The Mechanism Of Different Depositional Patterns In Different Caves Under The Same Climate Conditions

Nanjing Hulu Cave (32°03’N,119°02’E) is well known for its last glacial climate record from the stalagmites, but no Holocene stalagmites and no modern secondary carbonate deposit were found in this cave. By contrast, there is possessing a large number of modern secondary carbonate deposits in Penglaixian Cave(30°14’N,117°32’E), Anhui Province, which locates at the same climate conditions with Hulu Cave. Investigate the mechanism of different depositional patterns in the two caves, will help us to further know and understand the mechanism(s) of cave secondary carbonate deposition, and will have important meanings in application of paleoclimate records and interpretation of the substitution indexes of paleoclimate from stalagmites. Based on monthly in-situ monitoring of hydrogeochemistry and cave environment at Hulu Cave and Penglaixian Cave for nearly one hydrological year, we have contrasted the conditions of modern secondary carbonate deposition in the two caves, and discussed the possible cause(s) for loss of active speleothem in Hulu Cave and for the different depositional pattern of the two caves, preliminary conclusions are as follows:The cave air temperature、relative humidity、CO2concentration and cation concentration of drip water in the two caves have little differences, and even show a character that there would be more modern secondary carbonate depositing in Hulu Cave, it’s difficult to explain the reason why there’s a large number of secondary carbonate deposition in Penglaixian Cave but rarely/no in Hulu Cave. Although the drip rate of drip water in the two caves are different, water chemical analysis results reflected that drip rate would not be the main factor leading to different depositional patterns in the two caves. Using two kinds of method to calculate the calcite saturation of drip water in two caves, we have found that the calcite saturation index in Hulu Cave drip water always greater than zero, similar to those in Penglaixian Cave drip water, showing that the calcite saturation in drip water is not a factor for loss of active speleothem in Hulu Cave, either.Monitoring results show that SO42-and Cl-content of the drip water in Hulu Cave are6.3and8.2times higher than those in Penglaixian Cave, respectively. Based on the existing research and practical simulation test results, we infer that the "salt-in effect" is probably one important reason for loss of modern carbonate sedimentary in Hulu Cave, and it may well be an important factor contributing to the different depositional pattern of the two caves.A comparative analysis of stalagmite lamilae from Hulu Cave and Shihua Cave shows they are similar, suggests the secondary carbonate in Hulu Cave and Shihua Cave is likely to have the same depositional pattern. We infer that the drip water is constant in Hulu Cave before Holocene, the water chemistry controlled by rock conditions is similar to today’s, so there’s rarely/no secondary carbonate deposits during summer while the SO42-and Cl-content in drip water is higher, but inversely, it could deposit in winter, when water in the cave drop constantly and the SO42-and Cl-content in drip water was lower.Furthermore, the method and results of our investigation may have certain significance in helping us better understand the reasons why the sediments are aging in so many Chinese caves, now.