Anoxic Events And Chemostratigraphy At The Wuchiapingian–Changhsingian Boundary In Zigui,Hubei Province

Two mass extinctions took place in the Permian,one at the beginning of the Wuchiapingian—the end-Guadalupian extinction—and the other at the end of the Changhsingian,the end-Permian extinction.During the eight million years between the two extinctions,the paleo-environmental and paleoceanic changes are unclear and the connection between these changes is need to be figured out.In this study,the systematic field investigation of the the Wuchiapingian-Changhsingian strata was conducted at Yanglin in Zigui city,Hubei Province.Carbonate C-O isotopic composition,carbonate-associated sulfate(CAS)sulfur isotopic composition,total organic carbon(TOC)content,total sulfur content,and framboid size distribution were analyzed to constrain the carbon-sulfur cycles and the paleo-ocean redox conditions changes.The carbonate carbon-isotope profile shows a rapid negative shift of2‰-magnitude from+4‰to+2‰at the top of the Wuchiaping Formation,and another rapid negative shift from+2 to-5.9‰in the lower Changhsing Formation,and then a rapid bound back to-2‰,and a gradual recovery back to?+2‰after a small negative excursion.Overall,the?¹³C profile displays a“V”shape negative excursion in the lower Changhsing Formation.This kind of negative excursion of carbon isotope is very pervasive in South China,and is the carbon isotope marker for the Wuchiapingian-Changhsingian boundary,and thus we can identify the Wuchiapingian-Changhsingian boundary in the lower Changhsing Formation at Zigui.The negative carbon isotope excursion at the Wujiaping-Changxing boundary in south China was mainly caused by the late eruption of Emeishan large igneous province(LIP)and oceanic enoxia.In the Yanglin section,framboidal pyrite is abundant in the bedded siliceous lime mudstone at the bottom of the Wujiaping Formation,with a small range of variation and an average framboid size of 6.8?8.5?m.Framboidal pyrite is abundant in thin-to–medium-bedded lime mudstone in the middle Wuchiaping Formation.Framboid size distribution is wide with a large range of variation.The average framboid size is 6.5?8.7?m,with a larger average particle size and a maximum size of larger than 25 m.The number of framboidal pyrite in the upper siliceous limestone of the Wujiaping Formation was large,with the average particle size ranging from 5.8 to 7.3?m,and the maximum particle size was small.Compared with theframboidal pyrite in the lower Wujiaping Formation,the particle size was smaller and the distribution range was narrower.In the bottom of Changxing Formation limestone containing more framboidal pyrite,framboidal pyrite in the siliceous limestone of the middle of Changxing Formation is more abundant,and framboidal pyrite particle size distribution is wide,range is bigger,average particle size of 4.2?7.2?m,while most of the sample average particle size less than 5?m.The average particle size is small,and the maximum particle size less than 20?m.The amount of framboidal pyrite in the thick bedded limestone in the upper Changxing Formation is small.According to the framboid size distribution,this study concluded that the middle Wuchiapingian paleoocean was oxic-dysoxic environments,the late Wuchiapingian-early Changhsingian paleocean was intermittent sulfidic environments,and the middle Changhsingian paleooceanoxic-dysoxic environments.The intermittent euxinic event at the–Wuchiapingian-Changhsingian boundary not only occurs in the Yanglin section,but also in other areas of South China,Canada and Greenland,which indicates that the anoxic event at the–Wuchiapingian-Changhsingian boundary is likely to be a global anoxic event.This anoxic event is environmental disturbance before the Permian extinction event.The intermittent occurrence of such environmental deterioration event may also be one of the causes for the end-Permian extinction.CAS concentrations and CAS sulfur isotope composition were abnormal around the Wuchiaping-Changhsingian boundary(WCB).CAS concentrations increased significantly at WCB,and CAS sulfur isotope was negatively excursion with a18‰-magnitude at the WCB.The negative excursion of CAS sulfur isotopes is at least a regional event.The reason of its negative excursion may be related to the volcanic eruption of EMeishan LIP,seafloor hydrothermal activity and oceanic anoxia.