Magnetostratigraphy Timescale In South China During The Paleozoic-Mesozoic Upheavals And Its Global Correlation

The latest Permian witnessed the largest mass extinction during the Phanerozoic eon,and was followed by an extended and tortuous biotic recovery during the Early Triassic epoch.The rebuilding of complex marine ecosystems with metazoan reefs and efficient nutrient recycling was delayed until the middle of Anisian Stage?lower Middle Triassic?.Meanwhile the marine ecosystems had dramatically evolved from the Paleozoic into Mesozoic types.One of the best places to investigate and understand this end-Permian mass extinction and its ecological recovery are the superb sections on the South China Block.Multiple stratigraphic researchers have established the important background,including conodont and other biostratigraphy,U-Pb radioisotopic dating,and geochemical proxies related to ecosystems and environment fluctuations.However,the ability to enable high-resolution correlations between these important reference sections on the South China block and the other facies still remains a challenge.One tool for global correlation is magnetostratigraphy.When the barcode of geomagnetic polarity chrons,which are a relative chronology pattern,are combined with biostratigraphy such as conodont index species,with periodic orbital-climate oscillations,and with stable isotope trends and excursions of carbon and oxygen,the resulting integrated geomagnetic polarity time scale can provide a correlation with terrestrial facies and non-fossiliferous marine facies.We have used eight sections in South China that span the Paleozoic-Mesozoic upheavals to compile astrochronologic geomagnetic polarity timescales for the Changhsingian?Meishan and Shangsi sections,plus terrestrial-facies of the Dayulin section in North China?,Lower Triassic?Xiejiacao,Xueyu Cave,Youping and Laren,plus the Dayulin section?,lower-Middle Triassic Boundary?Wantou as a GSSP candidate,and the Youping sections?,Anisian?Sidazhai section?for correlation.1.Meishan D section?N.Zhejiang?hosts the GSSPs for both the base of the Changhsingian Stage and the base of Induan Stage?Paleozoic-Mesozoic boundary?.However,previous efforts to obtain a geomagnetic polarity pattern yielded significantly different interpretations,therefore were unsuitable as a tool for global correlation.Therefore,we applied more advanced demagnetization in magnetic-shielded lab instruments to analyze the magnetostratigraphy from the adjacent Meishan C section to verify at the bed-to-bed level which portions of the earlier studies of Meishan D section were consistent.The resulting geomagnetic polarity zones MS0-MS4 were combined with the published conodont biostratigraphy to create a bio-geomagnetic polarity time scale spanning the uppermost Wuchiapingian to lowest Induan.2.Shangsi section?N.Sichuan?,a former candidate GSSP for the base Induan,has also been studied by different paleomagnetists to yield slightly different interpretations.We compiled a more consistent polarity pattern from those studies that was partly based on analyzing the original raw data in those studies to enable precise stratigraphic placement of the most reliable polarity zones of SS1-SS3n relative to standardized conodont zonation.This scale also spanned the uppermost Wuchiapingian to lowest Induan.3.Xiejiacao section?E.Sichuan?is a kilometer-thick carbonate succession punctuated by evaporite low stands.Our magnetostratigraphy of XJ1-XJ7 polarity zones,when combined with the extensive carbon-isotope signatures and limited conodont biostratigraphy,span the uppermost Changhsingian to upper Spathian.The polarity zonation has several anchors for correlation:latest Permian carbon-isotope negative excursion-N1 to the base of magnetozone XJ1n;peak of Induan-Olenekian boundary?IOB?positive excursion-P1 to the base of XJ3n;peak of Smithian-Spathian boundary?SSB?positive excursion-P2 and conodont Nv.pingdingshanensis to the middle of XJ5n;and the onset of upper Spathian positive excursion-P3,to the middle of polarity zone XJ7n.The XY0r-XY5n polarity zone pattern from the similar facies at the Xueyu cave section?E.Chongqing province?can be correlated to that of the Xiejiacao section with XY3n-XY4n correlated to XJ2n.4.Youping section?N.Guangxi?is a composite of Youping I and Youping II sections.Polarity zones YPI0r-YPI4n from the Youping I section can be assigned to uppermost Changhsingian to middle Spathian based on previous carbon isotope and ammonoids-conodont biostratigraphy.This polarity pattern has several anchors:base of Induan conodont index Hi.parvus to the base of YPI1n;base of Smithian ammonoid index species to the base YPI2r.1n;and the peak of SSB positive excursion-P2 to near the top of YPI3n.In Youping II section,five conodont zones were established,in ascending order:Tr.brochus-Gl.carinata-Ch.gondolelloides-Ch.timorensis s.s.