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Modern systematics and environmental significance of stable isotopic variations in Wanxiang Cave,

This paper presents the stable isotopic compositions from the cave dripwater and actively forming soda straw stalactites collected from Wanxiang Cave, Wudu, Gansu, located on the Qinghai-Tibetan Plateau and Loess Plateau transition zone, China. The δ 18 O dw and δ D dw of dripwater samples in the cave plot directly on the local MWL, constructed by using GNIP data from 3 sites surrounding the cave regions (Lanzhou, Xi’an, and Chengdu), the nearest site to the cave, suggesting that there is a close relationship between the δ 18 O dw of the cave water and the δ 18 O of the pre-cipitations. Using the measured δ 18 O dw and δ 18 O mc values from the mid-farthest parts from the cave entrance and the carbonate paleotemperature equation, the calculated temperatures range from 8.9 to 12.4 ℃ , with the mean value of 10.7 ℃ and the temperature calculated at 8 locations in the farthest part of the cave is in the range of 10.1 — 12.4 ℃ , with the mean value of 11.5 ℃ , being consistent with the survey value(10.99 ℃ )in the cave, slightly lower than the mean annual temperature (14.4 ℃ ) in Wudu. This suggests that modern speleothems are forming under isotopic equilibrium and their isotopic composition accurately reflects the mean annual temperature at the surface, indicating that the isotopic composition of the modern speleothems records local temperature change with credibility.

At present, cave precipitates have presented huge potential in the study of past climatic change, e.g. precise dating of Dansgaard-Oeschger climate oscillations during the last glacial period [1] , the variation of the East Asian Monsoon intensity [2,3] and close relationship between solar variability and the monsoon intensity [4] . But there are multi-answers in explaining oxygen isotopic compositions of the speleothems from different regions, e.g. δ 18 Oc of speleothem calcite indicating the temperature effect dominant in Norway [5] and France [1] , which is negatively related to the temperature in Israel [6] , representing the amount of the precipitation; the δ 18 Oc from the region influenced dramatically by monsoon is more complex [2 4] , indicating the change of the monsoon intensity. Therefore, in order to interpret past stable isotope variations in speleothems, it is necessary to perform a thorough study of the modern carbonate-water system in the cave where they were formed [7,8] . Under equilibrium conditions, the δ 18 O of calcite is a function of the δ 18 O dw (dripwater) and temperature [9] . If the δ 18 O of both calcite and dripwater can be determined, the cave temperature can be obtained [10] , which has been shown to approximate the mean annual surface temperature [11] . The δ 18 O of cave water has, in general, been shown to be slightly enriched in 18 O relative to precipitation above the cave. The isotopic composition of cave waters, however, may exhibit complex variations on seasonal or even longer time scales, that is related to changes in annual precipitation amount, recharge rates, storm tracks, and evaporation in the epikarst [12] . To interpret δ 18 O variations in fossil speleothems, we must show ( ) the δ 18 O of modern speleothems reflects the δ 18 O of modern dripwater, and hence, modern precipitation, and ( ) the speleothem calcite was formed in isotopic equilibrium with the water. Therefore, the aim of this paper is to determine the principle controls on the calcite isotopic composition, by studying the stable isotopic compositions between the cave dripwater and actively forming soda straw stalactites collected from Wanxiang Cave, Wudu, Gansu, for obtaining the past climatic and environmental information from cave precipitates in the long period of time.

