Carbon Isotope Record Of Climatic Transition Period Of Carboniferous Ice In Xainza Region,Tibet

The study of carbon isotopes is an effective complement to lithostratigraphic and biostratigraphy in stratigraphic and global comparisons,as well as global climate change studies.And the fluctuation of inorganic carbon isotopes of carbonate rocks in geologic history is mostly related to the biosphere,in which the large negative offset fluctuations of carbon isotopes are associated with major geological boundaries,and many small fluctuations are related to the alteration of fossil belts.Except that,a substantial positive offset has a good correlation with icehouse climate.However,the regional comparative study,for carbon isotope curve in the non-carbonate rock region,is still in the embryonic stage.Significantly,the Carboniferous icehouse climate is one of the most important transition periods in the history of the earth's development.What's more,this icehouse climate is closest to the current climate condition.So,the more human understanding of it,the better people can respond to the future disaster events,including,the increase in the atmospheric CO₂ concentration,the increase in the average global surface temperatures,melting of polar and other glaciers and a global sea-level rise and others.In this paper,selecting the beginning stage of the Carboniferous ice climate(D₃-C₁)and the extinction stage of glacial maximum(C₃-P₁)as the research emphases,those age are correspond to the Chaguoluoma Formation(D1C1(?))and Laga Formation(C₂P₁l)in the tibet.Based on carefully collecting,screening,sorting the Upper Paleozoic Devonian-Lower Carboniferous Zhaguoluoma formation(D₁C₁(?)),the Lower-Up Carboniferous Yunzhug formation(C_1-2y),the Upper Carboniferous-Lower Permian Laka formation(C₂P₁l),Lower Permian Angjie formation(P₁a)and Middle Permian Xiala formation(P₂x),of Xainza area in Tibet,and refining the information of the data previously published about the lithology,fossil age attribution as well as this field investigation,this paper combines the petrological characteristics,edimentary facies evolution history and carbon isotope calculation,curve drift trend,and make a comparative study on each major stratigraphic areas in China and stratigraphic isotope curve value as well as the offset trend of the world's three largest domain in Late Paleozoic.On the basis of the above studies,the sedimentology,geochemistry and paleontological evidence of icehouse period are analyzed,and the driving factors of climate transition in icehouse period are discussed,and the following conclusions are obtained:1.The paper runs through the sedimentary geological process as the main line,geological chronology,stratigraphy,paleontology and stratigraphic theory as the instructions,and collect,screen and sort the former research on areabiological and chronostratigraphic data.In addition,the biostratigraphic Famennian and biostratigraphic Tournaisian near the Devonian Carboniferous boundary is complete,while the biostratigraphic Gzhelian and biostratigraphic Asselian near the Carboniferous-Permian boundary is complete.Since the complete biostratigraphy has laid a solid foundation for the comparison between domestic and international carbon isotope stratigraphy.2.Luogong section of Xainza in Tibet normally develops carbonate platform facies and shallow sea clastic rock neritic shelf facies from the bottom up.The carbonate platform facies can be divided into three deposition subfacies of open platform subfacies,platform edge shoal subfacies and platform front slope subfacies.The shallow sea clastic rock neritic shelf facies can be divided into inner shelf subfacies and outer shelf subfacies.Vertically upward,the sedimentary environment of the section gradually deepens in retrograde sedimentary sequence.Xiala section develops outer continental shelf clastic subfacies and carbonate platform facies from bottom to top.The vertical section of Xiala shows that the water gradually becomes shallower,and is generally a regression sea system and sedimentary-type sedimentary sequence.3.This paper quantitatively studies the main controlling factors of?¹³C_carband?¹³C_orgexcursion:The effect of nutrient phosphate on?¹³C_carb is 21.44 times the effect of temperature on?¹³C_carb.That is,the primary productivity is the main control factor of the excursion of the?¹³C_carb,and they have proportional relationship;The?¹³C_orgis inversely proportional to atmospheric p CO_2.The low value of greenhouse gas p CO₂corresponds to the positive deviation of?¹³C_org.4.Based on the drift comparison between the carbon isotopes of Devonian Carboniferous boundary and Carboniferous Permian strata of Xainza region in Tibet and that of coeval strata at home and abroad:it was found that the Devonian Carboniferous boundary layer of Xainza lies in the common boundary of upper pale biocalcarenite in Zhaguoluoma Group and bottom amaranth and yellowish brown sandy limestone in the overlying section of Zhaguoluoma Group;it was first found that the carboniferous permian boundary layer in Xainza lies between the third dark grey fine sandstone layer and the fourth yellowish brown siltstone layer at the bottom of Laga Group.5.Moraine analogs were spotted in Luogong section of Zhaguoluoma Group in Xainza,Tibet,indicating the beginning of icehouse climate;the continental glacier of gondwana land began to become active and influence Xainza in Tibet.There was an evident rhythmicity in Yongzhu Group and pebbly slate took on glaciomarine deposit.It showed that the period was the coldest for Laga Group is with exposed huge glacial boulders,and it was the late Paleozoic glaciation period.Gray bioclastic limestone with fine sandstone in Angjie Group indicates that the climate became warmer but it was still generally cold;while in Xiala Group,the amaranth carbonate rocks indicate that the climate was apparently warmer.The evaporite and coal seam only emerged in the Late Permian