Paleomagnetic Constraints on the Permian-Triassic Boundary in Terrestrial Strata of the Karoo Supergroup, South Africa: Implications for Causes of the End-Permian Extinction Event

The closure of the Palaeozoic witnessed the greatest biotic crisis in earth history. Surprisingly little is known about the effects and timing of the terrestrial counterpart of the well-described End-Permian mass extinction from known marine successions worldwide. In the present study, reliable paleomagnetic results were obtained from a PT boundary section in the terrestrial Karoo Basin of South Africa. Permo-Triassic aged mudstones from a locality in the Eastern Cape Province yielded two magnetic chrons, reverse followed by normal (with the boundary possibly close to the reversal). This extends to results from a previous study: thereby jointly identifying a R/N/R polarity pattern for this boundary interval. The PTB interval is constrained below the red mudstones of the Beaufort Group at the present locality and within reverse-magnetised green mudstone, implying a diachronic relation between the marine and terrestrial End-Permian mass extinction events.     

长江三角洲北翼ZKA4钻孔剖面第四纪磁性地层特征及其意义

对长江三角洲北翼南通地区ZKA4钻孔岩心进行了磁性地层学研究,结果表明,302.7 m的岩心记录了布容正向极性时（Brunhes）、松山负向极性时（Matruyama）和部分高斯正极性时（Gauss）。在系统古地磁样品采集、处理和测试的基础上,应用磁性地层、AMS 14C加速器测年等方法,结合岩性特征,对ZKA4钻孔剖面进行了详细地层划分,分别确定了下更新统、中更新统、上更新统和全新统的埋深及沉积厚度,其中Q/N界线位于291.72 m处,Qp1/Qp2、Qp2/Qp3、Qp3/Qh界线分别定位于189.39 m、132.44 m和26.14 m处。本项研究结果为该区域第四纪地层划分对比、古地理环境演化及岸线变迁等相关研究提供了可靠的地层年代框架。     

四川冕宁昔格达组磁性地层学初步研究及意义*

文章首次对川西安宁河流域冕宁昔格达组河湖相沉积物进行岩石磁学和磁性地层学初步研究。结果表明沉积物具有正常的沉积组构,适合磁性地层学研究;赤铁矿和磁铁矿是主要载磁矿物;沉积物记录的古地磁场方向基本都为正极性,主要形成于高斯正极性时,约4.18~2.58Ma。综合对比表明,分布在大渡河、安宁河和金沙江干流的昔格达组以典型黄灰色岩性组合为特征,具有基本相同的沉积序列和磁极性序列,主要形成于高斯正极性时。     

Timing and magnitude of tetrapod extinctions across the Permo-Triassic boundary

A review of the tetrapod (amphibian and amniote) record across the Permo-Triassic boundary (PTB) indicates a global evolutionary turnover of tetrapods close to the PTB. There is also a within-Guadalupian tetrapod extinction here called the dinocephalian extinction event, probably of global extent. The dinocephalian extinction event is a late Wordian or early Capitanian extinction based on biostratigraphic data and magnetostratigraphy (the extinction precedes the Illawara reversal), so it is not synchronous with the end-Guadalupian marine extinction. The Russian PTB section documents two tetrapod extinction events, one just before the dinocephalian extinction event and the other at the base of the Lystrosaurus assemblage. However, generic diversity across the latter extinction remains essentially the same despite a total evolutionary turnover of tetrapod genera. The Chinese and South African sections document the stratigraphic overlap of Dicynodon and Lystrosaurus. In the Karoo basin, the lowest occurrence of Lystrosaurus is in a stratigraphic interval of reversed magnetic polarity, which indicates it predates the marine-defined PTB, so, as previously suggested by some workers, the lowest occurrence of Lystrosaurus cannot be used to identify the PTB in nonmarine strata. Correlation of the marine PTB section at Meishan, southern China, to the Karoo basin based primarily on magnetostratigraphy indicates that the main marine extinction preceded the PTB tetrapod extinction event. The ecological severity of the PTB tetrapod extinction event has generally been overstated, and the major change in tetrapod assemblages that took place across the PTB was the prolonged and complex “replacement” of therapsids by archosaurs that began before the end of the Permian and was not complete until well into the Triassic. The tetrapod extinctions are not synchronous with the major marine extinctions at the end of the Guadalupian and just before the end of the Permian, so the idea of catastrophic causes of synchronous PTB extinctions on land and sea should be reconsidered.     

