扬子地区奥陶纪—志留纪转折期多事件耦合关系及有机质富集

Coupling relationship of multiple events and enrichment of organic matter during Ordovician-Silurian transition period in Yangtze region

从地质事件的影响角度出发,讨论扬子地区奥陶纪—志留纪转折期多地质事件的耦合关系、地质事件的环境响应以及对富有机质页岩发育的影响。结果表明:构造运动增强的大陆硅酸盐风化作用以及火山事件增加的碳埋藏都导致大气CO_(2)浓度的下降,当达到阈值以下便引发赫南特冰期事件;火山喷发出的硫会在平流层形成硫酸盐气溶胶,增加反射率,促进冰期的发展;冰期增大纬度温度梯度,促进上升流的形成;冰期降温的突发性、水体硫化缺氧环境的扩张以及火山事件所输入的汞和砷等有毒重金属是导致晚奥陶世生物大灭绝事件(LOME)的主要机制;火山事件输入的火山灰和上升流给海洋表层带来大量营养元素,以及冰期海平面迅速下降期间导致溶解性无机磷酸盐(DIP)浓度的增加,增强海洋初级生产力,促进有机质的生产;大量有机质在沉降过程中消耗氧气导致深层形成最小含氧带(OMZ),火山灰快速吸收溶解氧以及冰期结束海平面快速上升都会导致水体缺氧条件的扩张,从而有利于有机质的保存;主要的沉积作用是悬浮沉积,但局部也因构造运动、水体变浅以及陆源输入增强等因素发育部分重力滑塌、浊流、碎屑流、风暴流沉积。

From the perspective of geologic events,this study explores the interconnections between various geological occur-rences,their environmental repercussions,and their influence on the development of organic-rich shale during the Ordovi-cian-Silurian transition period in the Yangtze area.The findings indicate that continental silicate weathering intensified by tectonic movements and increased carbon burial due to volcanic events both contributed to a decline in atmospheric CO2 con-centration.This decrease in CO_(2) concentration triggered the Hirnantian glaciation when it fell below a certain threshold.Sul-fur released from volcanic eruptions formed sulfate aerosols in the stratosphere,increasing albedo and promoting the Hirnan-tian glaciation.The Hirnantian glaciation amplified the latitudinal temperature gradient,fostering upwelling.The abrupt cool-ing during the Hirnantian glaciation,the expansion of euxinia environments in the water column,and the introduction of toxic heavy metals like mercury and arsenic from volcanic events were the primary mechanisms leading to the Late Ordovician Mass Extinction(LOME).Volcanic ash input and upwelling from volcanic events brought substantial nutrients to the ocean sur-face.The rapid decline in sea level during the Hirnantian glaciation increased the concentration of dissolved inorganic phos-phate(DIP),boosting oceanic primary productivity and organic matter production.The consumption of oxygen by abundant organic matter during subsidence resulted in the formation of an oxygen minimum zone(OMZ)in deeper layers.The swift uptake of dissolved oxygen by volcanic ash and the rapid rise in sea level at the end of the Hirnantian glaciation expanded an-oxic conditions in the water column,aiding organic matter preservation.The primary sedimentation process was suspended sedimentation,although locally,gravity slumping,turbidity currents,clastic flows and storm surge deposits were also devel-oped due to tectonic movement,shallow water columns,and increased terrigenous input.