to the published WX42B δ¹⁸O record of Wanxiang Cave (Zhang et al., 2008) except for the period of AD1400~1850. Our study suggests that AMS ¹⁴C dating is an alternative method for stalagmites containing low U and high Th contents.