Late Permian-Middle Triassic Magnetostratigraphy And Secular Variation Of Terrestrial Ichnofossils In North China

The Permian–Triassic Mass Extinction was the severest biotic crisis in the Phanerozoic,resulting in the collapse of both marine and terrestrial ecosystems.Current studies have largely revealed the detailed timing and pattern of biotic extinction and the following recovery in marine ecosystem,yet it’s still unclear how terrestrial ecosystem evolved during the Permian–Triassic transition.Notably,strata on land are generally less well constrained by absolute ages and lack of continuous fossil records,which made it even more difficult to unravel the ecological evolution in non-marine facies.However,an understanding of the end-Permian extinction and its aftermath in continental settings is crucial to a full appreciation of the global biotic-environmental co-evolution.Relying on the well-developed terrestrial Permian–Triassic sequences in North China,this thesis aims to establish a detailed geological timescale based on magnetostratigraphy,and then discuss the biotic turnover using trace fossils.Accordingly,paleomagnetic samplings were systematically carried out at several sections to construct a composite polarity timescale in North China and compared with other marine and non-marine records,via the marine-terrestrial magnetostratigraphic correlation method.On the basis,the widely distributed trace fossils were also selected to uncover how the terrestrial organisms response to the end-Permian extinction and the following biotic restoration throughout the Early Triassic.Establishment of a reliable timescale is the most fundamental and important work for all studies on land during the mass extinction intervals.Consequently,magnetostratigraphic studies were systematically undertaken at the Shichuanhe section（SCH）in the southern part of the Ordos Basin,and Tuanbi（TB）and Zhaojiacun（PZ）sections from the northern part of the Qinshui Basin.In combination with a newly reported latest Permian high precision dating,we are able to construct a detail magnetic polarity pattern and chronostratigraphic timescale during the Permian and Triassic.Both hand-sampling and core-sampling methods are adopted to sample the continental red beds successively,including the widely developed mudstone intervals that are hard to be drilled.Analyzed paleomagnetic data pass the reversal test,the calculated paleomagnetic mean directions show great similarity with previously results obtained from coeval strata in North China,demonstrating that the magnetization is primary and obtained near the time of deposition.Regionally,magnetostratigraphy display good consistency,and can be compared with marine records.A long normal magnetozone has been recognized in the middle–upper portion of the Sunjiagou Formation from all sections,including SCH3n,TB3n and PZ3n,which are correlative to the earliest Triassic normal magnetochron LT1n,based on the latest Permian CA-ID-TIMS age of252.21±0.15 Ma obtained from 3.5 m below SCH3n at the Shichuanhe section.Consequently,the Permian–Triassic Boundary can be readily restricted within the lower part of magnetozones SCH3n,TB3n and PZ3n,on the bottom of upper Sunjiagou Formation,according to the integrated magneto-and carbon-stratigraphy.The upper Shihhotse to the lower Sunjiagou formations consist of reverse polarities from lower and upper parts,split with a normal magnetozone dominated interval in the middle.Combined magnetostratigraphy with the lithological features,the contact between these two formations is postulated to be conformable in the middle–southern part of North China,with the basal Sunjiagou Formation are roughly correlative to the Wuchiapingian–Changhsingian boundary.The reverse magnetozone dominated interval from the upper Sunjiagou to the middle Liujiagou formations,straddles from mid-Griesbachian to mid-Smithian,whereas the normal magnetozone from the upper Liujiagou to the upper Heshanggou formations,covers the late Smithian to late Spathian.Accordingly,the base of the Dienerian locates at the top of the Sunjiagou Formation,and the base of the Smithian and Spathian are within the lower and top of the Liujiagou Formation,respectively.The reverse magnetozone in the uppermost Heshanggou Formation represent the latest Early Triassic reverse magnetochron LT9r.The normal magnetozone of the lower Ermaying Formation are Middle Triassic in age,with the Early–Middle Triassic Boundary strata variably eroded,as seen in the field that the erosional contact between the Heshanggou and the overlying Ermaying formations.In short,integrated stratigraphic investigations provide a basic timescale for revealing the ecological evolution on land