|High-resolution δ 18 O and δ 13 C records of an AMS 14 C and 230 Th/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 230Th/U and AMS 14C methods. The Holocene parts of the stalagmites contain high Th contents, resulting large uncertainties of the 230Th/U ages. Two precise 230Th/U dates in the pre-Holocene part of XY07-8 agree well with the AMS 14C ages. The Holocene 14C dates of the two stalagmites reveal a 600-year initial 14C age which was probably introduced from the dissolved CO2 in the overlaying soil and transferred into the seepage water, supported by the AMS 14C dating on soil organic carbon in an overlying soil profile near the cave. Based on the AMS 14C and 230Th/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 δ18O and δ13C 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 δ18O record with the instrumental rainfall, historic dry-wet index and the stalagmite δ18O records from other caves in eastern China not only serve verification of the XY07-8 chronology, but also confirm the δ18O record as a monsoonal rainfall proxy. The XY07-8 δ18O 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 δ18O record reveals wetness changes probably corresponding to variations in total solar irradiance (TSI), with lighter δ18O excursions reflecting wetter episodes under less TSI that is opposite to the δ18O – TSI correlation of the stalagmite records from the caves in north and south China. Furthermore, unlike the δ18O record, the δ13C record does not show the Holocene “insolation decline” trend, but generally following the decadal-to-centennial variations of the δ18O record and the TSI record except the last 150 years. Since 1850 CE, the XY07-8 δ18O has an enriched trend which reflects an arid trend in climate, but the δ13C 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 δ18O 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. © 2017 Elsevier Ltd and INQUA
|accelerator mass spectrometry; geochronology; Holocene; paleoclimate; paleoenvironment; radiocarbon dating; spectral resolution; stable isotope; stalagmite; thorium; uranium series dating; China; Chongqing
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