Stratigraphy of the Middle Devonian boundary: Formal definition of the susceptibility magnetostratotype in Germany with comparisons to sections in the Czech Republic, Morocco and Spain

One major difficulty in geology is high-resolution correlation among widely separated sections, especially in the Paleozoic where magnetostratigraphy polarity is not well established because rocks are often remagnetized, where critical biostratigraphic zonation may be poor or lacking, or where structural complexities make correlations very difficult. To address this problem, we have been using magnetostratigraphy susceptibility measurements. Here, we report our work from the Middle Devonian in Europe and North Africa. The Middle Devonian (Emsian–Eifelian) global boundary stratotype section and point (GSSP), located in the Eifel Hills, western Germany, was ratified by the International Subcommission on Stratigraphy in 1985, after careful evaluation of the biostratigraphy for this and many other sections. The boundary interval has been characterized using biostratigraphy, and the beginning of the Eifelian stage has been specifically defined by the first occurrence of the conodont Polygnathus costatus partitus. We have collected the Eifel Hills section for magnetic susceptibility (MS) measurement and here we establish it as the magnetostratotype for the Emsian–Eifelian stage boundary, by formally defining the magnetostratigraphy susceptibility for the section. We then collected, measured and compared the magnetostratotype to four other sections for which conodont biostratigraphy has been studied and where P. costatus partitus is present; two Emsian–Eifelian sections in Morocco and two sections in the Czech Republic (including the Emsian–Eifelian parastratotype). Finally, we have measured the MS for the El Puerto Creek section in the Cantabrian Mountains of Spain and identified the location of the Emsian–Eifelian boundary within the section based on MS comparison to the GSSP in conjunction with excellent biostratigraphic indicators, primarily brachiopods. While the conodont zonation in the El Puerto Creek section is poorly defined, we believe that the correspondence between the MS and biostratigraphy in the section allows the identification of the Emsian–Eifelian boundary. These results indicate that this method can be successfully applied to marine sequences where ambiguities in correlation exist.     

Rapid vertebrate recuperation in the Karoo Basin of South Africa following the End-Permian extinction

The mass extinction that occurred at the end of the Permian Period approximately 251 Mya is widely accepted as the most devastating extinction event in Earth’s history. An estimated 75–90% of global diversity from both marine and terrestrial realms disappeared synchronously within at most one million and perhaps as little as 100,000 years. To date, most research has focused on the marine record and it is only recently that a few fully preserved terrestrial Permo-Triassic boundary sequences have been discovered. The main Karoo Basin of South Africa hosts several well-preserved non-marine Permo-Triassic boundary sequences that have been the focus of intensive research into the nature of the extinction and its possible causes. This study uses sedimentological and biostratigraphic data from boundary sequences near Bethulie in the southern Karoo Basin to make assumptions about the rates and timing of recovery of the terrestrial fauna in this portion of southern Gondwana after the extinction event. The biostratigraphic data gathered from 277 in situ vertebrate fossils allows us to define more accurately the temporal ranges of several taxa. These data also confirm a more precise extinction rate in this part of the basin of 54% of latest Permian vertebrate taxa, followed by the onset of a relatively rapid recovery, within an estimated 40–50 thousand years (based on the calculation of floodplain aggradation rates and compaction ratios) that included the origination of at least 12 new vertebrate taxa from amongst the survivors.     

