Clay Mineralogical And Geochemical Characterization Of The P-Tr Strata In South China And Their Paleoclimate/paleoenvironment Significance

The most catastrophic mass extinction event occurred during the latest Permian,when 90% of marine invertebrate,70% terrestrial vertebrate and most plant on land were wiped out.Despite of decades of efforts,no consensus has been reached on the trigger/killer mechanism?s?of this biotic crisis.One of the most popular hypotheses involves the intense volcanic activity as the ultimate trigger and its resultant deterioration of contemporaneous climate and environment,which includes global warming and acidification.However,a hypothesis needs geological evidence to be proven and the first step should be a thorough and comprehensive understanding of the paleoclimate/paleoenvironment condition during the Permian-Triassic interval.On regard of the palaeoclimate change during the P-Tr transition,the greatest breakthrough has been the quantitative estimation of the paleo-SST using biogenic oxygen isotopes,which has proved the onset of severe global warming close to the mass extinction event and that paleotemperature was lethally high during the Early Triassic.Nevertheless,other aspects of climate such as humidity,chemical weathering trend and possibility of acidification have remained questions on debate.With this on the background,this thesis aims to provide insights into how the above-mentioned ‘other' aspects of paleoclimate/paleoenvironment have evolved during the Permian-Triassic in South China by investigation into clay mineral assemblages,Chemical Alteration Index?CIA?etc.at high resolution from 4 marine P-Tr sections?northern edge of the Yangtze Platform: Shangsi,Ganxi sections;southern edge of the Yangtze Platform: Xinmin,Dongpan sections?and 1 terrestrial section?eastern Yunnan: Lubei section?.The following summarizes the main work and conclusions:?1?By comparing variation of stable major/trace element ratios?Al2O3/TiO₂,Zr/Hf,Zr/Sc,Nb/Ta,Th/Sc?,REE distribution patterns?and TOC profiles?,it is concluded that the sedimentary source and its vertical change are similar between Shangsi and Ganxi sections,which stayed relatively stable from conodont zone N.changxingensis onwards.The sources of Xinmin section and Dongpan section,on the other hand,are quite different with the former being more basic in composition and the latter more acidic.In addition,there appears to have been a major change in the sediment source of the Triassic strata relative the Permian at Xinmin,while little change has been noted at Dongpan.Strata of the terrestrial Lubei section have shown close affinity to the regional Emeishan Basalt with no major change of source material detected for the whole sampled section.?2?X-ray diffraction analysis?XRD?,SEM/TEM observation and Rock-eval analysis are combined to determine the clay mineralogical assemblages and their controlling factor.The results show clear regional variations.Clay minerals from the northern edge of the Yangtze Platform are relatively simple and mainly composed of illite and illite-smectite mixed layers,with an appearance and subsequent increase of chlorite approaching and beyond the P-Tr boundary,while those from the southern edge and the terrestrial Lubei section are comparatively complicated because of the addition of chloritic clays and various types of mixed layers?e.g.chlorite-smectite,chlorite-vermiculite etc.?.The reason that the clay minerals from the southern Yangtze Platform and the terrestrial Lubei section are more chloritic than those from the northern edge of the Yangtze Platform is that the sedimentary basins of the first two are close to a basaltic source.Taken together the rock-eval analysis,vitrinite reflectance?Ro,compiled from literature?,illite% in the bentonite illite-smectite mixed layers?Shangsi: 50-60%;Dongpan,Lubei: 70-80%;Ganxi,Xinmin: >90%?and SEM/TEM observation,it is considered that clay minerals from these studied sections are severely compromised by diagenetic alterations: illitization of smectite is the dominant diagenetic alteration form from the northern edge of the Yangtze Platform where the least complete transformation is noted in Shangsi section.In addition to illitization,chloritization of smectite is also common from both the southern edge of the Yangtze Platform and the Lubei section.However,despite of the moderate diagenetic stage,there is still R0-ordering illite-smectite left at the Dongpan section and considering the source required,a lack of K+ may be the reason that has retarded the illite transformation.The occurrence of R0-ordering illite-smectite at the Dongpan section provides an opportunity to understand that the primary clay mineral being smectite indicates a generally warm and semi-arid condition during the latest Permian.?3?After the evaluation of diagenetic effect,the clay minerals from the South China P-Tr successions still carry some paleoclimate information on progressive aridification towards the Triassic.On the northern edge of the Yangtze Platform,aridification is indicated by the appearance and an increasing of chlorite towards the Triassic,which is accompanied by a progressive increasing of illite crystallinity,while