晚始新世-早渐新世渭河盆地古气候变化的碳酸盐记录

PALEOCLIMATE VARIATIONS DURING LATE EOCENE TO EARLY OLIGOCENE IN WEIHE BASIN,CENTRAL CHINA,REVEALED BY CARBONATE MINERALS CONTENT

晚始新世-早渐新世转变期(约34 Ma, EOT)温度降低是新生代显著气候事件之一,对亚洲环境演变有深刻影响。本研究基于渭河盆地厚度为413.95 m的支家沟剖面34.89~31.51 Ma时段的河湖相沉积,获得高分辨率总碳酸盐、方解石含量变化序列,结合薄片及扫描电镜分析,尝试建立晚始新世-早渐新世渭河盆地气候和环境演变时间序列,探讨EOT期间渭河盆地古气候对全球温度变化的响应。镜下岩石薄片鉴定显示,沉积物粒度变化与岩相变化对应较为一致。34.89~33.89 Ma,高含量的总碳酸盐、方解石指示气候较为温暖干旱,沉积序列指示从滨湖相向湖泊三角洲相过渡;33.89~33.09 Ma期间,低含量的总碳酸盐、方解石指示气候偏凉且相对湿润,沉积环境为湖泊三角洲向滨浅湖过渡,水位逐渐上升;33.09~31.51 Ma,总碳酸盐、方解石含量变化与地层岩性变化有较好的一致性,此时沉积相为滨湖相,湖泊水位可能高于34.89~33.89 Ma,但低于33.89~33.09 Ma。结合全球温度变化序列,本研究认为渭河盆地始新世晚期-渐新世早期湿度变化可能受全球温度驱动的蒸发量控制,但明确的驱动机制有待进一步研究。

The Late Eocene-Early Oligocene climate temperature drop(ca.34 Ma,Eocene-Oligocene Transition,abbreviates EOT)is one of the significant Cenozoic events.To study the influence of EOT on climate change in East Asia,exploring the evolution of paleoclimate,and analysing the relationship between regional environmental change and the global climate transition events,are vital to understand the climate and environmental systems.The Weihe Basin(34°~35°40'N,107°~110°35'E),located in Shaanxi Province,Central China,covers an area of about 20,000 km’,with a watershed area of 130,000 km².The Cenozoic deposits in the Weihe Basin are good archives for recording climatic and environmental changes.The Bailuyuan Formation sedimentary sequence(34°16'03"N,109°12'26"E;635 m above sea level)in the Weihe Basin can be divided into three units from bottom to top:the lower part(413.95~346.75 m)is thick brown-red silty mudstone or siltstone with horizontal bedding,which is unconformity lying on the Honghe Formation.The middle part(346.75~233.5 m)is mainly composed of thick grey white sandstone interbedded with thin brown red siltstone or argillaceous sandstone.The upper part(233.5~0 m)is interlayered with grayish-white sandstone and brownish-red mudstone,and is in unconformity contact with the overlying Lengshuigou Formation with low angles.Based on the fluvial-lacustrine sediments of Zhijiagou section in Weihe Basin during the period of 34.89~31.51 Ma,we obtained a high-resolution sequence of total carbonate and calcite contents(836 samples),analyzed by the method of Fourier transform infrared spectroscopy transferring(FTIR).In addition,we observed the sediments by thin rock slice and Scanning Electron Microscopy(SEM).Microscopic examination of thin sections of rock shows that variations of sediment grain size correspond well with the lithofacies.During 34.89~33.89 Ma,high content of total carbonate and calcite(with average of 16.10%and 11.93%,respectively)indicates that paleoclimate is relatively warm and arid,and the sedimentary facies indicate a transition from shallow lake to lake delta facies.During 33.89~33.09 Ma,low content of total carbonate and calcite(with mean of 4.56%and 2.75%,respectively)indicates a cold and relatively humid paleoclimate,and the sedimentary environment is a transition from lake delta to shore-lake and shallow-lake facies,the paleo-water level gradually rises.From 33.09 Ma to 31.51 Ma,variations of total carbonate and calcite contents(with average of 14.94%and 11.20%,respectively)are in good agreement with variations of lithology.During this time,sedimentary facies was shore-lake facies,and lake level may be higher than that of 34.89~33.89 Ma,but lower than 33.89~33.09 Ma.The sub-Tethys Sea may not change significantly during the study period(33.89~31.51 Ma),there was not an intensive uplift of the Tibetan Plateau synchronized with the wet-dry variations in the Weihe Basin,therefore the sea-land configuration changes and surface uplift could have less influence on the wet-dry changes in the Weihe Basin during the EOT.Combined with global temperature series,we speculate that the humidity variations in the Weihe Basin from the Late Eocene to Early Oligocene may be controlled by evaporation which was driven by global temperature variations,however the exact driving mechanism needs further study.