利用荣玛地区温泉钙华δ^(18)O及微量元素重建西藏全新世以来古气候

Resuming the Holocene paleoclimate usingδ^(18) O and trace elements of travertine in Rongma area,Tibet

西藏荣玛温泉钙华发育,是研究古气候的天然地质载体。本研究在U系法定年的基础上,参照古里雅冰心δ18 O的气候记录,对钙华微量元素、稀土元素、氧同位素与古气候关系进行系统分析,进而挖掘荣玛钙华的古气候信息,总结出该区古气候变化规律:钙华母岩为灰岩,钙华锥以"半球面式生长模式"沉淀;钙华δ18 O与温度、降雨量具有较好的正相关性,表明钙华δ18 O受温度和降雨量影响;钙华Sr/Ca、Sr/Ba与降雨量具有较好负相关性,说明钙华Sr/Ca、Sr/Ba在一定程度上受降雨量影响;根据钙华δ18 O、Sr/Ca和Sr/Ba与气候的关系推测荣玛地区古气候变化规律为:35.87ka时期,为高温多雨气候;35.87~12.38ka时期,气温降低、降雨减少,为干冷气候;12.38~6.67ka时期,气温回升降雨量增大,气候由干冷变为湿暖;6.67~5.29ka时期,气温、降雨量波动降低,气候由湿暖变为干冷;5.29~0ka时期,气温、降雨量升高,由干冷变为湿暖气候。

Travertine, a natural geologic vehicle for the study paleoclimate, widely grow along the thermal spring at Rongma town of Tibet. On the basis of uranium-series dating technique and the climatic record in the Guliya ice core, we analyzed the relationship between the value of δ18O, Sr/Ca and Sr/Ba of travertine and the rainfall （or temperature） to reveal and make a summary of the paleoclimate. The following conclusions were obtained： （1） on the basis of the age of the sample, we present a hemispherical growing model of the Rongma travertine; （2） the data of trace element and REE indicated that the limestone was the initial rock of the travertine; （3） the similar variation trend and positive correlation between δ18O and the rainfall （or temperature） suggested that the δ18O of travertine was influenced by temperature and rainfall; （4） the opposite variation trend and negative correlation between the ratio of Sr/Ca （Sr/Ba） and the rainfall showed that the ratio Sr/Ca （Sr/Ba） was affected by the rainfall. It was megathermal and rainy at 35.87 ka, but changed to be dry and cold from 35. 87 ka to 12. 38 ka. On the contrary, it became warm and moist from 12. 38 ka to 6. 67 ka From 6.67 ka to 5.29 ka, it reversed to be dry and cold and then reversed to be warm and moist from 5.29 ka to 0 ka.