during the Permian–Triassic transition in North China.On the basis of the newly proposed time framework,detailed ichnological investigations have also been undertaken from six sections in North China.Totally,33ichnospecies in 17 ichnogenera,with 4 informally designed types and diverse rhizoliths,are identified.Trace fossils are mainly preserved in floodplain and shallow-playa lake facies,with a few occur within fluvial and fluvio-deltaic environments.Distributions of most traces are not affected by pedogenesis,which are constructed in inundated or periodically exposed,unconsolidated substrate conditions.However,some burrows,such as Skolithos and Taenidium,cut across carbonate nodules,which are suggested to be produced during the pedogenesis by air-breathers.Ichnoassemblages from floodplain and shallow-playa lake facies are dominated by Skolithos,Palaeophycus,Planolites,Taenidium,Beaconites and a few trackways such as Diplichnites and Kouphichnium,belonging to the Scoyenia ichnofacies.Scoyenia that are indicative of consolidated substrate under exposed conditions,are rarely encountered.Ichnofaunas of Gordia-Lockeia in fluvio-deltaic facies in the Liujiagou Formation are produced in relatively high energy environments,with periodically subaerial exposures,which could be a transitional Mermia-Scoyenia ichnofacies or a complicate type.Whilst the simple vertical Skolithos predominated ichnoassemblage in shallow lacustrine settings are ascribed to the Skolithos ichnofacies.Additionally,recognized crayfish burrows in the lower part of the Heshanggou Formation implying the occurrence of the Camborygma ichnofacies.Comprehensive ichnological,magnetostratigraphic and chronostratigraphic studies show that ichnofossils in North China display a three-phases evolutionally model of“Late Permian extinction-proliferated opportunists during the mid–late Smithian-rediversified ichnofauna in the Spathian”.Secular variations of ichnofossils cannot be purely explained by changes in sedimentary facies.The ichnological criteria,such as ichnodiversity,ichnodisparity,bioturbation intensity and burrow size,etc.,decreased significantly during the Late Permian,which were coincided with the demise of plant communities,intensified chemical weathering and increase in CO₂levels and temperature,representing the terrestrial ecological crisis in North China.Reappearance of trace fossils in the middle–late Smithian are characterized by low ichnodiversity,low bioturbation level and locally high density,indicating the opportunistic behavior of trace makers in the aftermath of extinction.In addition,locally developed penetrative burrows implicate that the fossorial behavior could also be living strategy to escape stressed environmental condition such as high temperature.Restoration of ichnological criteria and the occurrence of large burrows suggest re-diversified infaunal animals during the Spathian,which intensively disturb the sediments.The widespread rhizoliths are not only indicative of improved vegetational cover,roots that intertwine in the surfaces of Camborygma may also provide food sources for crayfishes.All lines of evidence signal the recovery of shallow-playa lake ecosystem during the late Early Triassic,which is early than that in deep lacustrine environments during the Middle Triassic.Overall,the extinction-recovery model of terrestrial ichnofossils is comparable with that in marine realms,although ichnodiversity during the late Smithian is quite low in non-marine facies.Meanwhile,the Permian–Triassic crisis not only led to the extinction of terrestrial ichnofossils,but also marks significantly turnover in terrestrial ichnofauna.Lake-margin ichnoassemblages in the late Early Triassic are characterized by locally abundant backfilled traces,presence of penetrative burrows,with a few arthropod trackways,which are more similar to those from the Mesozoic-type than the Permian suite.Previously such changeover was thought to happened during the mid–late Triassic,but the newly constructed timescale advanced this evolution to late Early Triassic.Moreover,multidisciplinary studies also allow to constrain the timing of the ecological crisis in North China during the Late Permian.In marine facies,the biotic crisis happened within the bottom–lower portion of magnetochron LT1n.However,loss in ichnodiversity and plant community predated the organic carbon shifts in the latest Permian,within a reverse magnetozone that immediately below LT1n.Furthermore,collapse of terrestrial ecological system in North China was proposed to be at least 270±150 kyr earlier than the marine counterpart based on the newly reported absolute age,which also preceded than that in South China and Sydney Basin.