桂林地区大气降水(大雨、暴雨)的δ18O特征与水汽来源的关系

摘要

现代大气降水中的稳定同位素组成是全球或地区性水循环研究的重要载体,同时也是冰芯、湖泊沉积物、石笋等研究领域中,运用稳定同位素来重建古气候的重要依据.本文研究了桂林地区2012年大气降水氢氧同位素组成的逐日变化,根据得到的132组氢氧稳定同位素组成建立了桂林局地大气降水线方程为δD=8.8δ18O +17.96,大气降水的δ18O波动范围在-13.56‰～+1.07‰,平均为-5.78‰;δD在-101.52‰～+16.02‰,δD平均为-41.03‰.利用降水稳定同位素资料,结合后向轨迹法(Backwards Trajectory)对桂林水汽来源进行追踪,发现夏季(5-10月)大气降水的水汽来源主要受来自孟加拉湾、南海海洋气团的水汽源的控制,降水的δ18O值偏负,平均为 8.02‰(共64组);冬季(11月至次年4月)大气降水的水汽来源主要受来自西太平洋暖湿气团、冬季风冷气团或西风环流所携带的大陆性气团的影响,不同程度地叠加了局地环流气团、蒸发水汽的补给的影响,降水的δ18O值偏正,平均为-2.86‰(共68组).研究结果表明,桂林大气降水的稳定同位素组成与降水的水汽来源、季风类型、降水云团来源和性质有关,来自远距离输送夏季风海洋性水汽团形成的降水δ18O值较低(或偏负),而大陆性气团或局地蒸发水汽循环形成的降水δ18O值较高(或偏正).不同的水汽来源是决定降水中δ18O值变化的主要因素,因此,通过降水中的δ18O值,特别是其季节变化的特征分析,可以反过来揭示当地降水的水汽来源.%It is significant to use stable isotopic of modern meteoric precipitation for the global or regional water cycle research,which is also important component in reconstructing paleoclimate in the studies of ice cores,lake sediments,stalagmites and the others.Oxygen and hydrogen stable isotopes (δ18O and δD) in the meteoric precipitation collected from each heavy rain event from January to December in 2012 in Guilin,are studied in this paper.The overall correlation between δ18O and δD is obtained as δD-8.8 δ18O +17.96 from 132 stable hydrogen and oxygen isotope components.During the monitoring period,δ18O values of the meteoric precipitation varied-13.56 ‰ to +1.07 ‰ with an average δ18O value of-5.78 ‰ (VSMOW) and the δD values of the meteoric precipitation are between-101.52 ‰ and +16.02 ‰ with an average of 41.03‰ (VSMOW).Using Backward Trajectory technology of the HYSPLIT model and the data of precipitation isotopes in Guilin,it was found that the water vapor sources of the meteoric precipitation in the summer time (May to October) is mainly controlled by the water vapor of the marine airmasses from the Indian Ocean or Gulf of Bengal and the South China Sea,secondly from the West Pacific Ocean.The δ18O value of the meteoric precipitation is more negative with average δ18O value of-8.02 ‰ (average of 64 groups) and accounts for 53.3％ of the total meteoric precipitation.And the water vapor sources of the meteoric precipitation in the winter monsoon (November to the next April) are mainly affected by the warm-moist airmass es of the West Pacific Ocean and the continental cold air mass flow carried by the westerly wind zone (or the winter monsoon) and sometimes superposed by the part of the local evaporation moisture circulation.In this period,the δ18O value of the meteoric precipitation is more positive with average of 2.86 ‰ (average of 68 groups) and accounts for 46.7％ of the total meteoric precipitation.The research results show that the stable isotopic composition of modern meteoric precipitation in Guilin is related to the monsoon type and source of the meteoric precipitation cloud and property of the meteoric precipitation.The δ18O value of the meteoric precipitation from far transmission of water vapor of the marine air masses carried by the summer monsoon is more negative.In contrast,the δ18O value is more positive while the meteoric precipitation is controlled by the continental cold air mass flow carried by the westerly wind zone,or by the overlay of winter monsoon with localised air moisture circulation.These results indicate that the different sources of water vapor have significant influences on the δ18O variation of the meteoric precipitation.Therefore,analysis of δ18O in the meteoric precipitation,especially its seasonal variation characteristics in turn reveals the water moisture sources of local meteoric precipitation.