鱼粪化石特征对早侏罗世托阿尔期湖泊生态系统的启示:以川东大安寨段为例

Enlightment of fish coprolites characteristics on lake ecosystem in Early Jurassic Toarcian:taking the Da’anzhai Member in northeast Sichuan Basin as an example

早侏罗世托阿尔期早期大洋缺氧事件(Toarcian Oceanic Anoxic Event,~183.8 Ma)是一次全球性的多幕式生物—环境事件,其在四川盆地下侏罗统自流井组大安寨段亦有显示。对采集自川东北地区大安寨段的鱼粪化石进行形态学描述和内含物分析,发现鱼粪化石中含有大量的磷质成分,多为未经消化的鱼骨化石,另有大量介壳类化石与有机质混杂在一起。据露头岩性和显微薄片特征认为:(1)粪化石呈螺旋状,生产该粪化石的鱼类为大型肉食性鱼类,其喜食小型鱼类和软体动物等,推测可能是肺鱼类(角齿鱼);(2)鱼粪化石形成于开放型淡水湖泊中的半深湖—深湖区,其中湖泊表层含氧量丰富,主要由生产者、消费者构成了研究区早侏罗世托阿尔期大型湖泊生态系统,食物链较为复杂,而湖底为水动力条件较弱的还原环境。该研究成果可为早侏罗世托阿尔期大洋缺氧事件在湖泊中的沉积响应及生态影响研究提供依据。

The Toarcian oceanic anoxic event(T-OAE,Early Jurassic)was an episode of profound environmental changes marked by a global carbon-cycle perturbation,marine mass extinctions,and black-shale deposits.Morphological description and inclusion analysis of fish coprolites collected from the Da’anzhai Member of the Lower Jurassic Ziliujing Formation in the northeast Sichuan Basin were carried out in this paper.We found that the interior of coprolites is composed of abundant organic matters,silts,incomplete clay minerals,phosphorous fossils,undigested fish bone,and a large number of ostracods.According to the morphology and microscopic examination of the coprolites,we speculate that they originated from lungfish.The fish coprolites were deposited in an intermediate to deep lake.During the Da’anzhai depositional period,the surface water of the Sichuan lake was enriched in oxygen,and the lake ecological system was healthy.The bottom of the lake was characterized by deposits of black shale,which implies an anoxic and reducing environment.The T-OAE fish coprolites may suggest a stratified water column in a Toarcian lake.The above results may provide a basis for the study of sedimentary response and ecological impact in the lake of Toarcian oceanic anoxic events of early Jurassic.