22. PALEOGEOPHYSICAL CHARACTERISTIC OF THE PALEOZOIC

22. PALEOGEOPHYSICAL CHARACTERISTIC OF THE PALEOZOIC* Original chapter is in: G.Tenchov, Paleogeophysics, 2013, HeronPress ISBN 978-954-580-337-6. Paleozoic is a part of the Earth's history in the period from 542MaBP to 251MaBP. In the Paleozoic occurs and widely spread first fossils with a hard shell. At the end of the period, large reptiles and modern plants settle at the continents. In the Paleozoic continental crust undergoes significant changes. Continental crust Cambrian is the earliest period of the Paleozoic in which we can determine with some confidence the position of the individual continental blocks. During this time, Rodinia (last supercontinent of the Proterozoic) continues to disintegrate. Large water areas open: northward - Panthalassic Ocean and southward - Iapetus and Rheic Oceans. In the Early Cambrian, individual blocks separate and situate around the tropics Fig.22.1a. Blocks from today's North America, Africa, Australia, Antarctica and India form the supercontinent Gondwana. It locates in the southern hemisphere of the Earth. The movement of continental plates is accompanied by collision between them. Typical of this period is an active expansion of the seabed and subduction between the blocks. As a result, active orogenesis takes place, known as the Pan-American Orogeny. At this time forms Caledonia-Hercynian Mountains. In the Ordovician, appears Taconian Orogeny. In the Late Ordovician, parts of Gondwana moves northwards Fig22.1b. In the Middle Paleozoic Gondwana moves in a complicated way around the South Pole. Different parts of Rodinia migrate to the equator. Some of these fragments take a part in the foundation of the supercontinent Laurasia. Australia and Scandinavia move to the tropics, while South America and Africa are in the Polar Regions Fig.22.1c. In the Silurian continues the movement of individual blocks to the equatorial areas. Baltica and North America actively interact, which leads to significant orogenesis. They form mountain ranges in today's North East America and in parts of the Baltic, known as Caledonian Orogeny. Parts of Gondwana moves to equatorial areas. Pan Thalassic Laurentia Siberia Siberia Gondwana Laurentia Gondwana Iapetus Ocean Baltica Oceanic stream Cold Warm Fig.22.1b Late Ordovician Fig.22.1a Cambrian Ural Siberia Namibia Gondwana Protethys Ocean Paleotethys Ocean Glaciers Fig.22.1c Middle Paleozoic Panthalassic Ocean Hercynian Orogeny Pangaea Paleotethys Ocean Tethys Glaciers Fig.22.1d Late Paleozoic In the Devonian, Gondwana continues to move toward the equator. Siberia and Africa also move towards the equator. Continues the formation of the supercontinent Laurasia In the Permian, all part of the continental crust, with the exception of East Asia unites in supercontinent Pangaea. It is on both sides of the equator, and extends to the South Pole. It begins the formation of the Tethys Ocean. World Ocean In the Paleozoic, the area of the World Ocean reduces by 4-5%. Sea level has several changes, initiate by glaciation and melting of glaciers at the Polar Regions. Fig.22.2 shows the average sea level and standard deviation (shaded part) of the world ocean. 500 Carb Pm Dev S Cm Ord Sea level, m 400 300 200 100 0 -100 -200 200 250 300 350 400 Age, MaBP 450 Fig.22.2 Sea level of World Ocean in the Paleozoic Reference: 500 550 Condie and Sloan (1997), Hallam (1989), Haq, B. et al.(1987), Ross, C.A. and J.R.P. Ross (1987, 1988). At the beginning of the Cambrian, Sea level rises by about 300m. In the Middle Paleozoic it varies within +/- 100m. Then, monotonous decreases reaching the minimum values at the end of the Permian. There outline the following cycles in the change of the sea level in the Paleozoic (in parentheses is their relative amplitude): 40Ma (27%), 20Ma (27%) and 319Ma (26%). Climate Fig. 22.3 shows the surface temperature of the World Ocean (OST) and the global average annual temperature of the Earth (GAT) during the Paleozoic. Idea of the temperature of seawater in the Paleozoic is obtained in analysis of the isotopic content of oxygen 18O (Craig, H. 1961; Craig, H and L. Gordon, 1965). At the beginning of the Cambrian start the first temperature cicle of the Earth climate in Paleozoic. The OST raises reaching maximum in the Middle Ordovician. Probably warm water streams from equatorial part