Climate and environmental changes during the past millennium in central western Guizhou, China as recorded by Stalagmite ZJD-21

摘要

Stalagmite ZJD-21 (12.3-cm long) was collected from Zhijin Cave in Zhijin County, Guizhou, China. Its ~(210)Pb profile and seven ~(23O)Th/~(234)U dates indicate that the stalagmite has grown continuously for the past 1100 years. The δ~(18)O record of ZJD-21 indicates that δ~(18)O in the stalagmite was mainly influenced by rainfall amount and/or summer/winter rain ratio, with lighter values corresponding to wetter climatic conditions and/or more summer monsoonal rains. The ZJD-21 δ~(18)O record suggests: (1) dry/warm climates during AD 950-1100 (overlapping with most of the Medieval Warm Period, MWP, in Europe); (2) strengthening of the summer monsoon from the MWP toward the beginning of the Little Ice Age (L1A) at AD 1250; (3) relatively wet/cold conditions occurred between AD 1250 and 1500, shown by relatively light δ~(18)O values; (4) the summer monsoon intensity strongly declined referred by the increase δ~(18)O trend from AD 1500 to AD 1600, perhaps resulting in dry/cold conditions; and (5) a strongly enhancement of the summer monsoon intensity appeared from AD 1700 to 1950, reflecting wet/cold conditions during the late period of the LIA. On decadal scales the monsoonal climate of central western Guizhou can be either warm/wet and cold/dry, or warm/dry and cold/wet. The δ~(13)C variations in ZJD-21 on decadal-to-centennial scales respond mainly to vegetation changes with heavier values reflecting lesser amount of forest coverage. Prior to AD 1700, the δ~(13)C generally co-varied with δ~(18)O reflecting the expected more extensive vegetation growth (lighter δ~(13)C) under wetter climate (lighter δ~(18)O). However, during the past 300 years the δ~(13)C increased sharply showing an opposite trend to that of δ~(18)O. This observation strongly suggests that a decline of surface vegetation due to an artificial deforestation might have occurred - an occurrence coincident with the large-scale immigration into central western Guizhou in connection with copper-mining activities during the reign of Emperor Yongzheng of Ojng Dynasty. Since the late 1890s, especially in the past 50 years, population surge has led to serious karst rocky desertification in the area.