Sampling and experimental
( ) Geographic environment and site. The study sites are located in the Qinghai-Tibetan Plateau and Loess Plateau transition zone, China, a key geographic location on the eastern edge of the Qinghai-Tibetan Plateau and the southwest edge of the Loess Plateau ( Fig. 1), near the modern limit of the summer monsoon, being very sensitive to monsoon change. Therefore, close relationships between cave precipitates and climate will be considered to be more significant in study. Wanxiang Cave (33°19′N, 105°00′E, 1200 m a.s.l.) in Wudu, Gansu, one of the largest caves in the northern China, is formed primarily in the Silurian limestone, and cave precipitates are shown by the way of large and various type of stalagmites and stalactite and stone-column etc., presenting a splendid sight. Loess deposits consist of the overlying soil outside cave. Therefore, it is of ideal conditions and advantageous geographic site in study of modern and past climatic changes.
( ) Samples collecting and cave detecting. We systematically collected some samples of cave dripwater from entrance to the end of the cave during the fall (October) of 1999, at the same time some actively dripping, fine, un-consolidated and/or un-crystal growing soda straw stalactites were collected, sagged from the coping of the cave. And several glass slides were also put in the cave at the dripping location. The data-logger for detecting the temperature and relative humidity was set in the farthest site from the entrance.
( ) Analytical method. Isotopic compositions of all samples were completed in the Center of Isotope Geochemistry, University of California, Berkeley. The δ notation represents hydrogen, oxygen data, (1) δ 18 O and δ D analysis of the water samples. δ 18 O of cave dripwater samples was measured by the equilibrium method between water CO 2 Vacutainers; δ D of all water samples will be measured using the method described by Venneman and O'Neil [13] , and analyzed with a VG Prism II isotope ratio mass spectrometer. δ 18 O and δ D values will be reported relative to VSMOW. The precision for each analysis is ±0.01‰ for δ 18 O and ±0.1‰ for δ D.
(2) δ 18 O analysis of the carbonate. 0.1 mm thickness of down-edge calcite scraped from un-consolidated and/or un-crystal growing soda straw stalactites with knife-blade was put into the preparation system an Isocarb automated carbonate device attached to a mass spectrometer, reacted with 100% of phosphoric acid (following the method of McCrea [14] ), and CO 2 produced was measured by an Optima mass spectrometer. Oxygen isotopic data are relative to the VPDB standard for carbonate samples, and the precision for each analysis is ± 0.1‰ for oxygen.

Results and discussion
( ) The local meteoric water line (LMWL). To characterized approximately modern precipitation, we are utilizing data from the last 15 years, obtained from the WMO/IAEA Global Network for Isotopes in Precipitation (GNIP) [15] , from 3 sites surrounding the cave region, Lanzhou (36°N, 104°E, 1517  The local δ D-δ 18 O relationship differs slightly from the Global Meteoric Water Line (GMWL) of Craig [16] and from the Meteoric Water Line in China [17] , with close slope and very different interceptors.
The δ D and δ 18 O values of the dripwater samples collected in the cave during 1999 plot directly on the local MWL (Fig. 2), suggesting that the cave water has not been significantly affected by evaporative processes in the epikarst [18] . Therefore, this result indicates that the isotopic The smaller depletion of δ 18 O dw in the dripwater from Wanxiang cave relative to the precipitation in Xi'an and Lanzhou (ca. 2‰) reflects either a seasonal bias in the drip composition, the depletion of local precipitation relative to Xi'an and Lanzhou due to local topographic variations, or a combination of these factors, and the bigger depletion relative to the precipitation in Chengdu (ca. 3.3‰) results from the altitude effect, i.e. δ D and δ 18 O values of precipitation decline with the increase of the altitude [19] , except for the depletion caused by either a seasonal bias in the dripwater composition, or the depletion of local precipitation.
( 2 a by the calculation, i.e. the dating of the down-part calcite is less than 2 a, and carbonate formed during 1 2 a. Therefore, our results are exactly reflecting the isotopic system in modern water-carbonate of Wanxiang Cave. Furthermore, according to the fractionation condition between the system of actively forming carbonate and liquid of the Hendy and Wilson [20] , the isotopic equilibrium was formed in the stalagmite from the Wanxiang Cave, and its isotopic composition could indicate the annual average temperature outside the cave closely, being slightly lower than the annual average temperature (14.4 ) during 1961-1990 (the data from the weather station of Wudu, Gansu).

Conclusion
There is a close relationship between the δ D dw and δ 18 O dw values of the dripwater in Wanxiang Cave and local precipitation, indicating that the isotopic composition of the local precipitation has been influenced weakly on the cave dripwater by the overlying limestone and surface overlying soil. And the data from the δ 18 O dw of the dripwater and the δ 18 O mc of the down part in modern soda straw stalactite are closely related to the mean annual temperature outside cave, suggesting that modern speleothems are forming under isotopic equilibrium with water.