The Jurassic/Cretaceous boundary in northern Siberia and Boreal-Tethyan correlation of the boundary beds

There is no international consensus regarding the GSSP for the Berriasian, the basal stage of the Cretaceous System. Any of the events discussed by the international expert community can be regarded as a marker of the Jurassic/Cretaceous boundary: a phylogenetic change of taxa, paleomagnetic reversal, or isotopic excursion. However, the problem of identification of this level in Boreal sections can be solved only using a combination of data obtained by paleontological and nonpaleontological methods of stratigraphy (bio-, chemo-, magnetostratigraphy, etc.). With any of the accepted markers, the Jurassic/Cretaceous boundary in Siberian sections will be within the upper part of the regional Bazhenovo Horizon. The least interval of the uncertainty of the position of this boundary in Siberian sections will be ensured by the selection of one of two markers: biostratigraphic (base of the Pseudosubplanites grandis Subzone) or magnetostratigraphic (base of the M18r magnetozone).     

Timing and magnitude of tetrapod extinctions across the Permo-Triassic boundary

A review of the tetrapod (amphibian and amniote) record across the Permo-Triassic boundary (PTB) indicates a global evolutionary turnover of tetrapods close to the PTB. There is also a within-Guadalupian tetrapod extinction here called the dinocephalian extinction event, probably of global extent. The dinocephalian extinction event is a late Wordian or early Capitanian extinction based on biostratigraphic data and magnetostratigraphy (the extinction precedes the Illawara reversal), so it is not synchronous with the end-Guadalupian marine extinction. The Russian PTB section documents two tetrapod extinction events, one just before the dinocephalian extinction event and the other at the base of the Lystrosaurus assemblage. However, generic diversity across the latter extinction remains essentially the same despite a total evolutionary turnover of tetrapod genera. The Chinese and South African sections document the stratigraphic overlap of Dicynodon and Lystrosaurus. In the Karoo basin, the lowest occurrence of Lystrosaurus is in a stratigraphic interval of reversed magnetic polarity, which indicates it predates the marine-defined PTB, so, as previously suggested by some workers, the lowest occurrence of Lystrosaurus cannot be used to identify the PTB in nonmarine strata. Correlation of the marine PTB section at Meishan, southern China, to the Karoo basin based primarily on magnetostratigraphy indicates that the main marine extinction preceded the PTB tetrapod extinction event. The ecological severity of the PTB tetrapod extinction event has generally been overstated, and the major change in tetrapod assemblages that took place across the PTB was the prolonged and complex “replacement” of therapsids by archosaurs that began before the end of the Permian and was not complete until well into the Triassic. The tetrapod extinctions are not synchronous with the major marine extinctions at the end of the Guadalupian and just before the end of the Permian, so the idea of catastrophic causes of synchronous PTB extinctions on land and sea should be reconsidered.     

Magnetostratigraphy of Paleogene deposits in Kamchatka

Paleomagnetic characteristics of several Paleogene sections in Kamchatka (Il’pinskii Peninsula, Bering Island, Chemurnaut Bay, Mametcha Bay) are considered. The sections are correlated with due account for biostratigraphic data, and possible correlation of magnetic polarity zones distinguished in the sections with the international magnetostratigraphic scale is presented.     

四川理塘甲洼组地层时代与环境演化

川西理塘甲洼盆地中的甲洼组是一套厚度在300m左右的松散状河湖相沉积物。磁性地层结果表明,B/M界线位于剖面中部157.0m处,剖面底部记录了奥尔都维正极性亚时。对比Cande和Kent的极性年表,理塘甲洼组的始沉积年代约为2.10MaBP,结束于0.10MaBP.孢粉记录显示该地经历亚高山针叶林植被→高山草甸植被的演变过程,沉积环境经历了河湖相→冲积相的转变过程。      

New data on the magnetostratigraphy of the Jurassic–Cretaceous boundary interval,Nordvik Peninsula (northern East Siberia)