rise OST. It reaches its maximum of about 280 . Probably this is the reason that GAT is lower than OST at this time. At the end of the Ordovician, Laurentia and Siberia block equatorial warm sea water. Water from poles invades and a significant cooling of sea water. The minimal temperature of around 6-70 take place. From the Middle Ordovician began cooling of both in OST and GAT. Global annual temperature falls down to about 100 . The second temperature cycle starts on the End of Ordovician and reaches maximum at the end of the Devonian. The beginning of the Carboniferous indicates by a significant cooling. The third cycle starts on the beginning of the Permian with substantially warming of sea water. The global average temperature but stays low. Probably sea water is blaocked around the equator. It reaches its maximum in the middle of the Permian and ends with cooling at the end of the Permian. The average temperature of the Ocean in the Paleozoic is around 180 . It highlights the following more important cycles in temperature variations of the Paleozoic: 80Ma (37%) and 40Ma (19%). Carb Temperature, deg Pm Dev S Cm Ord 35 OST 30 GAT Scotese et al. 25 20 15 10 5 Third cicle First cicle Second cicle 0 200 250 300 350 400 450 500 550 Age, Ma Fig. 22.3 Ocean surface temperature OST and the average annual temperature of the Earth GAT during the Paleozoic. (Scotese, CR, Song, H, Mills, et al. (2021) Phanerozoic Paleotemperatures: The Earth’s Changing Climate during the Last 540 million years. Earth-Science Reviews, 215. 103503. ISSN 0012-8252) Asteroids Carb Pm Dev S Cm Ord 1,E+06 Number Mass 10 1,E+05 8 1,E+04 6 1,E+03 4 1,E+02 2 1,E+01 0 1,E+00 200 250 300 350 400 450 500 550 Mass of asteroids, Mt/20Ma Number of asteroids, n/20Ma 12 600 Age, MaBP Fig.22.4 Asteroids in the Paleozoic Fig.22.4 shows the number and mass of asteroids found in the continental crust with ages between 250MaBP and 550MaBP. Data are averaged over intervals of 20Ma. The reason for this is the relatively small number of large asteroids and error of their age. There is some tendency that the number and mass of asteroids to increse from the Erly to the Late Paleozoic. The number of large asteroids found for periods of 20Ma ranged from 4 to 10. There are some cyclic phenomena in the number of asteroids. Most often such events occur at about 64Ma (31%) and 107Ma (27%). As was noted, due to significant inaccuracies in determining the age of asteroids can be assumed that about every 100Ma large asteroids impact the Earth. Earth rotation Data from the study of corals shows that during the Paleozoic the average angular velocity of the Earth’s rotation decreases by about 1.10-5 rad.sec-1 or relatively by about 10% (Fig.22.5) (Tenchov, G.G. 2007). Angular velocity of of the Earth’s rotation ω+ can be approximated by the equation: ω+ = -2.025E-11A2 + 4.139E-08A + 6.721E-05, (22.1) 1,0E+07 8,8E-05 1,0E+06 8,6E-05 1,0E+05 8,4E-05 1,0E+04 8,2E-05 1,0E+03 8,0E-05 1,0E+02 7,8E-05 Mass of asteroids per 20Ma omega 1,0E+01 1,0E+00 7,6E-05 7,4E-05 Carb Pm Dev S Cm Ord 1,0E-01 200 Omega, rad/s Mass of Asteroids, Mt/20Ma where A is the age of the Earth in Ma. 7,2E-05 250 300 350 400 450 500 550 600 Age, MaBP Fig.22.5 Earth rotation in Paleozoic A major reason for slowing down the Earth’s rotation is energy loss in the Earth-Moon system. Probably, the main loose of energy is due to the interaction of tidal ocean waves with the continental crust (Tenchov, G., 2013). Much less is the effect caused by friction in the propagation of the gravity waves in the Earth's crust and in those in the Moon. Another factors are formation of glaciers in the polar regions, changes in the shape of the continental crust and the colaps with large asteroids. A rise in world ocean level leads to an increase in the Earth's momentum of inertia as well. As a result, the rotation of the Earth slows down and the Earth-Moon distance increases. When, the world ocean level slows down the Earth's rotation increases Fig.22.6. Mass of asteroids increses from the Early to the Late Paleozoic and it contributes to slowing down of the Earth’s rotation. Fig.22.6 shows the change in Sea level and Domega. Domega is the change in Earth’s rotation in respect to trend line on Fig.22.5. Precambrian begins with steep increase in the sea level by near 300m. This is the probable reason as Domega to decreses by 4E-06 rad/s at the beginning of Cambrian. The decreases in the sea level in the Middle Ordovician is not accompanied with the increases of Domega (probably because no sufficient data for ω+ at that time). From the end of Ordovician to the Middle Permian green arrows confirm the above said idea of increasing of Domega by slowing down the sea level. The pink arrows confirm the Domega slowin down by increasing the sea level. 