Terrestrial Organic Carbon Isotopes And Carbon Emissions During The Permian-Triassic Transition

The Permian–Triassic mass extinction(PTME)represents the most severe biotic crisis of the Phanerozoic,and it was accompanied by profound environmental change,which destroyed both marine and terrestrial ecosystems.The global carbon cycle perturbations were caused by large carbon emissions,as indicated by global negative carbon isotope excursions(CIE)and increase of atmospheric CO₂ concentration(pCO₂).The researches on the carbon cycle perturbation were mainly from the marine records.The sedimentary records from continental facies are essential to a full understanding of global carbon cycle changes during the PTME.Here,we apply terrestrial organic carbon isotopes from bulk organic matter and C₃ plant remains to establish the complete terrestrial carbon isotope trend,which is used to develop the framework of chemostratigraphic correlation between terrestrial and marine settings.Then,organic carbon isotope of C₃ plant remains are applied to reconstruct the continuous pCO₂change during the PTME.Based on the high-resolution carbon isotopes and pCO₂ data,we employed the earth system model to simulate the carbon emission during the PTME,including the carbon source,emission rate and amount of the added carbon.In order to reveal the secular trend of terrestrial organic carbon isotopes(?¹³C_org)during the PTME,four sections from the southwestern China and three sections from North China are involved to establish high-resolution terrestrial organic carbon isotopes in bulk organic matters(?¹³C_TOC)and C₃ plants(?¹³C_p).The?¹³C_TOC and?¹³C_p from the four studied sections in southwestern China exhibit nearly identical secular trends that can be divided into four stages.In the pre-CIE stage,?¹³C_org records from the Xuanwei Formation are characterized by steady values around?25.0‰.The synchronous,prominent onset of negative CIEs with peak values of?32‰occurs at the bottom of the Kayitou Formation,which is accompanied with disappearance of the late Permian Gigantopteris Flora and coal beds.Subsequently,the onset of the CIE stage is followed by a prolonged interval with sustained low values through the whole Kayitou Formation.A recovery to slightly higher?¹³C_org values(?28‰to?26‰)starts in the uppermost part of the Kayitou Formation and the Dongchuan Formation.The?¹³C_TOC from the three studied sections in North China exhibit similar secular trends that can also be divided into four stages.In the pre-CIE stage,?¹³C_TOC records from the lower Sunjiagou Formation are characterized by relatively steady values.The?¹³C_TOC values drop rapidly from?22‰??23‰to?25‰??26‰at the boundary between the lower and middle Sunjiagou Fromation,which is accompanied with the extinction of the late Permian Pseudovoltzia-Germatopteris Flora in North China.Subsequently,the CIE body stage that is characterized by a prolonged interval with sustained low values occurred through the middle Sunjiagou Formation.A recovery to slightly higher?¹³C_TOCvalues starts in the upper part of the Sunjiagou Formation.This four-stage pattern is also found in the terrestrial?¹³C_org of other terrestrial basin around the world.Besides,a total of 10 global-distributed marine Permian–Triassic boundary sections with both high-resolution?¹³C_carb and conodont biostratigraphy were integrated as a global marine?¹³C_carbprofile,which also shows a four-stage pattern.The four-stage pattern in the marine?¹³C_carb and terrestrial?¹³C_org profiles facilitates the correlation among different Permian-Triassic terrestrial basins.Furthermore,the framework of chemostratigraphic correlation between terrestrial and marine settings during Permian-Triassic boundary is developed,which implied the Permian-Triassic boundary in southwestern China and North China is likely placed within the lower Kayitou Formation and the middle Sunjiagou Formation,respectively.In addition,this framework of chemostratigraphic correlation reveals that the extinction of the Gigantopteris Flora in southwestern China and the conifer flora in North China happened prior to the marine mass extinction.Based on the high-resolution correlation between the marine-terrestrial carbon isotope profiles,we present a continuous pCO₂ record across the PTME reconstructed from the high-resolution?¹³C of C₃ plants from southwestern China.Our estimates show that the pCO₂ was moderately low(426+133/?96 ppmv)in the late Permian.Subsequently,the pCO₂began to increase rapidly in the Clarkina yini zone,reaching the maximum level(2507+4764/?1193 ppmv),immediately above the Permian–Triassic boundary,which is about six-fold higher than it before.The pCO₂ remained high levels(ca.1500 to 2500 ppmv)immediately after the onset of the CIE,with only one transient drop(down to ca.1300 ppmv).This pCO₂ increase shows a close coupling with the coeval global negative CIE and increase of sea surface temperature.In addition,we perform some analyses of other effects(e.g.diagenesis,chemical treatment,plant taxa and precipitation)that could influence the C₃ plant carbon isotopes,and sensitivity analysis of some key input parameters,which could ensure the reliability of pCO₂reconstruction.In addition,elevated atmospheric pCO₂ was likely responsible for amplifying the magnitude of the CIE in the terrestrial?¹³C_p record.In order to unravel the details of carbon emission,the earth system model c GENIE was applied in our study to the reconstruction of the carbon emission during the onset of negative CIE stage.The model experiments and sensitivity analysis performed by c GENIE further reveal that the?¹³C of added carbon(?¹³C_added)is about?12‰.This estimate implies that the mixture of organic matter and mantle CO₂ is likely the main carbon source,and that volcanic CO₂ or biotic CH₄ as the single carbon source seems to be impossible.In addition,the secular trend of?¹³C_added profile can be divided into two stages.The?¹³C_added is relatively low with lower carbon emission rate in the early stage,and then it is replaced by the late stage characterized by higher?¹³C_added values and higher carbon emission rate.This two-stage pattern probably implies that the Siberian Traps volcanism was dominated by the intrusion into organic matter that slowly released thermal CO₂ and CH₄ in the early stage.Subsequently,the intrusion is gradually replaced by eruption with fast and large mantle CO₂ released.Furthermore,the carbon emission model experiments show that the peak carbon emission rate could reach 0.7 Gt C yr^-1 with 28,000 Gt C released during the onset of negative CIE stage.Carbon emissions at the geological time seems different from the faster and shorter anthropogenic carbon emission,which might imply that the environmental change caused by anthropogenic carbon emission in the future would be more severe than those during the PTME.