西藏地区二叠系-三叠系界线剖面有机碳同位素异常研究

The Tibet was located between Southern Alps and Southern China, and characterized by the successive carbonates sediments in the Permian-Triassic. Furthermore, the inorganic carbon isotopes showed gradual and sudden shift，the former is mainly founded in south Alps area of western Tethys, however the latter is mainly founded in South China area of eastern Tethys, they are perfectly comparative under the chronostratigraphic framework that is based on the conodont sequence. Therefore, the carbon isotopes have been used to correlate such strata outcropping in the Southern Alps and Southern China. carbon isotope is a major method to contrast The Permian-Triassic event stratigraphy, Nevertheless, the inorganic carbon isotopes indicate significant differences in the one strata of one section,without third-party phenomenon for Identity verification. However, the organic carbon isotopes could avoid reform from diagenesis, epigenesis, and hypergenesis to keep more primary geological information,then have been made full used in the Permian -Triassic boundary. Based on the carbon isotopes, this project will focus on the Wendangsesang and Selongxishan sections in Tibet for undertaking isotopes including inorganic carbon and organic carbon in order to discuss their anomalies and mechanism, then get the model.

二叠纪-三叠纪之交，西藏地区位于南阿尔卑斯和华南地区之间，具有连续碳酸盐岩沉积，无机碳同位素具有南阿尔卑斯典型的渐变式和华南地区典型的突变式两种异常表现样式，因此开展碳同位素研究可以建立以上两个地区碳同位素地层对比的桥梁。虽然无机碳同位素作为二叠系-三叠系事件地层对比的重要手段，但往往同一剖面相同层位存在较大差异，且难以得到第三方同一性验证，而源于生物体先质的有机质在地质历史中具有相对稳定的演化规律，可避免成岩作用、后生作用和表生作用等改造，其同位素更好保存了原始信息，显示出有机碳同位素在地层研究中的优越性，并且其应用范围不再局限于碳酸盐岩地层，在碎屑岩细粒沉积物中也能够得到广泛应用。本项目拟选择西藏地区文不当桑和色龙西山剖面作为基干剖面，同步开展无机碳和有机碳同位素分析，研究二叠纪-三叠纪过渡期碳同位素异常信号和驱动机制，分析有机碳和无机碳分配和循环过程，建立碳同位素异常解释模型。

项目针对青藏高原和华南二叠系-三叠系地层开展了调查研究工作，重点选取改则文布当桑剖面、江西赣丰1井岩心剖面、安徽皖泾地1井岩心剖面开展详细实测和描述，在建立岩性地层、生物地层、锆石年代地层对比格架下，详细研究了二叠系-三叠系界限剖面碳氧同位素变化样式。通过国内外典型二叠系-三叠系界限剖面对比研究表明，碳同位素变化样式受到沉积相的控制，其中泻湖相和斜坡相更有利于富轻碳同位素组分的有机质保存导致PTB地层无机碳同位素快速负异常；而台地相、台地边缘向和陆棚相PTB地层无机碳同位素缓慢负异常。有机质类型不但控制了碳同位素变化样式，而且有机碳长时间的高速埋藏可能会造成海平面的下降，同时形成大量的烃源岩，控制现今油气资源格局，形成了下扬子二叠系孤峰组、龙潭组和大隆组是页岩气勘探重要目标层；三叠系殷坑组致密油勘探重要目标层的勘探部署思路。此外，有机质长时间的高速埋藏和宇宙撞击事件可能对二叠纪末期生物灭绝具有重要的影响作用。