The results of this study were used to identify a reversed polarity magnetozone, referred to as M17r, in Berriasian sections of the Nordvik Peninsula (northern East Siberia) within the normal polarity magnetozone (M18n) from previous studies. The new magnetozone embraces the Volgian–Ryazanian boundary (Chetaites chetae/C. sibiricus zonal boundary). It was also found that the former magnetozone M17r at Nordvik, which includes the C. sibiricus/Hectoroceras kochi zonal boundary should correspond to magnetozone M16r. Using magnetostratigraphic and biostratigraphic criteria proves that the Boreal C. sibiricus Zone is correlated with at least the major part of the Tethyan Tirnovella occitanica Zone, and the Boreal H. kochi Zone is correlated with the lower part of the Malbosiceras paramimounum Subzone of the Tethyan Fauriella boissieri Zone.     

黔西滇东地区二叠纪-三叠纪之交有机碳同位素和生物地层对比

二叠纪-三叠纪之交海、陆相地层对比研究对陆相二叠系-三叠系界线的定义以及全面认识该全球性重大生物与环境突变事件具有重要意义,是当前国际古生物学与地层学研究的重点和难点.选择贵州六盘水仲河二叠系-三叠系界线剖面为研究对象,系统研究了该剖面的化石面貌和有机碳同位素演变特征.结合黔西滇东地区二叠纪-三叠纪之交良好的陆相、海陆过渡相和浅海碎屑岩相地层记录,初步搭建了海、陆相生物地层与有机碳同位素地层对比框架.值得关注的是,综合已有研究的陆相和海陆过渡相剖面植物有机碳同位素和海相剖面无机碳同位素数据,发现均存在相同的碳同位素演变特征,且与生物地层对比方案一致.据此,认为高分辨率的有机碳同位素化学地层是实现海-陆相地层对比的有效手段之一.      

Geomagnetic reversal history

The history of geomagnetic polarity reversals in the Cenozoic and Late Mesozoic is well known since the Late Jurassic (Oxfordian). A continuous record of polarity has been derived for this time interval from the interpretation of oceanic magnetic anomalies. Most of the polarity chrons in this oceanic record have been verified and dated in coordinated magnetostratigraphic and biostratigraphic studies. This has led to the generation of progressively refined and improved geomagnetic reversal time-scales that provide a framework for absolute dating of palaeontological zonations. By serving as a basis for statistical analysis of reversal frequency they provide information relevant to processes in the Earth's core. The rate of reversals since the Late Cretaceous shows a steady increase on which a cyclical variation appears to be superposed. A stochastic model for reversals predicts a Poisson distribution of polarity interval lengths. The polarity time scales contain many fewer short (± 50 kyr) polarity chrons than a Poisson distribution, and it has been suggested that a gamma renewal process with index greater than unity is a more appropriate statistical model. The statistical arguments give no convincing reason for abandoning the model and other, physical reasons must be sought to explain the incompleteness of the reversal record. The discovery and verification of short chrons in the oceanic record may best be investigated by deep-tow magnetometer surveys. The reversal history before the Late Jurassic is not well known. Magnetostratigraphy in coeval Early Jurassic sections has not given correlatable records and it has not been possible to compile a definitive polarity sequence. Evaluation of geomagnetic polarity history for the Early Mesozoic and the Palaeozoic will require unambiguous magnetostratigraphy in well-dated sections where verification of the polarity pattern is possible at the fossil zone or stage level.     