350 4,0E-06 Sea level 300 3,0E-06 DOmega 2,0E-06 ? 200 1,0E-06 150 0,0E+00 100 -1,0E-06 50 -2,0E-06 0 DOmega, rad/s Sea level, m 250 -3,0E-06 Pm Carb Dev S Cm Ord -50 -4,0E-06 240 280 320 360 400 440 480 520 560 Age,MaBP Fig.22.6 Sea level (data from Condie and Sloan, 1997) and change in the Earth rotation Domega in Paleozoic time. Condie K. and Sloan, R.1997 Origin and evolution of Earth. Principles of Historical Geology. Prentice Hall, NJ, 498 G.G. Tenchov, 2013 Growth of the continental crust and its influence on the Earth-Moon system Academia.edu Earth magnetic field The Earth's magnetic field (EMF) during the Paleozoic can be characterized by: 1. Polarity of the EMF. 2. Frequency of change of polarity for a period of time or known as “ivents” 3. Virtual dipole momentum (VDM) of the EMF. Polarity of the EMF presents by modern (0) and revers (1) polarity. Fig.22.7 shows polarity of the EMF during the Paleozoic. The general impression is that there is a transition in the Cambrian of inverse (1) to modern (0) polarity. In Ordovician there is a trend to dominate the modern polarity. From Silurian begin a stable trend of modern polarity with some exeption at the beginning of Carboniferous. At the end of Permian dominate the inverse polarity of the EMF Modern polarity Inverse polarity Trendline Pm Carb Dev Sil Ord Cmb Polarity of EMF 1 0 240 280 320 360 400 440 480 520 Age, MaBP Fig.22.7 Polarity of the EMF during the Paleozoic Change of polarity of the EMF is labeled “ivent”. Fig. 22.8 shows the frequency of ivents in Paleozoic for periods of 10Ma. As noted in previous chapters, change the polarity of the EMF is due to different reasons: change the level of the ocean, intense orogenesis, redistribution of land masses, volcanic activity and impakts with extraterrestrial bodies. The average value of ivents in Paleozoic is about 2 ivents/10Ma. It can be pointed on several cycles: about 40Ma (25%), 20Ma (22%) and 107Ma (20%). 10 Carb Ivents/10Ma Pm Dev Ord S Cmb 8 6 4 2 0 250 300 350 400 Age, MaBP 450 500 550 Fig.22.8 Ivents in Paleozoic for periods of 10Ma. Magnetic field of a sphere can be approximated as a magnetic field of a dipole inside the sphere. It is labeled as Virtual Dipole Momentum (VDM). Fig.22.9 shows VDM of the EMF in Paleozoic. Data are scarce and presented in VDMU (1VDMU=1022Am2). It is added a sample from Precambrian time. 14 Pm Carb Dev S Ord Cmb St dev >1 Single samples St dev<=1 12 VDM*1022Am2 10 8 6 4 2 0 200 250 300 350 400 450 Age, MaBP 500 550 600 Fig.22.9 VDM of the EMF in Paleozoic Green points are single samples. Triangles are samples with standard deviation less than 1VDMU, and black points are samples with standard deviation above 1VDMU. The overall conclusion is that VDM is 3-4 times less than modern value (8.1022Am2). From Precambrian, VDM decreases and reaches a minimum in the Late Ordovician and has a local maximum in Devonian. From the Middle Carboniferous VDM increases. Data from single samples overflow the modern VDM. From the Middle Permian VDM decreases substantial. In the Middle Carboniferous sea level fall dawn dramatically by 300m (Fig.22.6). It is accompanied by the change in the Earth’s rotation. At the end of Permian sea level raises by 150m. Numbers of asteroids with huge mass meet the Earth (Fig.22.4). Probably all this catastrophic phenomena generate such behaviour of the VDM at that time. For now there is no relevant hypotheses about behavior of the VDM in Paleozoic. Extinction, explosion and temperature of the ocean When studying the ancient atmosphere and climate we will pay particular attention to the extinction, which is a biological characteristic, but it gives information about the climate in this period. Paleozoic, as already was noted, is characterized by the flourishing of the biosphere. The number of genera in the sea increase sharply. From the beginning of the Cambrian animals and plants are invaded on land. On Fig.22.10 is shown a