兰州-民和盆地八盘峡剖面河口群沉积物色度纪录的140.66～124.19Ma间气候变化

Climate Change during 140.66～124.19 Ma Recorded by the Color of the Sediments of the Hekou Group from Lanzhou-Minhe Basin

随着新生代全球气候变化研究的不断深入,迫切需要了解前新生代气候变化背景,特别是白垩纪作为距今最近的"温室地球"时期,其气候变化已成为当今研究的重点课题。本文通过对兰州-民和盆地八盘峡剖面河口群沉积物采样与色度测量,获得了兰州-民和盆地140.66～124.19Ma期间的色度变化序列。对色度值变化机制分析发现,河口群总体为一套红色色调的碎屑沉积岩石组合,有机质含量极低,色度值变化与风化作用及沉积环境有关,亮度值的高低主要与沉积物中钙质胶结物的含量有关,而红度(黄度)值的变化主要与赤铁矿含量多少有关。综合分析色度值及沉积环境变化显示盆地沉积时处于暖湿气候环境,进一步分为140.66～137.43Ma气候相对湿热、137.43～127.41Ma气候相对温湿和127.41～124.19Ma气候相对干热3个阶段;这种阶段性变化以气候的快速转型为界,而且比海洋沉积物记录的全球海平面温度阶段性变化的起始时间提前约1～1.3Ma。

With the increasing research of global Cenozoic climate change,it is imperative to understand the background of climate change prior to the Cenozoic,especially the Cretaceous climate which is characteristic of the ＂greenhouse Earth＂ alike present-day＇s climate.The Cretaceous climate change has been attracting a magnitude of attention.This study obtained the sequence of color variation of the Lanzhou-Minhe basin during 140.66～124.19 Ma by sampling and color measurement for the red nonmarine sediments from the Lower Hekou Group along the Bapanxia section in the Lanzhou-Minhe Basin.Analysis of the mechanism for color variation shows that Hekou Group is generally one set of red detrital sedimentary rocks assemblage,with the least amount of organic material.The color variation was predominantly controlled by the weathering of the source area and sedimentary environment.The level of brightness（L＊） values was related mainly to the content of the calcium cement（the sediments are poor in the organic matter） and the redness and yellowness（a＊ and b＊） to the content of the hematite.The stepwise variation of the color together with the sedimentary settings suggests that the basin was in a wet-warm climate environment,which can be further divided into three stages：relatively wet hot-warm stage at the 140.66～124.19 Ma,relatively wet and warm stage at 137.43～127.41 Ma,and dry-warm stage at 127.41～124.19 Ma.The climate transition was bounded by the rapid climate shift,and the initiation of this shift was approximately 1～1.3 Ma prior to the age observed from the record of the marine sediment.