Speleothem δ18O and δ13C records from Fengyu Cave in south Guilin of China: Climate and environmental changes during the past 65 Ka

Two stalagmites from Fengyu Cave (24o25’N, 110o17’E, 150 m a.s.l), ~100 km south of Guilin, have been studied: the 14-cm long stalagmite F-4 grew from 1480 AD to 1997 AD when the growth was active during the sampling; and the 221-cm long stalagmite F-1 covers 4~65 Ka. F-4 was dated by 230Th/U TIMS and 210Pb methods, whereas F-1 was dated by 230Th/U ICPMS in two labs. The F-1 record with 26 age dates and 3698 isotope measurements provides high-resolution, continuous variations of regional climate and local environmental conditions which exhibit all Heinrich cold events, most D-O warm events and Holocene Optimum, generally supporting the Hulu and Dongge records. The climatic and environmental conditions during 4~65 Ka in the studying site are cold/dry with less vegetation intensity under weak summer monsoon during the Heinrich episodes except H6, and warm/wet with abundant vegetation coverage under stronger summer monsoon in the D-O intervals except the Bølling-Ållerød. The climatic condition during the Younger Dryas in Fengyu Cave area might be cold and wet. The comparison of the speleothem, ice core and marine records reveals strong linkage between N. Atlantic temperature and the Asian summer monsoon. The speleothem records in eastern China resemble GRIP ice core record on centennial-to-millennium scale during the last glacial, indicating strong linkage between N. Atlantic temperature and the Asian summer monsoon. Changes in high latitude ice sheet and/or N. Atlantic temperature led to changes in meridional temperature gradients and caused shifts in the monsoonal boundary and the ITCZ position. The strong similarity of the three cave records implies that the moisture source effect may be the dominant factor to control the stalagmite δ18O. After the deglaciation, as the continental ice sheet shrunk, the forcing factor on the Asian monsoon was changed. The regional disparity of the summer monsoon influence may lead to the discrepancies among the stalagmite δ18O. On orbital scale, NH solar insolation seems the main forcing factor to influence the monsoonal variability. However, both marine and speleothem records in the monsoonal region indicate the coldest and driest conditions occurred at 16 Ka during the past 65 Ka, with a 5-6 kyr lag to the minimum NH solar insolation during LGM. The δ18O values of F-4 are heavier than these of the Holocene section of F-1, reflecting weaker summer monsoon in the late Holocene. The summer monsoon strength during the past 500 years shown by the F-4 record were relatively weaker during 300-500 yr BP than 100-300 yr BP, and continuously decrease since 1900 AD. The δ13C record of F-4 registers influence of human activity on the local vegetation.