on the southern edge of the Yangtze Platform,it is manifested by the gradual reduction of kaolinite and R0-ordering illite-smectite mixed layers and also an increase of illite crystallinity.At the Lubei section,a decreasing trend of kaolinite is also evidence of aridification,which can also be indicated by the gradual decreasing occurrence of coal seam/line,replacement of the sub-tropical Gingantopteris flora by the aridity-tolerate Peltaspermum flora.On a whole,drying-out during the latest Permian-earliest Triassic is gradual and subtle and it is not until ⁰.08 Ma later?rough estimation from the Lubei section?that rapid aridification took place.?4?Calculation of the CIA is not very successful for the northern edge sections and Xinmin sections from the south,the reason being large quantities of chemical precipitation of carbonate.In comparison,samples from the Dongpan and Lubei sections are more suitable for CIA calculation given their fine-grained characteristics,lack of biogenic carbonate component and stability of sediment source.The result shows that CIA from both sections show a very subtle decrease towards the Triassic and rapid decrease of CIA is not demonstrated until ⁴0m upper at the Lubei section,which indicate that chemical weathering remained unchanged or slightly decreased during the P-Tr transition and rapid decrease of chemical weathering happened only ⁰.08 Ma afterwards.?5?More evidence?s?of aridification is provided by the environmental magnetic experiments,which demonstrate a replacement of a predominance of magnetite in the Permian strata by that of hematite?and maghematite?in the Triassic at the Lubei section.Similar conclusions have also been reached by environmental magnetism studies at the Shangsi,Xiakou sections on the northern edge of the Yangtze Platform and the Dongpan section to the south of the Yangtze Platform.Integrated with the clay mineral change and variation of CIA values,it is envisioned that the Lower Triassic red beds in South China should be a result of the arid and oxygenated environment rather than of burial diagenesis.?6?The CIA values show extreme variation in the Triassic at the Lubei section,the pulsive high values of which are in correspondence with fine-grained sediments,higher susceptibility values and more magnetite content,which altogether indicate intermittent high precipitation of monsoonal character.This kind of seasonality was never indicated in the Permian strata,which means that seasonality was apparently stronger in the Triassic as is well manifested by the stronger alternation of lithology in South China.?7?Chemical weathering intensity has been assumed to be stronger in the Triassic because enormous release of volcanic CO₂ provides sufficient reactant for the decomposition of silicate minerals and with the increase of temperature,decomposition is accelerated.However,another aspect is left out in that scenario,which are the prevailing monsoons that are expected because of from-pole-to-pole mass distribution of the Pangea supercontinent and that are expected to create a more arid climate condition.As a result,whether the trend of climate goes towards more arid or a more humid condition depends on the compromise between volcanic effect and the monsoons and based on the present study,it is more likely that moonsoons took the dominance during the P-Tr transition,at least in South China.?8?The presence of vermiculite is proposed as a proxy of acidification during the P-Tr transition.Vermiculite is a type of expandable clay mineral that is usually formed in acidic soils usually with a pH range between 4 and 6.And HIM is a type of special expandable clay mineral that features “gibbsite-like” islands in the interlayer region and usually formed in a relatively more acid environment than vermiculite.Among the studied sections,vermiculite is present in 3?Shangsi,Dongpan and Lubei?and is almost completely in association with the most negative carbon isotopes and HIM is present at the Lubei section,also in association with the negative carbon isotopes.Considering the low pH required for the formation of vermiculite and HIM and their association with the most negative carbon isotopes that are usually envisioned as the result of volcanic emission of CO₂,it is logical to consider volcanic CO₂-induced acidification occurring at that critical time interval.?9?As much asattention the link between the paleoclimate/paleoenvironment change and the mass extinction during the P-Tr transition gets,it remains an unresolved question.As far as this thesis is concerned,although aridification from the end Permian to the earliest Triassic is gradual and subtle,the widespread wildfires that are close to the End-Permian plant extinction event may be a potential cause of the extinction as well as ecosystem collapse on land.Rapid global warming,on the other hand,could have been one of the reasons to kill the majority both on land and at sea,but disagreement still exists on this conclusion at the present.Acidification,is recovered to be either below or above the extinction horizon from different localities,thus it is still too early to come to a final conclusion as to the role it played in the mass extinction.