general picture of biodiversity in the Paleozoic. The number of genera increases during the Cambrian and is strongly developed in the end of Ordovician. With some, sometimes significant variations it remain at high level in the Silurian, Devonian and in the Carboniferous. At the end of the Permian life on the Earth is imposed on real catastrophe. A better understanding of biodiversity in the Paleozoic can be presented by the relative extinction. That is the ratio of the number of missing to the number of existing genera (Fig.22.10). Extinction of organisms is a natural process. With certain reservations it can be assumed that the average extinction in the Paleozoic is at the order of 15±5%. This value in this case can be considered as a normal extinction characteristic of the Paleozoic. Extinctions over 20% and under 10% we accept as anomalous. Pm 2000 Dev Carb Sil Cmb Ord 80 genera extinction 70 Gener a 50 1000 40 Normal extinction 30 500 20 Extincted genera, % 60 1500 10 0 250 300 350 400 Age, MaBP 450 500 0 550 Fig.22.10 Biodiversity in the Paleozoic As was noted, in the Early Cambrian appear first organisms with shells. Important ecosystem during this period is the marine reef areas inhabited by different organisms. They are located in shallow tropical sea. Inhabitants of them are highly sensitive to changes in the marine living conditions, changes in the temperature and in the sea level (Fig.22.11). The period within 540MaBP-530MaBP is characterized by an increase in sea temperature. Sea water temperature is set to scale back. So it is easier to correlate with the relative extinction and mass extinction (dark arrows) and with the flourishing of life (light green arrows). The first significant mass extinction (1) (red numbers) is between 530MaBP and 520MaBP. Around 40% of the organisms in the rift zones and shallow seas dye. The most probable hypothesis is that sea level is lowered due to a general climate cooling. In the Polar Regions are accumulated significant ice formations. From there raid cold water masses. In the recession of sea level, living organisms move on land. They are not adapted for life in such conditions. They are subjected to active influence of ultraviolet radiation. According to other hypotheses cold and oxygen poor water up welled from the deep sea. The absence of oxygen leads to mass extinction of organisms. Between 520MaBP and 510MaBP sea is cool. The number of living organisms is decreased, but the relative extinction does not exceed 15%. The next mass extinction (2) is in the Late Cambrian and in the Early Ordovician (about 500MaBP). There are killed almost 40% of the species. Probably most of the still fragile species die, although a sea surface temperature is not below 170C. During the Ordovician, for a relatively short period of time (about 492MaBP) occurs the next mass extinction (3) that kills about 30% of the species. Sea temperature is about 170C. Probable cause for the extinction of some species is the struggle for existence between organisms. From 490MaBP to 470MaBP Sea temperature rises significantly from 170C to about 0 27 C. This causes a reduction of extinction and preparing to mass explosion of life. Life on the Earth enters in a significant boom (light green arrow). Over 1200 new family enrich the biodiversity. From 480MaBP to 450MaBP the sea temperature monotonically decreases and with thus offers the possibility for the next mass extinction. Carb Pm 40 Dev Ord Si Cmb 80 sea water temperature extinction 30 60 20 40 10 20 0 250 7 8 300 6 5 350 4 400 450 3 2 1 Extinction, % Sea water temperature, deg big asteroids 0 500 Age, MaBP Fig.22.11 Extinction and temperature of the ocean in the Paleozoic The most significant extinction in the Early Paleozoic (4) occurs at the end of the Ordovician (448MaBP-438MaBP) where 30% of all sea creatures die. It is believed that the reason for this is the glaciation of Gondwana and decreases in the sea level. Evidences are found in the Saharan desert, and data from paleomagnetic study. The latter confirms that Africa is around the South Pole at that time. Sea temperature drops to anomalous low value of 70C. Some authors call this part of Earth's history snowball Earth. During the end of Silurian, the Earth enters in a new period of warming the climate. The Sea temperature grows monotonically from 7-100C in the late Ordovician to 12-130C