THE GEOMAGNETIC POLARITY TIME SCALE FOR EARY PALEOZOIC: DATA AND SYNTHESE 1 2 1 Vladimir Pavlov and Yves Gallet 2 Institute of Physics of the Earth, Institut de Physique Russian Academy of Sciences INTRODUCTION EARLY CAMBRIAN: Kirschvink and LONG STANDING CONROVERSY: Constructing the Geomagnetic Polarity Time Scale (GPTS) through the geological history is of crucial importance to address Rozanov, 1984 TWO PRIMARY POLES FOR ONE SIBERIAN PLATFORM? several major issues in Earth sciences, as for instance the long-term evolution of the geodynamo. The GPTS is also an Khramov et al., 1982 important chronological tool allowing one to decipher the age and duration of various geological processes. Moreover, GPTS 509 Ma is widely used in prospecting for commercial minerals and in petroleum geology Kirschvink’s direction OR Khramov’s direction? Series Regiostage . N N The chronology of the geomagnetic polarity reversals since the Upper Jurassic is rather well known thanks to the magnetic Kirschvink and Pisarevsky anomalies recorded in the sea floor. For more ancient epochs our knowledge of the GPTS is still very fragmentary and much Rozanov, 1984 et al., 1997 more uncertain. However significant information has been obtained for several time intervals, in particular for the Triassic and the beginning of the Phanerozoic. Hirnantian Khondelen We will focus our presentation on recent developments made in the determination of the GPTS during the Early Paleozoic P-T Botoma - Toyon Botoma - Botoma - Toyon Botoma - 516 Ma (Siberia) (Cambrian and Ordovician). Siberian APWP Ashgill Epoch 2 Epoch 2 D3-C1 экватор E30° Е90° Е150° Botmoynak Dolborian (Tianshan) Late Ediacarian - Hyperactivity of the Earth magnetic field? Early Cambrian Kirschvink’s pole Early Cambrian O2-O3 Cm3-O1 Khramov’s pole S30° Moyero N Cm2 N N Low (very low?) reversal frequency (Siberia) Pavlov et al., Pavlov et al., 2018 S60° unpubl. Upper Ordovician Moderate (high?) reversal frequency Toluk (Tianshan) Tommot Atdaban Tommot Atdaban 525 Ma Katian Terreneuvian Terreneuvian Caradoc SYNTHES: both directions are primary (i.e. have been recorded either during the formation of rocks or soon after). But observed data can't be reconciled in the framework of the Geocentric Axial Dipole hypothesis! N N Almaly Rozhkova N (Tienshan) Chertovskian Baksanian (Siberia) Gullhogen (Sweden) Kudrino Sandbian (Siberia) White sea area, Arkhangelsk region, Late Ediacarian, 558-555 Ma, siltstone and claystone, about 60 reversals in 420 m, estimated revrsal rate ~ 20 RMa-1 Southwestern Siberian platform, Late Ediacarian, To explain seemingly contradictory outcomes of the numerous paleomagnetic studies we By the beginning of the Sandbian age (Late Llandeilo-Early Caradoc) geody- South Urals, Late Ediacarian, 545-550 Ma ~550 Ma, siltstone and sandstone, 58 polarity zones siltstone and sandstone, 40 polarity zones in 60 m, estimated reversal rate > 30 RMa-1 consider a hypothesis of anomalous non-uniformitarian geomagnetic field during Early Camb- namo returns to “normal reversing state”, however, soon, the new long magnetic in 110 m, estimated revrsal rate ~ 20-25 RMa-1 rian. In this case the geomagnetic field could be characterized by occurrence of two quasi- polarity interval (but of normal polarity) is, probably, established. Existence of stable generation modes, which replaced each other in turn. this Late Ordovician - Early Silurian superchron would be in a good aggrement, The first mode would correspond to prolonged period during which an axial mostly mono- with the hypothesis of “double - superchron” of Algeo (1996) and with the Available data indicate that on the eve of and around the Precambrian-Phanerozoic transition geodinamo had operated Khorbusuonka river section polar dipole field predominated. The second mode would correspond to relatively short hypothesis of sudden transitions between geodynamo operation modes (Gallet in a hyper-activity reversing mode (Gallet and Pavlov, 2016) characterized by an extreme geomagnetic reversal frequency. epochs when a reversing circumpolar or midlatitude dipole predominated. and Pavlov, 2016). Further studies are needed to check these hypotheses. Magnetostratigraphy from the Middle Cambrian through the Middle Ordovician Ordovician LocationOur of samplingmain studied in the sections Siberian platform Early Paleozoic Geomagnetic polarity time scale: tentative compilation (Siberian platform) Upper Cambrian The Middle Cambrian was mostly characterized by very high reversal frequency. Beginning from the end of the Middle Cambrian through Early Ordovician (Trema- Upper Cambrian doc) the geodinamo was in its “normal reversing mode” (Gallet and Pavlov, 2016) with medium to low reversal frequency. Between the Late Tremadocian and the Ma Silurian Ma Kulumbe Middle Llandeilian (begiining of the Sandbian age) the superchron Moyero occurred. This