二叠系－三叠系研究的进展

介绍了近年来二叠系、三叠系年代地层学的研究趋势及最新的年代地层表与磁性地层表。在二叠系、三叠系界线方面报道了新的底界方案及四个层型候选剖面，以及与之有关的生物地层学进展。界线事件地层学的，总趋势是球外事件研究趋于沉静而缺氧事件、海侵事件及火山事件的综合作用导致生物大绝灭的观点已占主导地位，其中界线缺氧事件的确立以及海侵始于二叠纪末的新观点是引人注目的发展。在层序地层学方面对于二叠系的全球海平面变化一般趋向于分四个旋回，但对于三叠纪则尚未统一。早二叠世的全球冰期－海平面升降旋回及三叠纪的米兰柯维奇旋回在我国均有可能发现和研究。文章最后提出了层序地层界线与年代地层界线不一致所产生的理论问题并探讨了解决方法。     

Magnetostratigraphy of the Maastrichtian continental record in the Upper Aude Valley (northern Pyrenees,France): Placing age constraints on the succession of dinosaur-bearing sites

The first detailed stratigraphic succession of the Upper Cretaceous continental record from the Upper Aude Valley (southern France) is presented together with a magnetostratigraphic study. The combined stratigraphy and magnetostratigraphy of the Marnes rouges inférieures Fm (Lower Red Marls), constrained by biochronological markers such as charophyte occurrence and revised dinosaur eggshells, results in a succession of fluvial red beds dated from chron C32n to the top of chron C31r. It implies an earliest Maastrichtian age close to the C32n.1n-C31r reversal for the majority of the dinosaur sites including Bellevue. In contrast, the upper Maastrichtian is likely represented by a short interval within the lacustrine-palustrine Calcaires et argiles de Vignevieille Fm (Vignevieille Limestones), or it might even not be recorded. The proposed age indicates that the marine to continental transition, as a result of the Late Cretaceous transgression, took place earlier in the north Pyrenean basin than in the southern area.     

二叠系-三叠系研究的进展

介绍了近年来二叠系、三叠来年代地层学的研究趋势及最新的年代地层表与磁性地层表。在二叠系、三叠系界线方面报道了新的底界方案及四个层型候选剖面，以及与之有关的生物地层学进展。界线事件地层学的总趋势是球外事件研究趋于沉静而缺氧事件、海侵事件及火山事件的综合作用导致生物大绝灭的观点已占主导地位，其中界线缺氧事件的确立以及海侵始于二叠纪末的新观点是引人注目的发展。在层序地层学方面对于二叠系的全球海平面变化一般趋向于分四个旋回，但对于三叠纪则尚未统一。早二叠世的全球冰期—海平面升降旋回及三叠纪的米兰柯维奇旋回在我国均有可能发现和研究。文章最后提出了层序地层界线与年代地层界线不一致所产生的理论问题并探讨了解决方法。     

柴达木盆地大浪滩梁ZK05钻孔的磁性地层研究

本文展示了柴达木盆地西北地区大浪滩盆地梁ZK05钻孔上部330m岩芯的最新磁性地层结果。梁ZK05钻孔的磁极性序列记录了4个正极性亚时,分别对应于布容期、Jaramillo、Cobb Mountain和Olduvai。磁极性序列底部未出现Reunion亚时,其年代应小于2128ka,B/M界限深度位于94m。根据钻孔的平均沉积速率,我们推算出钻孔最顶部的年代为111ka,钻孔在330m深度的年代为2046ka。由此可知,钻孔330m以下的卵砾石层沉积的结束年代为2Ma,代表阿尔金山的这次强烈隆升应早于2Ma,与青藏运动B幕相当,其启始时间为2.6Ma,结束时间大致为2Ma。另外,梁ZK05钻孔记录了3次极性漂移事件,位于51~58m、207.5~212m和249~252m,分别对应于Calabrian Ridge2(515~525ka)、Gardar(1465~1485ka)和Gilsa(1567~1575ka)。我们在沈振枢等人(1993)的8个钻孔磁性柱基础上,加上梁ZK05钻孔和ZK02钻孔的磁极性序列,辅以最新的国际标准磁极性年表,建立了柴达木盆地最新的磁性地层年代框架。     