-Ma.alexanderi.This conodont succession can be correlated globally and are associated with carbon isotope excursions.The first occurrence?FO?of Ch.timorensis s.s.was slightly higher than magneto-subzone YPII1n.2n and below the peak of upper Spathian positive excursion-P3.5.Laren section?N.Guangxi?yielded a geomagnetic polarity zonation of LR0r-LR1n.The previous conodont/ammonoid biostratigraphy implies that the lowest occurrence of Nv.pingdingshanensis?conodont index datum for base of Olenekian?is slightly higher than the top of magnetozone LR1n,and that the Laren section spans nearly the entire Smithian Substage.6.Wantou section?N.Guangxi ca.50 km SE of Youping?has been published by us as a GSSP candidate for the Anisian.In this section,the WT1-WT2 polarity zones,conodont zones and datums,and the carbon isotope excursions span the uppermost Spathian to lowermost Anisian.There are several anchors for global correlation,including the onset of upper Spathian positive excursions-P3 to the upper part of WT1n,etc.In the Wantou section,four conodont zones were established,in ascending order:Tr.brochus-Gl.carinata-Ch.timorensis s.s.-Gl.tethydis.In addition to correlation to Youping II,this conodont succession can be correlated globally.The lowest occurrence of Ch.timorensis s.s.,our preferred marker for the GSSP of the Olenekian-Anisian boundary?OAB?was slightly higher than magneto-subzone WT1r.4n and below the peak of upper Spathian positive excursion-P3.7.Sidazhai section?SW.Guizhou?yielded a polarity pattern SDJ1-SDJ5.Previous conodont biostratigraphy indicates that the Sidazhai section spans nearly the entire Anisian Stage.The main correlation anchors are the FO of Ch.timorensis s.s.at slightly below magnetozone SDJ1n and the FO of Paragondolella bulgarica at the base of SDJ3n.8.Dayulin section?S.Henan province,N.China block?is a terrestrial succession of fluvial to paleosol deposits.Polarity zones DY1n-DY5n were interpreted,and their geologic age is partly constrained by previous research on organic carbon isotope trends in the lower Sunjiagou Fm.We suggest that the sampled Dayulin section spans the time interval of late Changhsingian to middle-late Smithian,and that it will provide the chronological scale for correlating terrestrial North China settings to marine facies stratigraphy.These sections have provided the fundamental integrated geomagnetic polarity time scale?GPTS?integrated with chemo-biostratigraphy for the Paleozoic-Mesozoic transition?Changhsingian to Anisian stages?.These new composites are more consistent with other regional magnetostratigraphy compilations,and provide both verification and an improved bio-,cycle-and radiometric-age control on the geomagnetic polarity time scale.1.The lower Changhsingian GPTS is dominated by normal polarity?chron LP2n of Hounslow,2016?with two reversed-polarity subchrons within the middle Clarkina wangi to middle Cl.subcarinata conodont zones.The upper Changhsingian is dominated by reversed polarity,with the onset of chron LP2r of Hounslow at about 25%up in the Cl.changxingensis conodont zone.The biostratigraphic placement of normal-polarity chron LP3n of mid-upper Changhsingian at both Meishan and Shangsi sections indicates that it is near the top of Cl.changxingensis zone.Portions of the middle Changhsingian polarity pattern are yet to be verified in multiple sections that have the required good conodont record and magnetic preservation.The latest-Permian mass extinction?base of Cl.meishanensis zone?is essentially coeval with the onset of the normal-polarity chron?LT1n?that continues through the lower half of the Griesbachian substage of basal Triassic.The Permian-Triassic boundary,as defined by its GSSP at Meishan that is bracketed by radiometric ages on volcanic ash beds,is approximately 50 kyr after the beginning of that normal-polarity chron LT1n.This placement is consistent with its revised placement within the cycle-magnetostratigraphy from the Germanic Basin;and this relative placement between the base of chron LT1n and the PTB is consistent with other sections in different facies.2.The integrated GPTS for the Induan Stage mainly follows the bio-cycle-magnetostratigraphy scale that had been previously derived from the Chaohu section?Anhui province?;but with revision of some sub-chron,such adding LT1n.1r and LT1n.2r at the lower and upper portions,respectively of chron LT1n,and clarifying the precise relationship between the FO of Nv.waageni eowaageni?IOB index conodont specie?and the base of chron LT3n.3.The major reference sections for the GPTS spanning the Smithian sub-stage had been carbonate facies at Guandao?South China?and the clastic-dominated facies in the Arctic?Ellesmere Island of Canada and Svalbard,north of Norway?with approximate scaling by Germanic basin cycles.This previous Smithian GPTS had interpreted chrons of LT3n-LT6n.The Smithian magnetostratigraphy from the Xiejiacao-Laren-Xueyu cave composite in this study lacks LT4n and has a shorten LT5n.However,the relative placements of LT3n at the base of Smithian and of L6 at the base of Spathian are the same.The previous interpreted longer duration of LT5n may have been caused by an accidental assignment to the main fining-upward sediment packages counted in the Germanic Basin to a100-kyr short-eccentricity periodicity,whereas there is a dominance