华南埃迪卡拉纪硫化海洋环境演变--来自深水相区的黄铁矿证据

Euxinic Evolution of Ediacaran(Sinian) Ocean in South China——Evidence from Analysis of Pyrite Morphology

为进一步认识华南埃迪卡拉纪(震旦纪)海水硫化条件的演变过程,本文对斜坡相沉积的陡山沱组中的黄铁矿进行形态与粒度分布研究。结果表明陡山沱期沉积的多数黄铁矿是早期成岩作用形成,以自形、半自形晶体为主;个别层位中草莓状黄铁矿粒径大于10μm的比例较高(89%～96%),反映是在海水、沉积物界面以下生长的,不能充分指示水体硫化状态。但在陡山沱组中部和上部发育有数层从水体中析出的同沉积草莓状黄铁矿(96%以上的莓球粒径小于10μm),反映出海水硫化条件发育。黄铁矿铁与高活性铁的比值(FePY/FeHR)作为判别海洋硫化状态的重要指标,可能会因次生黄铁矿的叠加效应影响而产生偏差,应结合黄铁矿矿物形态—粒度分析和其他手段加以校正。通过这2种方法并结合碳、硫同位素等研究表明,陡山沱组沉积旋回Ⅰ顶部和旋回Ⅲ底部沉积期间华南古海洋曾经历了两幕规模较大的间歇性硫化时期,在此期间硫化水体的覆盖范围至少包括陆架至斜坡中部相区。硫化水体的形成和时空演变主要受大气含氧量、海水硫酸盐浓度、细菌硫酸盐活动(BSR)及海平面变化等因素控制。重要的生物类群主要出现在非硫化间隔期,表明海水硫化条件是限制生物发展的重要环境因素之一。

In order to better understand euxinic evolution of the Ediacaran ocean in South China,a study of authigenic pyrite morphology and size distribution from the Doushantuo formation in deep-water area has been conducted.The result shows that most pyrites in the Doushantuo formation were formed below sediments/water interface during early diagenesis.They are generally euhedral or semi-euhedral in shape,some also form framboids,but most（89%~96%） of them are larger than 10μm in diameter.These diagenetic pyrites may not reflect euxinic water condition.There are several intervals of syngenetic framboidal pyrites（More than 96% with diameters 10μm） occurring in middle and upper Doushantuo formation,which indicate euxinic condition.FePY/FeHR ratio in sediments as an important proxy alone for environment euxinia may be biased due to the superimposition of secondary pyrites generated within sediments during early diagenesis.Therefore it is better to use this proxy in combination with other methods,such as the analysis of pyrite morphology.Using these methods along with carbon and sulfur isotope analysis,the present study shows that the Ediacaran ocean in South China had experienced two episodic euxinic intervals during upper sedimentary cycle I and lower sedimentary cycle III of Doushantuo Formation.Euxinic water body may have expanded to slope facies area in extent during these intervals.The temporal and spatial evolution of the euxinic environments seemed to have been mainly controlled by the variation of ocean oxidation,sulfate concentration,bacterial sulfate reduction（BSR）,sea-level fluctuation and their interactions.The known major fossil groups in this period largely occurred in the non-euxinic intervals,which might means that,in addition to ocean oxidation,the euxinic condition also played an important role on the biotic evolution in this period.