元谋人时代研究评述——兼论我国早更新世古人类时代问题

我国早更新世古人类遗址时代的确定对于认识早期人类起源、扩散与演化具有重要意义。对元谋人化石的各种年龄测定结果和认识分歧进行了综合评述。综合分析认为，在翔实的岩石地层学和古生物地层学基础上，开展高分辨率磁性地层学研究是解决包括元谋人在内的这一时期哺乳动物时代科学问题的最有效的研究途径。     

四川省纳溪县上马剖面侏罗系至白垩系古地磁研究

运用磁性地层学研究岩石单元磁性特征,是一种有效的地层对比方法。上马剖面按地层序列磁性特征的分异把磁极性一致或极性反向频率大致相当的区段组合在一起,由此划分了３３个磁性地层极性带,３个极性亚超带,２个极性超带,可与国际标准磁极性年表对比。     

Stratigraphy, biostratigraphy and C-isotopes of the Permian–Triassic non-marine sequence at Dalongkou and Lucaogou, Xinjiang Province, China

Measured lithostratigraphic sections of the classic Permian–Triassic non-marine transitional sequences covering the upper Quanzijie, Wutonggou, Guodikeng and lower Jiucaiyuan Formations at Dalongkou and Lucaogou, Xinjiang Province, China are presented. These measured sections form the framework and reference sections for a range of multi-disciplinary studies of the P–T transition in this large ancient lake basin, including palynostratigraphy, vertebrate biostratigraphy, chemostratigraphy and magnetostratigraphy. The 121 m thick Wutonggou Formation at Dalongkou includes 12 sandstone units ranging in thickness from 0.5 to 10.5 m that represent cyclical coarse terrigenous input to the lake basin during the Late Permian. The rhythmically-bedded, mudstone-dominated Guodikeng Formation is 197 m and 209 m thick on the north and south limbs of the Dalongkou anticline, respectively, and 129 m thick at Lucaogou. Based on limited palynological data, the Permian–Triassic boundary was previously placed approximately 50 m below the top of this formation at Dalongkou. This boundary does not coincide with any mappable lithologic unit, such as the basal sandstones of the overlying Jiucaiyuan Formation, assigned to the Early Triassic. The presence of multiple organic δ¹³C-isotope excursions, mutant pollen, and multiple algal and conchostracan blooms in this formation, together with Late Permian palynomorphs, suggests that the Guodikeng Formation records multiple climatic perturbation signals representing environmental stress during the late Permian mass extinction interval. The overlap between the vertebrates Dicynodon and Lystrosaurus in the upper part of this formation, and the occurrence of late Permian spores and the latest Permian to earliest Triassic megaspore Otynisporites eotriassicus is consistent with a latest Permian age for at least part of the Guodikeng Formation. Palynostratigrahic placement of the Permian–Triassic boundary in the Junggar Basin remains problematic because key miospore taxa, such as Aratrisporites spp. are not present. Palynomorphs from the Guodikeng are assigned to two assemblages; the youngest, from the upper 100 m of the formation (and the overlying Jiucaiyuan Formation), contains both typical Permian elements and distinctive taxa that elsewhere are known from the Early Triassic of Canada, Greenland, Norway, and Russia. The latter include spores assigned to Pechorosporites disertus, Lundbladispora foveota, Naumovaspora striata, Decussatisporites mulstrigatus and Leptolepidites jonkerii. While the presence of Devonian and Carboniferous spores and Early Permian pollen demonstrate reworking is occurring in the Guodikeng assemblages, the sometimes common occurrence of Scutasporites sp. cf. Scutasporites unicus, and other pollen, suggests that the Late Permian elements are in place, and that the upper assemblage derives from a genuine transitional flora of Early Triassic aspect. In the Junggar Basin, biostratigraphic data and magnetostratigraphic data indicate that the Permian–Triassic boundary (GSSP Level) is in the middle to upper Guodikeng Formation and perhaps as high as the formational contact with the overlying Jiucaiyuan Formation.