Six ichnogenera,Chondrites,Palaeophycus,Planolites,Rhizocorallium,Skolithos,and Thalassinoides,were identified from the drilling cores near the Global Stratotype Section and Point(GSSP) of Permian-Triassic boundary at Meishan,Zhejiang Province.The ichnological indices,including abundance,bioturbation index,and disturbed depth of trace fossils,show two episodes of intense changes near the Permian-Triassic Boundary(PTB).Episode I occurred in Beds 25-27b when the ecologically complicate forms such as Chondrites,Skolithos,Rhizocorallium,and Thalassinoides disappeared hereafter,the bioturbation index reduced from 1-5 to 1-3,and the disturbed depth declined from 5-66 to 2-5 cm.Episode II took place at the base of Bed 33 with the disappearance of Palaeophycus and Planolites,and subsequent absence of trace fossils and bioturbation till the middle-upper part of Bed 41 when the disturbed structures reoccurred,but they are only tiny Planolites and the bioturbation index was never higher than 3 and the disturbed depth less than 4 mm.Episode I shows an intense change,corresponding to the main stage of the end-Permian mass extinction,whereas Episode II is relatively weak,corresponding to the epilogue of the mass extinction of trace makers in the Early Triassic.Subsequently,ichnofossils were dominated by surface tracks in simple ecological habit and structures.This phenomenon indicates that the Early Triassic benthonic fauna is changed from sessile benthic system to mobile benthic system after the end-Permian mass extinction.In other words,the evolution of the trace fossils across the Permian-Triassic transition had an episodic process similar to the body fossils.In addition,the change of ichnofabrics is well coincided with the negative excursion of carbon isotopes and the expansion of cyanobacteria.As the results of physical and biogenic processes,trace fossils provided unique materials for the study of the biotic and environmental events,as well as their coupling evolution through the great Permian-Triassic transition.
The West Pingdingshan Section in Chaohu, Anhui Province, has been extensively studied in recent years and become one of the classic Lower Triassic sequences well-clarified in multiple stratigraphies. Also it is an important section that defines the Smithian-Spathian (S-S) boundary within the Olenekian Stage. The S-S boundary strata at the section are restudied in high-resolution conodont biostratigraphy and carbon isotopes. The refined S-S boundary defined by the FAD of conodont Neospathodus pingdingshanensis is at 30 cm above the base of Bed 52, corresponding to a rapid diversification of conodonts. A sharp positive shift of δ13Ccarb curve co-occurs at the S-S boundary and it can be used as a key reference to define the boundary. The defined S-S boundary position and carbon isotopes curve can be well correlated globally.
The stratigraphic sequence of calcimicrobialite facies at the Permian-Triassic (P/Tr) boundary has well recorded the biotic and environmental transition across the end-Permian catastrophic events. The biostratigraphy, microfacies, carbon isotopes, and fossil records across the P/Tr boundary have been studied at the Kangjiaping Section in Cili County, Hunan Province. Three biostratigraphic zones, Palaeofusulina-Colaniella Zone, Hindeodus parvus Zone, and Isarcicella staeschei Zone, are identified. The excursion of δ13Ccarb exhibits a sharp negative shift in the calcimicrobialite at the P/Tr boundary, which is roughly accordant with the abrupt bioclastic decline. In addition, five types of microfacies are recognized, including algal-foraminifer bioclastic limestone, algal-laminated calcimicrobial limestone, oolitic grainstone, vermiculate limestone, and intraclastic wackstone. The results indicate that the changeover of ecosystem from metazoan reef to calcimicrobialite in Cili is a classic case of marine ecological evolution during the Paleozoic-Mesozoic transition.
Carbonate carbon isotope δ(13Ccarb) has received considerable attention in the Permian-Triassic transition for its rapid negative shift coinciding with the great end-Permian mass extinction event.The mechanism has long been debated for such a δ13Ccarb negative excursion through the end-Permian crisis and subsequent large perturbations in the entire Early Triassic.A δ13Ccarb depth gradient is observed at the Permian-Triassic boundary sections of different water-depths,i.e.,the Yangou,Meishan,and Shangsi sections,and such a large δ13Ccarb-depth gradient near the end-Permian mass extinction horizon is believed to result from a stratified Paleotethys Ocean with widespread anoxic/euxinic deep water.The evolution of δ13Ccarb-depth gradient combined with paleontological and geochemical data suggests that abundant cyanobacteria and vigorous biological pump in the immediate aftermath of the end-Permian extinction would be the main cause of the large δ13Ccarb-depth gradient,and the enhanced continental weathering with the mass extinction on land provides a mass amount of nutriment for the flourishing cyanobacteria.Photic zone anoxia/euxinia from the onset of chemocline upward excursion might be the direct cause for the mass extinction whereas the instability of chemocline in the stratified Early Triassic ocean would be the reason for the delayed and involuted biotic recovery.
SONG HaiJunTONG JinNanXIONG YanLinSUN DongYingTIAN LiSONG HuYue
Series of large conodont samples with 20 species and 3 similar species in 3 genera were collected from the Permian-Triassic(P-T) boundary sequence in a shallow carbonate facies at Yangou(沿沟),Leping(乐平) County,Jiangxi(江西) Province,South China.On the basis of the distributions of the identified species,seven conodont zones have been recognized in ascending order as follows,Neogondolella changxingensis zone,Neogondolella yini zone,Hindeodus changxingensis zone,Neogon-dolella taylorae zone,Hindeodus parvus zone,Isarcicella staeschei zone,and Isarcicella isarcica zone.The successive occurrences of Hindeodus changxingensis,Neogondolella taylorae and Hindeodus parvus serve as proxies for defining the P-T biostratigraphy boundary at the base of Sub-bed 21-4 of Bed 21 in the Yangou Section.Correlations with the Meishan Section are also discussed in terms of conodont bio-stratigraphy.Three successive conodont faunal assemblages are grouped through the P-T transitional interval to examine the evolution of conodonts across the great transitional event.