Epoch indicates that the late Paleozoic ice age has ended in this period and Xainza entered a period with warm climate.6.Tethys warm water reef-building coral dominated the Tournaisian Age of Early Carboniferous Epoch in Xainza,indicating that Xainza was warm in the Tournaisian Age of Early Carboniferous Epoch,and it also indicates that the beginning of the great ice age of the late Paleozoic should had been within the late Devonian period.The phenomenon of warm and cold mixing in representative cold water small scale free plate coral(the dominance)and the presence of brachiopod fossils indicate that the cold climate was on the way,with glacial periods and warm periods alternating.Fossils are sparse in Laga Group,indicating that alarge number of organisms migrated into warmer regions with lower latitudes in extreme cold period in Xainza.Brachiopod in Angjie Group are cold water organisms,indicating that the climate was still cold and hadn't changed;the quantity of corals,brachiopods and fusulinid recovered in Xiala Group with Tethys warm water types as the dominance,indicating the gradual warming of the climate.7.According to the evidence of geochemical characteristics in climate transition period of Carboniferous icehouse climate in Shenzha,Tibet,Famennian Age of Late Devonian Epoch-Capianian Age of Late Permian Epoch can be divided into 9 positive drifts.D1 are located in the stage of?³C_carb,?¹³C_org and?¹⁸O all with positive deviation;in addition,tillite analogs in Luogong section of Zhaguoluoma,cold water organisms and temperature of Tournaisian period 11.97?according to whole rock oxygen isotope calculation indicate that late Paleozoic Glaciations had extended to the Lhasa in Late Devonian period.The C1 with fluctuating frequently positive offset of?¹³C_org,and all so the C2 taking on positive offset of?¹³C_org.C1 and C2 groups are at the lower part of Yongzhu Group,its bottom pebbly slate is with evident rhythmicity;in addition,there is a phenomenon of warm and cold mixing in Brachiopoda of Yongzhu Group,with cold water small phosphorus free coral as the dominance.The above characteristics show that the two positive drifts can be amid the glacial epoch of the Serpukhovian Age of Early Carboniferous Epoch-Bashkirian Age of late Carboniferous Epoch,and it can be indicated that the late Paleozoic Glaciation between Visean Age of Early Carboniferous Epoch-Kasimovian Age of late Carboniferous Epoch was in discontinuous development.The C3 with positive deviation trend of?¹³C_org is more obvious compared to that of?¹³C_carb and?¹⁸O;this can be related to the fact that there is less sample distribution in the section.The?¹³C_org value of C4-C6 is with 1.91‰and 2.66‰positive offsets,which are strong drifts.The temperature range according to whole rock oxygen isotope calculation is from 11.07 to 17.52?;though it is not belemnite or foraminiferal oxygen isotope,the extreme cold temperature of the period is indicated.Huge glacial boulders ranging from 5 to 6m were developed in Laga Group;in addition,there is few fossil,indicating a low temperature.C3 to C6 indicate that the Gzhelian stage of late Carboniferous epoch-Artinskian stage of early Permian epoch was a great ice age during the late Paleozoic glacial epoch with petrology,geochemistry,and paleontology evidences.The climate was the coldest in this period.P1 is located in the upper part of the Angjie Group,the positive offsets of?¹³C_carband?¹⁸O are 2.5‰and 2.6‰respectively,and the lower part is rather stable.Angjie Group features biocalcarenite and fine clastic rocks,as well as cold water brachiopods.This indicates that glacial climate persisted in the Kungurian Age of early Permian Epoch,but its intensity weakened.P2 is located at the bottom of Xiala Group,and the positive offsets of?¹³C_carb and?¹⁸O are 2.6‰and 3.8‰respectively.Xiala Group features amaranth limestone,warm water corals,brachiopods and fusulinid,and their quantity had recovered.In the Roadian Age-Capitanian Age of middle Permian Epoch,the climate became warmer and the icehouse climate had disappeared in Lhasa.8.The p CO₂of the carbonate rocks in the upper of Chaguoluoma Formation(D₁C₁(?))in the Xainza of the Tibet range from 1577.34?586.19ppmv,the average value is 1090.26ppmv,and show the rapidly declining trend.The variation range of p CO₂in the carbonate rocks of Xiala Formation(P₁a)is 454.76?303.28ppmv,the average value is 386.23ppmv,which showing slowly down with concussively.The organic carbon source of the Late Devonian-Early Permian in the Shenzha area of the Lhasa area is a mixed source,with a slightly more land source.The organic carbon burial rate reached the highest peak at the bottom of the Laga Formation(Gzhelian Age of Late Carboniferous-Artinskian Age of Early Permian),indicating that the ocean buried a large amount of carbon isotopes,resulting in a change in the sea-air balance system,and the ocean absorbs a large amount of CO₂ from the atmosphere.Vascular plants flourished in late Paleozoic Carboniferous,the concentration of CO₂,which is also known as a greenhouse gas,reduced to the minimum value of the Phanerozoic,at the same time,gondwana was in clockwise movement,and its northwest section collided into Laurasia,taking up most of the high latitudes of the southern hemisphere(near the South Pole;the temperature reached the minimum and the climate was in a great ice age.In Permian,Pangea moved northward and the main part of gondwana also moved northward to the area with latitudes 30 to 60 degrees,which was the region of temperate climate;meanwhile,elevated O₂ concentration caused frequent occurrences of forest fires,termite stomachs generated digestive bacteria and prevented a portion of carbon from being buried and converted it into greenhouse gas:methane,which returned to the atmosphere;this stopped the spread of glaciers and a minor reversal emerged,increasing the albedo and making"icehouse climate"gradually switching to"greenhouse climate"instead.