The Marine and Continental Environmental Changes after Nantuo Glaciation in Xiushan, Chongqing

Key words: 
• Snowball Earth /
• Nantuo glaciation /
• acid-insoluble residua /
• environmental changes /
• geochemistry

Abstract: It is believed that the Nantuo glaciation represented by sediments in the Yangzte Platform could be as a Snowball Earth event and restricted to the Marinoan glaciation in age. The carbon, oxygen and strontium isotopes, and elements of whole-rock and acid-insoluble residua of the~2.5 m-thick Doushantuo cap dolostone and shale from the Changheqiao section in Xiushan, Chongqing were measured and their geochemical features indicated some specific environmental co-changes of ocean and continent during the post-Nantuo Glaciation warming. The δ¹³C_PDB varied from -3.4‰ to -5.2‰, which could be explained by the model of stratification and mixing of ocean. The ratios of U/Th suggested that the sedimentary environments were rapidly changed from anoxic to oxic during the post-glacial warming. These changes might be related to the upwelling and oxidizing of the anoxic deep seawater. The lower 0.3 m samples have strong positive Eu anomaly, that may be related to the frequent hydrothermal activities in the Ediacaran ocean or proximal hydrothermal activity. The negative Ce anomaly in the cap dolostone might be due to oxic conditions. The values of Y/Ho and ⁸⁷Sr/⁸⁶Sr probably displayed that a large number of terrigenous residuas were carried into ocean by the increasing surface runoff, during the post-glaciation. The acid-insoluble residuas of cap dolostone can be used to study the background of the average degree of continental chemical weathering. The CIA of acid-insoluble residuas ranges from 71.2 to 72.9, which indicated a warm and wet environment and medium chemical weathering.