by 32.8-kyr obliquity cycles during part of this time interval,as documented in the Chaohu section.The amplitude of the obliquity cycles,hence its importance relative the eccentricity signal,has a theoretical1.2-myr modulation.Therefore,the duration of polarity chron LT5n in this study is 0.12-myr,in contrast to the previous cyclostratigraphic-interpretation of 0.35-myr from the 3?sediment packages in the Germanic Basin.This 1.2-myr modulation of the relative importance of obliquity-cycle amplitude relative to the eccentricity signal also occurred during the times of polarity chrons LT1n and LT2n;therefore,these chrons also spanned less time than had been previously interpreted from counting the main Germanic sediment packages.The Spathian polarity chrons of the Youping sections can also be linked the previous magnetostratigraphy from Chaohu,the Germanic basin,and the Arctic?Norway,Canada?.The Youping polarity pattern verified that the lower part of the Spathian Substage is dominated by normal polarity?LT6-LT9 succession?and the upper part by reversal polarity?chron LT9r?.4.Wantou and Youping sections provide a detailed high-resolution integrated calibration of the Early-Middle Triassic boundary?OAB?succession of lithostratigraphy,volcanic episodes,conodont zonation,ammonoid biostratigraphy,geomagnetic polarity chron,inorganic carbon isotopes,sea-surface temperature derived from conodont-apatite oxygen isotopes,and ID-TIMS U-Pb radioisotopic dating.The late Spathian?latest Early Triassic?GPTS is characterized by normal polarity?chron LT9n?that encompasses the FOs of the typical Spathian conodonts Triassospathodus homeri and Gladigondolella carinata,the late Spathian Neopopanoceras haugi ammonoid zone,and the beginning of a progressive positive shift in inorganic carbon isotopes.The overlying reversed-polarity LT9r contains two brief normal-polarity subzones?MT1n and MT2n?that can be recognized in several other marine and terrestrial sections.The FO of conodont Chiosella timorensis sensu stricto,our proposed base-Anisian global marker,is near MT1n and near the end of the positive?¹³Ccarb excursion.The lowermost Anisian at Wantou and Youping is dominated by normal polarity?MT3n,with the presence of one major reversed-polarity subzone MT3n.1r?,contains the FO of typical Anisian conodonts?Gladigondolella tethydis,Magnigondolella alexanderi?,and has the onset of a plateau in inorganic carbon isotopes values?stabilizing around+4‰?.The combination of the FO of conodont Ch timorensis s.s.,the brief normal-polarity subchron?MT1n?and the last portion of the rising carbon-isotope trend are suitable as primary proxies for global correlation of the Early-Middle Triassic boundary?base of Anisian?to other marine and non-marine settings.5.The Youping section integrated-stratigraphy researches suggested the lower Anisian is dominated by normal polarity,and the YPII2n-YPII4r can be correlated to GPTS MT3n-MT4r.The Sidazhai section biostratigraphy from conodonts and correlation to the Guandao section implies it provides a nearly complete Anisian section for future detailed studies.The FO of Pa.bulgarica?used for a conodont-placed base of the Pelsonian substage?is close to the base of the local magnetozone SDJ3n,which is interpreted as the base of"global"magnetozone MT4n of Hounslow and Muttoni?2010?.This correlation completes the Anisian GPTS which had been mainly based on the Guandao section.This comprehensive,integrated chemo-bio-magneto-stratigraphy timescale enables improved high-resolution correlation of different facies across the Paleozoic-Mesozoic transition upheaval,and provides a framework for comparing regional responses to the lethally high-temperature greenhouse episodes and intervals dominated by obliquity-forcing of climate during the Early Triassic in the aftermath of the latest Permian mass extinction.1.The magnetostratigraphy of the Dayulin section in a terrestrial setting in North China can be correlated to marine successions that have paleo-temperatures derived from conodont-apatite oxygen isotopes.These correlations imply that the style of its non-marine clastic deposition during warm phases was dominated by red mudstone and siltstone;whereas during cool phases,the sedimentation was dominated by sandstone and coarse sandstone.2.The sections in South China,Himalayan region,and the Dolomites were in low-to mid-paleolatitudes during the latest Permian through Early Triassic.Their magnetostratigraphy framework,coupled with conodonts and carbon-isotope trends can now be correlated to the high-paleolatitude sections in the Arctic?Svalbard basin north of Norway and Sverdrup basin of Canada?.These correlations imply that the independently interpreted sea-level changes and primary sequences are sub-isochronous among the multiple facies and paleolatitudes.In addition,the 1.2-myr modulation of obliquity forcing of amplitude of climate cycles can be detected in these different facies and paleolatitudes;and there is a suggestion that the main cooling phases enhanced the rates of ocean circulation with theoretical enhanced oxidization of the ocean.