贵州中西部洞穴水系与碳酸钙的稳定同位素意义

Stable Isotopes of Water and Carbonate Samples from Caves in Central Western Guizhou:Implications of Paleoclimate and Paleoenvironment

在贵州中西部四个洞穴取得水样和碳酸钙样品,其中地表水与洞穴水的δD和δ^(18)O_(SMOW)的平均值分别为-51．1‰±6．2‰和-7．48‰±0．88‰,代表了该地区的大气降水δD和δ^(18)O的平均值。一方面,尽管只是一次取样,多数洞穴滴水的δD和δ^(18)O值还是反映了地表水的同位素年平均值。而另一方面,在个别滴水点可能由于洞顶包气带很薄,能够灵敏地反应季节性雨水δD和δ^(18)O值的变化,那么,短期的滴水测量不一定反映地表水的年平均值。石将军洞的年轻鹅管δ^(13)C_(PDB)值(-1．6‰)远比织金洞(-7．00‰)的重,是由于植被覆盖率低、严重石漠化造成的,织金洞上覆植被由于受到人为的保护,恢复较好。其他洞穴碳酸钙沉积物的δ^(18)O和δ^(13)C显示,全新世时期的植被发育较好,δ^(13)C值轻;而冷干的冰期时,植被覆盖减少,δ^(13)C值偏重。因此,洞穴沉积物的δ^(13)C可以作为重建古植被和贵州喀斯特地区石漠化演变的重要依据。

23 water samples and 24 carbonate samples were obtained from four caves located in the central- western Guizhou for analyses of δD and δ^18O. The average δD and δ^18O of the surface and cave waters are -51.1±6.2%0 and -7.48±0.88%0 （SMOW）, which represent these of the annual precipitation in this region. The cave water δD and δ^18O at most sites reflect the annual mean values of the surface precipitation, except for a few specific sites where the dripping water δD and δ^18O can sensitively reflect the changes of δD and δ^18O in seasonal rainfall, perhaps because of a thinner vadoes zone above the cave. The δ^18O of modern soda straw from Shijiangjun Cave in Anshun is nearly 2‰ heavier than that of the modern soda straw from Zhijin Cave, whereas the δ^18O values of dropping water in both caves are similar. The discrepancy is difficult to be explained by the δ^18O of atmospheric precipitation and depositional temperature. The phenomenon is also observed on the comparison of Holocene δ^18O records between Dongge Cave and Qixing Cave in the same region. The explanation of the discrepancy remains unclear. The δ^13CPDB（-1. 6‰） of modern soda straw in Shijiangjun Cave is heavier than the δ^13CPDB （-7.00‰） of modern soda straw in Zhijin Cave, which may be caused by serious karst-desertification and poor vegetation coverage in the former site. Around Zhijin Cave, vegetation and environment have been protected since 1982 when the cave became a national scenic area. Since then, the vegetation coverage and environmental condition have been well recovered in the area. The δ^18O and δ^13C of other speleothem samples show that vegetation condition in Anshun was better during warm and wet Holocene reflected by lighter δ^13C; and was poor during cold and dry glacial period indicated by heavier δ^13C. Therefore, δ^13C of speleothem appears to be a sensitive proxy for reconstruction of paleo-vegetation and evolution of karst-desertification in the region.