神农架石笋氧同位素的海拔效应

Altitude effect on δ^(18)O in stalagmites from Mt.Shennongjia,central China

作为古气候代用指标,石笋氧同位素为亚洲季风演化研究提供了一系列可靠的证据。然而,关于石笋氧同位素指示的气候意义一直存在争论。本文选取了位于神农架地区海拔1 250~2 250m的黑龙洞、青天洞和三宝洞3个洞穴进行对比分析,发现不同海拔洞穴石笋δ18O在波动形式上相同,但绝对值存在约-0.1‰/100m的系统差,而大气降水的在该地的海拔效应约为-0.2‰/100m。为此,我们认为,石笋δ18O除了受季风降雨的影响外,不同海拔的温度梯度也对石笋δ18O具有较大的贡献。结合洞穴年均温度,初步估算石笋δ18O的温度效应约为-0.25‰/℃,与O’Neil的同位素平衡分馏计算结果相同。

Proxy data δ18O reflecting the oxygen isotope composition of meteoric precipitation are widely used in reconstructions of continental paleoclimate.Nevertheless,there has been a general debate that precipitation amount or temperature dominates the speleothem δ18O signal.To assist interpretation of speloethem δ18O time series,it is necessary to understand the impacts of main oxygen isotope fractionation during moisture transfer.Here we present the δ18O records of stalagmites from three caves（Heilong,Qingtian and Sanbao caves） along a transect of progressively increasing altitudes（1 250~2 250m） in Shengnongjia（31.5~31.7°N,110.2~110.6°E）,central China.We confirm the effect of altitude on stalagmite δ18O that mean stalagmite δ18O values from two different same time period decrease with increasing cave altitude at a rate of about-0.1‰/100 meters,indicating that the oxygen isotope compositions of precipitation undergoes fractionation during the moisture transfer from low to high altitude.In fact,modern meteorologic precipitation data show that altitude effect has an effect on the δ18O change at a rate of-0.2‰/100 meters,which exceeds more the speleothem result.Consequently,in addition to the isotopic influence of precipitation on stalagmite δ18O,temperature differences between altitudes contribute greatly to δ18O variation.In combining with annual averaging temperature in caves,an average d δ18Op/dT of approximately-0.25‰/℃ was calculated,which agrees with the result of isotopic equilibrium fractionation by O＇Neil.