superchron of reversed polarity would have a duration of ~20 Myr. 499 443.8 section Khorbusuonka Hirnan- tian Moyero Epoch 3 Kulumbe section: Uppermost Middle Cambrian- Lowermost Ordovician Turukhansk Upper Cambrian - lowermost Ordovician Mayan stage Comparison of magnetostratigraphy of sections Ashgill region Popigay from Siberia, Australia and China Kulumbe Middle Cambrian Composite Stages Pyasina section Fomich o Kheta Upper 70 Kotuykan Magan Djogdjo Lena Ordovician Арениг Kulumbe Anomocarioides limbataeformisA.truncata L.laevigata - NW Siberia Rakhmet Cordylodus Caradoc Olenek Cordylodus Sandbian Katian Kotuy Enisey S.quadra- angulatus Enisey J+ angulatus Arenig (Floian-Darriwilian) plicatus Gallet and Pavlov, 1998 Paroistodus proteus C.herfurhi H+ I+ I+ Cordylodus Cordylodus Drepanoistodus C.angulatus H+ J+ lindstromi lindstromi delfiter Markha Тремадок H+ V Aktay iluy C.intermedius G5- I+ Nyayan 458.4 Viluy Uchur-Maya G4+ ? C.proavus Cordylodus Tremadoc H+ Anomocarioides limbataeformis prolindstromi Cordylodus G3- G- proavus Cordylodus lindstromi region Hirsutodontus F3+ Llandeilo simplex G2+ Cordylodus intermedius Nijnyaya Tunguska F1+ Ungur F3+ Loparian G2+ Cordylodus proavus Tangchan, C.minutus D+ F3+ Mayan stage Cordylodus G1- C.primitivus Epoch 3 Northern China proavus F1+ Po Eoconodontus (E.) dka Aldan F3+ F1+ m B+ Mansi en Yang et al., 2002 alisonae na H.discretus F2- E- y Eoconodontus (E.) a F1+ D+ D+ notchpeakensis Batyrbay T Eoconodontus u T.fissus - P.sacheri T.fissus Epoch 3 H.appresus ng P.muelleri o uska B3+ Hirsutodontus ani .gibbus 60 Middle Lena T Middle Cambrian E- Darriwilian C.perforatus- A.henrici C- B1+ H.resimus Westergaardodina Enisey Ketyian Eoconodontus Amga stage amplicava Kuonam- kites P.dahlmani- Vitim Olekma D+ Llanvirn Middle Cambrian P.memorabilis B3+ Aksay Enisey Angara Oryctocara Mulleri Lena Protoconodontus T.fissus- P.sacheri Teridontus P.postero- nakamurai C- B2- costatus Ridge Upper Cambrian Rozhkova B1+ .gibbus Amga stage T Superchron Proconodontus Cretaceous Postero- costatus Saks Yurakian A- P.tenui- serratus “Moyero” Jurassic Toyon stage Toyon 1200 Khorbusuonka Black Mountain, Permian and Triassic Dayangcha, Australia Ayusokkan Polovinka section NW China Paleozoic Entsian Ripperan and Kirschvink, 1992 Middle Ordovician Baykal Lake Epoch 2 Ripperdan et al., 1993 Madyian 0 Precambrian Mayan 100 Kudrino Lower Cambrian Lower Tavgian Botoma Nganasanian Arenig Middle cambrian Floian Dopingian Rakhmet 477.7 Three Distinct Reversing Modes in the Geodynamo (Gallet and Pavlov, 2016) Middle Cambrian Magnetostratigraphy of the Arenig and the Llanvirn Magnetostratigraphy of the Middle Ordovician Aktay 10 Serie Serie Lower Ordovician Lower Tremadoc Siberian platform East-European platform Amgian 485.4 Кукрусе 510 Regiostage 8 Sandbian Sandbian Composite South Sweden Киренско- кудринский Composite section ? Ungur Graptolite zones Graptolite zones Scale (Torsvik and Trench, Kudrino, Rodionov et al.,2001; Лландейло Moyero 1991a) Лландейло This study 6 Rozhkova Polovinka, Gullhogen, Trench and Torsvik, Caradoc Kulumbe Ухаку Pavlov et al.,2000 Sandbian 1991 Profile 2 Profile 1 Sandbian Волгинский N.gracilis N.gracilis Moyero, Regiostage 4 ЛМГ Toyon Caradoc Caradoc Gallet and Pavlov, 1996. Chertovsky no data Alexeevka, Batyrbay reversal frequency (rev/Myr) ? this study ? ? ? 2 Lava ? Stolbovaya Азери Муктейский (Smethurst et Kulumbe ? ? al., 1998) Magnetic ЛланвирнЛланвирнЛланвирн 0 Лланвирн Aksay Tosna Botoma ? 0 100 200 300 400 500 600 Kudrinsky Daprriwilian (Smethurst et Daprriwilian al., 1998) Normal Кунда Вихоревский polarity Normal Reversing mode Time (Ma) Kukruse ? Superchron mode Hallekis, Saks Trench and Torsvik, Reversed 1991 Atdaban Hyper-Active Reversing mode Lower Cambrian Lower Darriwilian Darriwilian G.teretiusculus G.teretiusculus Upper Cambrian Upper Волхов polarity ? Llandeilo Llandeilo Llandeilo Кимайский Dapingian Dapingian Kirensky Anomalous ? interval Арениг Арениг The geodynamo operates in three distinct reversing modes: (i) a “normal” reversing mode generating Биллинген Tommot 526 Ayusokkan 499 geomagnetic polarity reversals according to a stationary random process, with on average a reversal rate Uhaku Floian Floian ? of ~3 rev./Myr; (ii) a non-reversing “superchron” mode characterizing long time intervals without reversal; Угорский Хуннеберг (iii) a hyper-active reversing mode characte-rized by an extreme geomagnetic reversal frequency. Volginsky Ediacarian Middle Cambrian Europe Lena, 59.8N, 118.1E, The transitions between the different reversing modes would be sudden, i.e. on the Myr time scale. Siberia Torsvik et al.,1995 We suggest that in the past, the occurrence of these transitions has been modulated by therma Няйский Варангу conditions at the core-mantle boundary governed by mantle dynamics. Tremadoc Тремадок Llanvirn Llanvirn Llanvirn.