at the end of the Silurian. There appear about 500-600 new genera. In the Silurian continues the evolution of marine life. There appears first freshwater fish and those with jaws. It is found the first well-preserved traces of life on land: spiders, centipedes and plants with a capillary system. The temperature continues to increase. At the end of the Devonian it reaches 25-270C. Warm and shallow seas are located in the equatorial part of the Earth. In the Devonian appear sharks and fish with a hard skeleton. Corals are spread widely and form large reef areas. Insects and amphibians appear. Modern plants emerge of land and form the first forests. From 385MaBP to 385MaBP occur two brief cases of mass-extinction (5 and 6). From 385MaBP to the end of the Devonian (375-360MaBP) sea temperature continuously is increased, reaching 25-270C. At the end of the Devonian (365MaBP-350MaBP) a moderate extinction take place. Probable reason for this is a relatively sharp drop in temperature of the sea by nearly 150C. Another assumption is that a large meteorite falls at that time. This leads to significant changes in the atmospheric composition, solar radiation and photosynthesis. From the beginning of the Carboniferous, sea temperature is decreased monotonically from 26-270C in 360MaBP to 11-130C at 350MaBP. However, biodiversity increases on land. About 325MaBP the sea temperature drops below 150C. Biodiversity decreases significantly by about 600 genera, which is the next mass extinction (7). At the end of Carboniferous sea temperature drops to 10-120C. In the Early Permian starts a monotonic increase in the sea temperature from about 120C in 300MaBP up to 280C at 277MaBP. Biodiversity is not altered substantially. Then, for about 20Ma the sea temperature falls sharply by over 100C. At the end of the Permian is the last mass extinction in the Paleozoic (8). Then about 90% of marine genera die which is about 5055% of all genera. There are several hypotheses about the causes that gave rise to mass extinction in the Paleozoic: 1. Glaciation of Gondwana, reduction in the sea level and alternating periods of significant warming, glaciations and drought. It is confirmed by studying sediments in the Permian; 2. Formation of Pangaea accompanied by reducing the area of shallow seas. This is the reason for intensifying the struggle between species for living space; 3. Volcanic activity that leads to the invasion of large amount of sulfates in the atmosphere. Large clouds of silica dust decreases the solar radiation, and hence the average temperature on the Earth. There are outlined the following major cycles of extinction: 20Ma (39%), 319Ma (34%), 80Ma (24%) and 160Ma (22%). At the end of the Middle Paleozoic, the composition of the atmosphere changes significantly (Fig.22.12). Carb Pm Dev S Ord Cmb i 4000 40 CO2 30 2000 20 1000 10 0 O 2, % CO 2, ppm O2 3000 0 200 250 300 350 400 450 500 550 Age, MaBP Fig.22.12 Composition of the atmosphere in the Paleozoic Devonian forests consume substantial quantities of CO2 and emit oxygen. This process continued until The Early Permian. Oxygen reaches maximum values on the atmosphere- 3236% and the level of CO2 is close to modern values. Towards the end of the Paleozoic concentration of oxygen in the atmosphere decreases and CO2 increases slightly. Probable cause is the mass extinction, which affects the flora in the Paleozoic. Analysis of geophysical fields in the Paleozoic outlines some cycles. On Fig.22.13 are shown cycles with relative amplitude greater than 20%. There are outlined 5 major cycles in the evolution of Paleozoic: 20Ma, 40Ma, 64-80Ma, 107Ma and 319Ma. With most significant amplitudes are the temperature variation of sea water at (80Ma) and the mass extinctions at (20Ma and 319Ma). Then follow impact events with large asteroids (64Ma and 107Ma) and change in the global sea level (20Ma, 40Ma and 319Ma). 50 Relative amplitude, % 40 30 20 Sea level Ocean temperature Asteroids 10 + EMF polarity Extinction 0 10 100 1000 Period, Ma Fig.22.13 Cycles of geophysical fields in the Paleozoic The changes in the Earth’s rotation and in the polarity of the Earth magnetic field are with smaller relative amplitude. They are distributed at all time intervals. The cycle of 319Ma probably represents the Paleozoic (with total length of 300Ma) as an integral process in the evolution of the Earth.