Oxygen and carbon isotopic systematics of aragonite speleothems and water in Furong Cave, Chongqing, China

To understand oxygen and carbon stable isotopic characteristics of aragonite stalagmites and evaluate their applicability to paleoclimate, the isotopic compositions of active and fossil aragonite speleothems and water samples from an in situ multiyear (October 2005-July 2010) monitoring program in Furong Cave located in Chongqing of China have been examined. The observations during October 2005-June 2007 show that the meteoric water is well mixed in the overlying 300-500-m bedrock aquifer, reflected by relatively constant delta(18)O, +/- 0.11-0.14 parts per thousand (1 sigma), of drip waters in the cave, which represents the annual status of rainfall water. Active cave aragonite speleothems are at oxygen isotopic equilibrium with drip water and their delta(18)O values capture the surface-water oxygen isotopic signal. Aragonite-to-calcite transformation since the last glaciation is not noticeable in Furong stalagmites. Our multi-year field experiment approves that aragonite stalagmite delta(18)O records in this cave are suitable for paleoclimate reconstruction. With high U, 0.5-7.2 ppm, and low Th, 20-1270 ppt, the Furong aragonite stalagmites provide very precise chronology (as good as +/- 20s yrs (2 sigma)) of the climatic variations since the last deglaciation. The synchroneity of Chinese stalagmite delta(18)O records at the transition into the Bolling-Allerod (t-BA) and the Younger Dryas from Furong, Hulu and Dongge Caves supports the fidelity of the reconstructed East Asian monsoon evolution. However, the Furong record shows that the cold Older Dryas (OD) occurred at 14.0 thousand years ago, agreeing with Greenland ice core delta(18)O records but similar to 200 yrs younger than that in the Hulu record. The OD age discrepancy between Chinese caves can be attributable to different regionally climatic/environmental conditions or chronological uncertainty of stalagmite proxy records, which is limited by changes in growth rate and subsampling intervals in absolute dating. Seasonal dissolved inorganic carbon delta(13)C variations of 2-3 parts per thousand in the drip water and 5-7 parts per thousand in the pool and spring waters are likely attributed to variable degrees of CO(2) degassing in winter and summer. The variable delta(13)C values of active deposits from -11 parts per thousand to 0 parts per thousand could be caused by kinetically mediated CO(2) degassing processes. The complicated nature of pre-deposition kinetic isotopic fractionation processes for carbon isotopes in speleothems at Furong Cave require further study before they can be interpreted in a paleoclimatic or paleoenvironmental context.</p