Reconstruction of paleo-salinity and lake-level fluctuation history by using boron concentration in lacustrine mudstones

Abstract

根据封闭湖泊古盐度与湖面变化之间具有反向变化的特点,以沱沱河地区通天河剖面为例,利用泥质岩的硼元素含量作为古盐度计,重建了青藏高原北部地区晚渐新世-早中新世湖泊沉积的古盐度变化曲线,讨论了应用湖水古盐度变化曲线追踪过去湖平面变化历史的适用范围和局限.结果表明:雅西错组上部古湖泊水体的盐度一般处于咸水湖分布区,盐度变化于微咸水到超咸水范围内,指示渐新世晚期气候干燥,湖平面长期处于低水位期,但中新世早期五道梁组沉积期,高原气候开始向湿润方向发展,湖水古盐度明显下降,湖水位快速上升.;The relationship between lake-level changes and lake water salinities has been confirmed as a covariance in closed-basin lakes. According to this principle, lake-level fluctuation history over time can be reconstructed using quantitatively calculation of paleo-salinities in boron concentration of lacustrine mudstone deposition. In this article, boron contents in lacustrine sediments as a salinity index, examples from the Tongtianhe section in the Tuotuohe area of northern Tibetan plateau, vertical lake-level curves inferred from paleo-salinities calculation data was reconstructed during late Oligocene-early Miocene. At the same time, applicability and limitation of this method were also discussed. The results showed that the salinity of paleo-lake water was from brackish to saline water during the upper Yaxicuo Formation deposition. It indicated that late Oligocene was an arid climatic period and lake level was a long low water stand in this area. Conversely, paleo-lake water salinity obviously dropped and reflected water level rise in lower Wudaoliang Fromation. It indicated that Tibetan Plateau climate evolved to enter a humid stage in early Miocene.