Ostracod Fauna Near The Permian-Triassic Boundary In Chongyang, Hubei Province And Its Palaeoenvironmental Significance

The end-Permian biotic event is the greatest mass extinction in the Phanerozoic. More than 95% of marine species disappeared during the event, while many microorganisms thrived, which formed the microbialites vastly distributed in the carbonate platform in South China. The fossils in the microbialites are mainly composed of calci-microbes, with some micro-gastropods, bivalves, foraminifers and ostracods. They constitute a simple and special ecological system which called bacterium-alga system.Ostracods were thriving after the mass extionction event and became one of the "disaster form" for their widely distribution and strong adaptability in every aquatic environment. They played an important role in the bacterium-alga system. Therefore, systematic study of ostracods near the mass extinction boundary has important value for understanding the evolution of ostracods and the marine environment at that time.But until present, the study of ostracods in the microbialites is focused in the slice observation. The systematic study of the ostracods is much less and just limited in Chongqing (Crasquin-Soleau et al.,2005), Jinya/Waili (Crasquin-Soleau et al.,2006) and Dajiang (Forel M-B et al.,2009). More work need to carry out.We have done much work on the ostracods involved in the microbialites in Chongyang, Hubei province. Nine samples from the bioclastic limestone below the mass extinction boundary, 15 samples from the microbialite and oolite above the boundary were analyzed. After processing the samples by the method of "hot-acetolysis", more than 600 spcimens were extracted from 5 samples. Thirty species ascribed to 10 genera were identified, of which 20 species of 6 genera discovered in the Changhsingian bioclastic limestone and 11 species of 7 genera found in the microbialite.The ostracod fauna in Chongyang section is mainly compose of Family Bairdiidae both in the abundance and in the diversity. Hollinella tingi (Patte) is the only filter-feeder in the fauna and others are deposit feeders. Lack of filter-feeder reflects the palaeoceanic water is rich in oxygen. The ostracod assemblages identified at Chongyang section are obviously different from those preciously reported in the contemporaneous microbialites in Guangxi,Guizhou and Chongqing areas, not only in the components of ostracods but also in the ratio of filter-feeders to deposit-feeders, an indicator of oxygen levels of the seawater. This spatial difference in ostracode assemblages might reflect the diversity of marine environment after the end-Permian mass extinction.The variation of the carbon isotope composition, an indicator of palaeoceanic environment shows a comparable pace with the change in the diversity of ostracod species, but not with the change in the abundance of ostracods. In general, the data of the carbon isotope are relatively low in the whole microbialite sequence. However, the ostracods are almost absent in the lower part of the microbialite but enriched in the upper part. The lack of the relationship between the ostracod abundance and the carbon isotope composition might have resulted from the development of the laminar structure in microbialites. At the Chongyang section, the lower part of the microbialites is featured by laminar structures while the upper part displays graniphyric fabric. The ostracods are found to be fewer when the laminar structures are more developed. This observation was similar to those reported elsewhere, such as the microbialites in Ziyun, Guizhou province.Some ostracods are found to disappear long before the main mass extinction horizon at the Chongyang section, inferring the presence of the initial biotic event prior to the main mass extinction. The stable carbon isotope composition of carbonate is observed to coincidently shift at the 200cm below the main mass extinction horizon, suggesting the deterioration of marine environments. The first episodes of the mass extinction happened near the mass extinction horizon with 16 species of 5 genera disappeared, while the other episodes happened at the top of the microbialite with 8 species of 5 genera disappeared.Our study would provides more materials for the understanding the evolution of ostracods and discussing the variations of biology and environment during the geological transitional stage.