Sedimentology Of The Late Permian Terrestrial Strata In Eastern Yunnan:Implications For Palaeoecological Evolution

It is well known most severe biotic crisis in the geological history occurred at the transition of Permian and Triassic(BP?252Ma).At present,the most plausible causes of this extinctions is that the globally volcanic eruptions at the end of Permian caused a massive of environmental deterioration,then leading to global ecosystem disturbance,even collapse,and until triggering the end-Permian extinction.Thus,deeply keeping on the paleoenvironmental and paleoclimatic evolution research of terrestrial ecosystem is of great significance for revealing the extinction patterns and mechanism of the end Permian severe biotic crisis in detail.Nowadays,under the circumstance that the research progresses between marine and terrestrial ecosystem are unbalanced and marine research is more advanced,researches on terrestrial field should be strengthened.In view of this,the studies on isotope geochemistry,element geochemistry.coal petrology,palynology and cyclostratigraphy are carried out on the Late Permian terrestrial fine-grained sediments and coals from eastern Yunnan and western Guizhou in China,and some conclusions about paleoenvironmental and paleoclimatic evolution as well as special geological event in the terrestrial ecosystem during the Late Permian have been obtained as follows:(1)The atmospheric carbon dioxide(CO2)content during the Late Permian was reconstructed according to the organic carbon isotope of the terrestrial sediments in eastern Yunnan.It shows that the CO2 content was stable and relatively low in the whole sedimentary period of the Xuanwei Formation,and then it increased rapidly at the beginning of the Kayitou Formation period and gradually decreased later.When it started at the Dongchuan Formation period,CO2 content rapidly reached up to 0.23%and after then it returned back to ca.0.04%.Considering that the potential influence of Siberian volcanic activity on the atmospheric CO2 content during the Late Permian,we estimated the magmatic activity state that the eruption was stable and slow in the whole sedimentary period of the Xuanwei Formation,and it erupted violently in a short time during the period of the Kayitou Formation,then at the beginning of Kayitou Formation period,the state became the most intense and later returned back to the stability.(2)Geophysical logs of the Xuanwei Formation coals in Fuyuan County of eastern Yunnan Province are analyzed employing spectral analysis.The result shows that the mineral matter content(ash yield)in the coal was possibly influenced by Milankovitch cycles of 123 kyr(eccentricity),35.6 kyr(obliquity),and 21.2 kyr(precession)during the Late Permian.Using this time-frame,the Lopingian tropical peatland NPP level is calculated to be 360?450 g C·m-2·a-1,which is apparently correspond to the lower end of the range of the Holocene tropical peatland NPP.An overall analysis supports that terrestrial productivity is mainly controlled by temporal atmospheric O2 and CO2 levels,especially the CO2 content.(3)Geochemistry and palynology study on terrestrial fine-grained sediments from eastern Yunnan and western Guizhou reflected that the paleoenvironment and paleoclimate of this terrestrial area had experienced the transition from warm,humid and reducing in the sedimentary period of the Xuanwei Formation to warm,arid and weak oxidizing in period of the Kayitou Formation,and then to hot,dry and oxidizing at the beginning of the Dongchuan Formation period.This transition contains 'double-stability and double-turbulence' four turbulent evolution stages.Referring to other previous research,we deem that the reasons for the paleoenvironment and paleoclimate evolution during the Late Permian were possibly from the combined effects of Siberian volcanic activity and atmospheric environment as well as biosphere.(4)The contents of inertinite macerals compositions and combustion-derived PAHs of the Late Permian coals increased upwards from the bottom to the top of the Xuanwei Formation,implying that the Late Permian peatland had suffered from growing frequent wildfires.It also estimates the atmospheric O2 level reached up to 27%near the top of the Xuanwei Formation,challenging the hypoxia theories advocating for the end Permian mass extinction.The estimation of burning temperatures of palaeo-wildfire,derived from inertinite reflectance,suggested that the fire types overall dominated by surface fires,and ground fires gradually varied to crown fires upwards in the Xuanwei Formation.Combining this wildfires variation trend in frequency and temperature with the abundance variation of spore and pollen,it supports the proposal that,in the Xuanwei Formation,the reason for the vegetation reduction was possibly from the more frequent and hotter wildfires under a stable warm and humid climate condition.(5)In this study,it is concluded that the terrestrial ecosystem disturbance during the Late Permian was due to the combined effects of atmospheric environment,paleoclimate,wildfire event,and so on.Among those,the forest wildfire event could aggravate the destruction of vegetation system and result in the rapid disturbance and even collapse of the whole terrestrial ecosystem.Therefore,we advocate that,in subsequent study on the procedure of terrestrial ecosystem disturbance,the initiation and evolution of forest wildfire event during the Late Permian mass extinction should be paid more attention.