Ecological Modeling Of Marine And Terrestrial Paleocommunities In South China And Xinjiang During Permian And Triassic And Marine Redox Variations

The Permian-Triassic(P-Tr)transition is a critical period of biotic evolution,during which there are several mass extinctions and environmental catastrophe events,re-shaping the evolution process of ecosystems.The exploration of the causes,consequences and interaction mechanisms of these extreme events is one of the research hotspots in the earth sciences.This paper devoted to reveal evolution pattern of biology and environment in the key point based on stratigraphic record and fossil database from North Xinjiang(Northwest China)and South China.Biological evolution is driven by interspecific interactions in communities.The composition and structure of paleocommunities have a significant influence on its stability and resilience,determines the stability of communities to resist external environmental perturbation and the direction of biological evolution.This paper focus on Permian-Triassic paleocommunities,introducing the concept of guild and paleo-food web,reveals the interspecific relationships within the paleocommunities and the relationship between biodiversity and community stability,through the latest food-web modeling method—Cascading Extinction on Graphs(CEG),modeling the paleocommunity dynamics,process,stability and resilience in this period.Pyrite framboid method is used to restore redox history of South China during Late Permian to Middle Triassic,and re-evaluated the validity of this method.The potential impact mechanism of marine extreme environmental changes on the stability of paleocommunities is discussed in the end.Five new understandings in biological and environmental research are as follows.(1)The stability of 14 terrestrial paleocommunities from Middle Permian to Early Jurassic in North Xinjiang is evaluated using mathematical modeling method.CEG modeling indicates the decline of collapse threshold closely related to three extinction events.The community stability significantly reduced before the end-Guadalupian(Middle Permian)extinction,this might relate to this gradual biological extinction pattern,which was proved in other marine/terrestrial biological extinction studies in the same period.There is a sharp decline of community stability after P-Tr mass extinction.The compositions of three Early Triassic communities are completely different with other communities.Permian type guilds disappeared and new guilds emerged,and the guild richness also reduced sharply.Such unique dynamics may have stemmed from the decoupled size variations of herbivorous and carnivorous in that time.The P-Tr mass extinction greatly distorted the original evolution direction of Permian paleocommunities and laid a foundation for the development of modern ecosystem.Community stability has restored to the Late Permian level in the Middle Triassic,corresponding to the full recovery of Marine ecosystems.Extinction and recovery pattern of community during P-Tr transition in North Xinjiang are comparable to that in South Africa,indicating a global recovery pattern.Community stability reduced obviously before Triassic-Jurassic(T-J)mass extinction but restored immediately and keep stable in Early Jurassic,this pattern is like that of in other Mesozoic extinction events.(2)The food web of marine paleo-food web has been established.Stability of 21 marine invertebrate paleocommunities in P-Tr transition,a total of nine marine invertebrate paleocommunities and seven marine vertebrate paleocommunities in Early-Middle Triassic have been quantificationally analyzed using mathematical modelling method.The results show that the communities' dynamic significant inconsistencies in the two phases of P-Tr mass extinction.The dramatical reduction of guild diversity,guild richness and community stability occurred after the second extinction phase,while the first stage didn't have a serious impact on structure and stability of paleocommunities.Combined with previous studies,it is further confirmed that the first extinction phase resulted in a sharp decline in biodiversity,while the second extinction phase caused the complete collapse of ecosystem.The stability of invertebrate community recovered to a relatively high level in Spathian,indicating that the invertebrate community may have recovered more rapidly.This is because the invertebrate community has a very simple community structure,which dominated by primary consumers.The stability of such communities is dominated by biodiversity rather than community strcuture.On the contrary,although complex community structure and large apex predators—marine reptiles have already occurred in the Early Triassic marine vertebrate community,their community stability stay in a low level till the Middle Triassic,indicating that marine vertebrate community recovered slowly and the guild structure of Early Triassic communities are not complete.Fragile community structure is difficult to maintain and continuously evolve in a disturbed environment.Full recovery has not been achieved until Anisian.