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石笋氧碳同位素古气候代用指标研究进展

孔兴功   

  1. 南京师范大学 地理科学学院,南京 210046
  • 收稿日期:2009-06-20 修回日期:2009-06-20 出版日期:2009-06-20 发布日期:2009-06-20

Advance in study of oxygen and carbon isotope variations in cave stalagmites as Palaeo-climate proxies

KONG Xing-gong   

  1. College of Geography Science, Nanjing Normal University, Nanjing 210046, China
  • Received:2009-06-20 Revised:2009-06-20 Online:2009-06-20 Published:2009-06-20

摘要: 洞穴石笋以其独立的绝对年龄和稳定的古气候代用指标而成为很好的陆地古气候研究材料。应用U 系定年,可以获得独立精确的日历年时间序列,结合年纹层统计,可以获得年际甚至是季节性变化古气候记录。作为古气候代用指标,石笋氧同位素具有全球可对比性,现已建立了氧同位素的全球气候变化曲线,显示了比深海氧同位素及极地冰芯氧同位素记录更多的优势。石笋δ18O 的影响因素很多,在东亚季风控制区和其它季风控制区,石笋的δ18O 反映了夏季风的强度或者是季风降雨的变化。这些全球可对比的具有高精度绝对定年的石笋δ18O 记录,突破了早期的单一温度控制机理,不仅为古气候学家提供了坚实的古气候变化时间序列,而且使古气候研究能从机理上探讨其变化规律。δ13C不如δ18O 具有全球可对比性,在同位素平衡的开放体系,石笋δ13C 主要反映了洞穴上覆土壤CO2 的同位素特征。因此,一般认为石笋δ13C可直接反映当地植被的特征,包括植被类型C3/C4 比率的变化及植被的茂盛与衰退,在一些地区也可作为大气降水和温度变化的指标。石笋δ13C 很易受到蒸发作用、滴水的快速去气、动力分馏、碳酸盐的先期沉积等影响而使得其数值偏正,应该予以重视。通过介绍石笋氧碳同位素的研究现状,使得读者对石笋的稳定同位素指标有一个初步认识。同时,对今后的研究方向给与一定的探讨。

Abstract: Stalagmites are now regarded as valuable archives in study of climatic conditions on the continents. They are ideal materials for precise U-series dating, yielding ages in calendar years, coupled with annual layer counting, the chronology of stable oxygen-isotope then be made with the resolution higher than year. These absolute precise dating and high resolution records offer a number of advantages relative to other continental climate proxy recorders such as lake sediments and peat cores. In many cases, stalagmite records may provide climate conditions that rival those obtained from marine sediments and ice cores. The controls on stalagmite δ18O are numerous, under equilibrium condition, the δ18O values of stalagmites are related to intensity of summer monsoon or amount of the monsoon precipitation at the typical monsoon area. This emphasis on global climate teleconnections has replaced attempts to constrain absolute palaeotemperature, allowing the scientific community to place global climate changes on a firm chronological and climate dynamical footing. Unlike the global comparison of δ18O, under equilibrium open system conditions, the δ13C of the dissolved species reflects the isotopic composition of the soil CO2, with no detectable isotopic imprint from the carbonate host-rock. Thus the stalagmite δ13C is mainly controlled by the vegetation covering the cave, normally by the C3/C4 ratio of the local vegetation. The δ13C can still be related to the change of precipitation and temperature at special region.We should pay special attention to these processes including evaporation, rapid degassing of cave dripwaters, kinetic fractionation,CO2 degassing of dripwaters and consequent calcite precipitation in the vadose zone above a cave that could affect the carbonisotope to a relatively heavier signature. The literature is reviewed briefly to provide for the readers a sense of the current research work of the stable isotope of stalagmite and provide some ideas for future research directions.