High-resolution δ18O and δ13C records of an AMS 14C and 230Th/U dated stalagmite from Xinya Cave in Chongqing: Climate and vegetation change during the late Holocene

Two stalagmites, XY07-8 and -11 collected from Xinya Cave in northeast Chongqing of China, were dated by Th-230/U and AMS C-14 methods. The Holocene parts of the stalagmites contain high Th contents, resulting large uncertainties of the Th-230/U ages. Two precise Th-230/U dates in the pre-Holocene part of XY07-8 agree well with the AMS C-14 ages. The Holocene C-14 dates of the two stalagmites reveal a 600-year initial C-14 age which was probably introduced from the dissolved CO2 in the overlaying soil and transferred into the seepage water, supported by the AMS C-14 dating on soil organic carbon in an overlying soil profile near the cave. Based on the AMS C-14 and Th-230/U dating results, a reliable chronology of the 22-cm part of XY07-8 has been established, revealing a continuous growth since 23 ka BP. The delta O-18 and delta C-13 records of the top 5.3-cm part of XY07-8 provide climate and vegetation changes over the past 4100 years. The comparisons of the XY07-8 delta O-18 record with the instrumental rainfall, historic dry-wet index and the stalagmite delta O-18 records from other caves in eastern China not only serve verification of the XY07-8 chronology, but also confirm the delta O-18 record as a monsoonal rainfall proxy. The XY07-8 delta O-18 record indicates that on millennial scale the intensity of East Asian Summer Monsoon was decreased from 4100 a BP to 2400 a BP following the decline of the summer insolation, then leveling out with decadal-to-centennial variations. On decadal-to-centennial scales, the XY07-8 delta O-18 record reveals wetness changes probably corresponding to variations in total solar irradiance (TSI), with lighter delta(18)Oexcursions reflecting wetter episodes under less TSI that is opposite to the delta O-18 - TSI correlation of the stalagmite records from the caves in north and south China. Furthermore, unlike the delta O-18 record, the delta C-13 record does not show the Holocene "insolation decline" trend, but generally following the decadal-tocentennial variations of the delta O-18 record and the TSI record except the last 150 years. Since 1850 CE, the XY07-8 delta O-18 has an enriched trend which reflects an arid trend in climate, but the delta C-13 has a depleted trend perhaps indicating human impacts on the vegetation and atmosphere CO2. This study calls for the attention to the discrepancies and physical meanings of stalagmite delta O-18 on different time scales ("monsoon intensity" on millennium scale vs. "rainfall index" on decadal scale) and to the spatial disparity of monsoonal rainfall in eastern China. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.