(3)The validity of pyrite framboid proxy in the reconstruction of marine paleo-redox condition has been evaluated.Compared with other geochemical proxies,the biggest advantage of the pyrite framboid method is that this method is still effective for the weathered samples.We found that many pyrite framboids are oxidized.However,this oxidation occurs only on the outer surface of framboid pyrite,and its interior is still composed of pyrite.Therefore,the EDS mapping method is recommended in composition analysis of pyrite,which can indicate the extent and extent of pyrite weathering.In addition,the oxidation of pyrite is a volume reducing process.Our statistical analysis suggests that the mean and medium oxidation-related reduction in size is less than 2.17% and 1.82% respectively,within the system error,thus having little effect on redox interpretations based on pyrite framboid sizes.We reconstructed the high-resolution marine redox history during P-Tr transition based on framboidal pyrite from the Meishan GSSP,South China and Guryul Ravine and Barus Spur sections,North India.Our study show that there are two anoxia stages during the P-Tr transition,the first stage correspond to the Clarkina meishanensis conodont zone to the C.taylorae zone,and oxic to upper dysoxic condition developed in the overlying Hindeodus parvus zone to the lower Isarcicella staeschei zone,the seconde anoxia stage correspond to the I.staeschei zone to I.isarcica zone.The two-stage anoxia coincide closely with metazoan extinction,two microbial blooms,two negative excursions in sulfur isotope as well as total organic carbon(TOC)concentration peaks from the Meishan section.The first anoxia stage coincided with a rapid increase in sea surface temperature.The two-stage anoxia pattern also recognized in Bulla(North Italy),Daxiakou,Chaotian,Xiaojiaba,and Changtanhe(South China)sections,indicating that this pattern is not just occurred in Paleo-Tethys ocean,it might be a global pattern.(4)A detailed,10 m.y.redox history of latest Permian to Middle Triassic oceans is reconstructed.Based on the framboidal pyrite evidence from four ramp sections in South China,the result shows that there are three major euxinia-anoxia episodes in Early to Middle Triassic,namely,the end Changhsingian to end-Smithian,middle to late Spathian,and early to middle Anisian.The variations of oxygen minimum zone(OMZ)are suggested as the driving mechanism.In the pre-extinction Clarkina yini zone,dysoxic condition developed in the margin of Yangtze platform and ramp settings,and euxinic condition developed in the Panthalassa.Marine anoxia/enxinia spread over habitats in almost all oceans during the first extinction pulse of Permian-Triassic mass extinction.In the earliest Triassic(H.parvus zone),the redox condition varies in different geological settings,oxic-dysoxic were pronounce in the ramp setting,euxinic-anoxic conditions were pronounced in the relatively shallow platform margin,however,some northwest Pangea shallow marine sections have yielded unusually diverse Early Triassic ichnofossil assemblages indicating some well-oxygenated refuges.In the second phase of the Permian-Triassic mass extinction and its aftermath,equivalent to the I.staeschei and I.isarcica zones,euxinic-anoxic water columns were pronounced in these settings and basins.During the Griesbachian-Dienerian transition,euxinic-anoxic conditions were detected in the relatively deep Sverdrup Basin,and both shallow platform margin and ramp settings,although samples from the Panthalassa Ocean suggest more dysoxic conditions at this time.Thus,deep-ocean anoxia weakened at this time while remaining intense in shallower settings.During the earliest Smithian,euxinic-anoxic water columns can be recognized in ramp settings,whereas dysoxic-oxic conditions prevailed in basins.Euxinic-anoxic conditions expanded dramatically in the late Smithian.Widespread and intense euxinia recurred in the middle–late Spathian.During the Anisian,oceans were generally oxygenated,coincides with the rapid recovery of marine biotas.(5)Despite the strong volcanism at the end of the Permian,the rapid warming of seawater,the large-scale ocean anoxia and acidification events have seriously affected marine biodiversity.These disaster events did not cause the stability of the paleocommunity to decline significantly in the first phase of extinction.Conversely,the high temperature,anoxic and ocean acidification events along with the second phase of Permian-Triassic mass extinction may lead to a complete collapse of the stability of the paleocommunity.After the extinction,the stability of invertebrate paleocommunity increased with the restoration of species diversity and reached the level of the Middle Triassic in the Spathian,indicating that this community seems does not to be attacked by the frequently environmental fluctuations in Early Triassic.On the contrary,the stability and resistance of marine vertebrate paleocommunities were more severely affected by the harsh environment of the Early Triassic,and it was not fully recovered until the various environmental factors of the Middle Triassic reached stability.