HOLOCENE VARIABILITY OF THE MONSOONAL CLIMATE IN CENTRAL CHINA INDICATED BY MULTI-PROXY RECORDS IN A SPELEOTHEM

A 22-cm long stalagmite (SF-1) was collected from Buddha Cave (33^o40’N 109^o05’E) located at ~80 km south of Xian in central China. Dating with ²¹⁰Pb, ²³⁰Th/²³⁴U and lamination counting shows that the top 16.5 cm of the stalagmite represents growth since early Holocene. There is a growth hiatus at 16.5 cm lasting from about 16 to 10 ka, below which the stalagmite grew much faster with rates of the order of 0.5 mm/y. This hiatus is clearly manifested from our measurements of d¹⁸O, d¹³C, and concentrations of Mg and Sr, which we use as proxies for climate changes. The proxy records can be divided into four segments/periods: I (22-16.5 cm; ~16.1-16 ka), II (16.5-9.5 cm; ~10-5 ka), III (9.5-4 cm; ~5-2 ka) and IV (top 4 cm; <2 ka). Records for period I indicate that during at least part of the last deglaciation, eastern China was under a weak summer monsoon regime with relatively dry and cold climate. The dryness is reflected by heavy d¹⁸O (-6.4±0.8‰, PDB) and d¹³C (-6.3±1.1‰, PDB) values, and the coldness, by comparatively small Mg/Sr (11.4±1.1, ppm/ppm) ratios. The onset of period II is characterized by a sharp decrease of d¹⁸O and a sharp increase of Mg/Sr. Afterwards, d¹⁸O, d¹³C and Mg/Sr averaged -10.9‰, –9.8‰ and 48.7, respectively, indicating a much stronger monsoonal climate than in period 1. Period III had average d¹⁸O, d¹³C and Mg/Sr of -10.0‰, -9.7‰, and 55.5 with relatively large variations on millennial and shorter scales indicating more varied climate conditions. Period IV showed d¹⁸O and d¹³C with strong positive correlations and an increasing trend toward the present, averaging -9.2‰ and -9.5‰. Correspondingly, its Mg/Sr decreases from 55.5 to 51.1 implying the presence of a slight cooling trend for the period. Our proxy records in SF-1 are in accordance with the following scenario: A strengthened East Asia Summer Monsoon accompanied the onset of Holocene warming paced by solar insolation forcing. There was generally an early-to-late Holocene decreasing trend in the East Asia Monsoon strength, due probably to a gradual southeastward retreat of the summer monsoon maximum caused by insolation decrease. This trend seems to find its equivalence in the Indian and African Monsoons as recorded by deposits in Qunf Cave (Oman) and Soreq Cave (Israel), respectively.