An integrated biostratigraphy (conodonts and foraminifers) and chronostratigraphy (paleomagnetic reversals, magnetic susceptibility, elemental chemistry, carbon isotopes and geochronology) for the Permian-Triassic strata of Guandao section

The chronostratigraphy of Guandao section has served as the foundation for numerous studies of the end-Permian extinction and biotic recovery in south China. Guandao section is continuous from the Permian-Triassic boundary to the Upper Triassic.Conodonts enable broad delineation of stage and substage boundaries and calibration of foraminifer biostratigraphy as follows. Changhsingian-Griesbachian: first Hindeodus parvus, and first appearance of foraminifers Postcladella kalhori and Earlandia sp. Griesbachian-Dienerian: first Neospathodus dieneri, and last appearance of foraminifer P. grandis. Dienerian-Smithian: first Novispathodus waageni and late Dienerian first appearance of foraminifer Hoyenella ex gr. sinensis. Smithian-Spathian: first Nv? crassatus and last appearance of foraminifers Arenovidalina n. sp. and Glomospirella cf. vulgaris. Spathian-Aegean: first Chiosella timorensis and first appearance of foraminifer Meandrospira dinarica. Aegean-Bithynian: first Nicoraella germanica and first appearance of foraminifer Pilammina densa. Bithynian-Pelsonian: after last Neogondolella regalis, prior to first Paragondolella bulgarica and first appearance of foraminifer Aulotortus eotriasicus. Pelsonian-Illyrian: first Pg. excelsa and last appearance of foraminifers Meandrospira? deformata and Pilamminella grandis. Illyrian-Fassanian: first Budurovignathus truempyi, and first appearance of foraminifers Abriolina mediterranea and Paleolituonella meridionalis. Fassanian-Longobardian: first Bv. mungoensis and last appearance of foraminifer A. mediterranea. Longobardian-Cordevolian: first Quadralella polygnathiformis and last appearance of foraminifers Turriglomina mesotriasica and Endotriadella wirzi.The section contains primary magnetic signature with frequent reversals occurring around the Permian-Triassic, Olenekian-Anisian, and Anisian-Ladinian boundaries. Predominantly normal polarity occurs in the lower Smithian, Bithynian, and Longobardian-Cordevolian. Predominantly reversed polarity occurs in the upper Griesbachian, Induan-Olenekian, Pelsonian and lower Illyrian. Reversals match well with the GPTS. Large amplitude carbon isotope excursions, attaining values as low as -2.9 per mil d13C and high as +5.7 per mil d13C, characterize the Lower Triassic and basal Anisian. Values stabilize around +2 per mil d13C through the Anisian to Carnian. Similar signatures have been reported globally. Magnetic susceptibility and synthetic gamma ray logs show large fluctuations in the Lower Triassic and an overall decline in magnitude of fluctuation through the Middle and Upper Triassic. The largest spikes in magnetic susceptibility and gamma ray, indicating greater terrestrial lithogenic flux, correspond to positive d13C excursions. High precision U-Pb analysis of zircons from volcanic ash beds provide a robust age of 247.28 ± 0.12 Ma for the Olenekian-Anisian boundary at Guandao and an age of 251.985 ± 0.097 Ma for the Permian-Triassic boundary at Taiping. Together, the new U-Pb geochronology from the Guandao and Taiping sections suggest an estimated duration of 4.71 ± 0.15 Ma for the Early Triassic Epoch.

关道剖面的年代地层学（chronostratigraphy）长期作为中国南方诸多二叠纪末生物大灭绝（end-Permian extinction）与生物复苏研究的重要基础。该剖面连续覆盖了从二叠纪-三叠纪界线至上三叠统的完整地层序列。牙形石（conodonts）可用于精准界定各阶与亚阶的界线，并对有孔虫生物地层学（foraminifer biostratigraphy）开展校准，具体划分如下：长兴阶-格里斯巴赫阶：以微小欣德牙形石（Hindeodus parvus）的首现为标志，伴生有孔虫Postcladella kalhori与Earlandia sp.的首现；格里斯巴赫阶-迪内尔阶：以迪内尔新棘牙形石（Neospathodus dieneri）的首现为标志，对应有孔虫P. grandis的末现；迪内尔阶-史密斯阶：以瓦根新棘牙形石（Novispathodus waageni）的首现为标志，迪内尔阶晚期出现有孔虫Hoyenella ex gr. sinensis的首现；史密斯阶-斯帕斯阶：以Nv? crassatus的首现为标志，对应有孔虫Arenovidalina n. sp.与Glomospirella cf. vulgaris的末现；斯帕斯阶-埃吉亚阶：以帝汶基奥斯牙形石（Chiosella timorensis）的首现为标志，伴生有孔虫Meandrospira dinarica的首现；埃吉亚阶-比提尼亚阶：以德国尼科拉牙形石（Nicoraella germanica）的首现为标志，对应有孔虫Pilammina densa的首现；比提尼亚阶-佩尔松阶：以皇家新舟牙形石（Neogondolella regalis）的末现之后、保加利亚副舟牙形石（Paragondolella bulgarica）的首现之前为界，伴生有孔虫Aulotortus eotriasicus的首现；佩尔松阶-伊利里安阶：以Pg. excelsa的首现为标志，对应有孔虫Meandrospira? deformata与Pilamminella grandis的末现；伊利里安阶-法萨尼安阶：以特伦皮布杜罗牙形石（Budurovignathus truempyi）的首现为标志，伴生有孔虫Abriolina mediterranea与Paleolituonella meridionalis的首现；法萨尼安阶-隆巴迪阶：以Bv. mungoensis的首现为标志，对应有孔虫A. mediterranea的末现；隆巴迪阶-科德沃利安阶：以多形四臂牙形石（Quadralella polygnathiformis）的首现为标志，对应有孔虫Turriglomina mesotriasica与Endotriadella wirzi的末现。该剖面保存有原生磁信号，在二叠纪-三叠纪、奥伦尼克阶-安尼阶以及安尼阶-拉丁阶界线附近频繁出现极性倒转。下史密斯阶、比提尼亚阶与隆巴迪阶-科德沃利安阶以正极性为主；上格里斯巴赫阶、印度阶-奥伦尼克阶、佩尔松阶与下伊利里安阶以反极性为主。上述极性倒转与全球极性时间尺度（GPTS）匹配度极佳。下三叠统与安尼阶底部发育大幅值碳同位素漂移，δ¹³C值最低可达-2.9‰，最高可达+5.7‰；安尼阶至卡尼阶期间，δ¹³C值稳定在+2‰左右，该碳同位素特征在全球范围内均有报道。磁化率（magnetic susceptibility）与合成伽马射线测井曲线显示，下三叠统的波动幅度较大，中三叠统至上三叠统的波动幅度整体呈下降趋势。磁化率与伽马射线测井的最高峰值对应陆源岩屑通量的增加，且与正δ¹³C漂移同步出现。对火山灰层（volcanic ash beds）中的锆石（zircons）进行高精度U-Pb同位素定年（U-Pb analysis），获得关道剖面奥伦尼克阶-安尼阶界线的年龄为247.28 ± 0.12 Ma，太平剖面二叠纪-三叠纪界线的年龄为251.985 ± 0.097 Ma。结合关道与太平剖面的新U-Pb年代学数据，估算早三叠世的持续时长为4.71 ± 0.15 Ma。