Was there indeed a time when the entire earth was covered, up to the equator, in a blanket of ice? Geological evidence suggests that it may be
By: Michael Bate and Dov Avigad, Galileo
The Snowball Earth theory, and in Hebrew the Earth as a snowball, holds that there was a time when the entire Earth was covered with an ice sheet, the thickness of which reached 1 km in the oceans. This event occurred at a time when the animal world was in the initial stages of its evolution. The theory was formulated in its current form in a series of articles by researchers Daniel Schrag and Paul Hoffman, and is intended to explain a set of phenomena that indicate glacial activity that occurred 540 million to one billion years ago - a period called the Neoproterozoic.
According to this theory, the Snowball Earth period consisted of a chain of events that included at least four ice ages, each of which probably lasted between 5 and 10 million years and involved the average temperature dropping to 50 degrees Celsius below zero. At the end of each of these ice ages, in a sharp transition, a short warming event began, on the order of tens of thousands of years, during which the average temperature of the Earth rose to 50 degrees above zero.
Unlike earthquakes, tsunamis and other natural phenomena that have occurred in the past and continue to occur in the present, the Snowball Earth period was a unique event, which did not repeat itself. This uniqueness made it difficult for the scientific community to accept the theory. However, the examination of the theory produced corroborating evidence, which led to a growing recognition of its correctness. In this article we would like to present the main points of the theory to the readers, and describe the manifestations of the event in our region.
The development of the idea
Is it possible that the Earth, also called the blue planet because of the blue color of the oceans, became a white star after being completely covered by a thick ice cover for millions of years? Does such an event, if it did occur, indicate the possible limits of climatic changes during the 4.5 billion year history of the Earth?
The beginning of the Snowball Earth theory is in Brian Harland's discovery in 1964. Harland found that rocks that were transported and deposited by glaciers in the oceans during the Neoproterozoic period appear on all continents. Harland relied on paleomagnetic measurements (see Box 2) to prove that these rocks were at that time in the tropics, around the equator.
The Russian geophysicist Mikhail Budyko studied the phenomenon and tested theoretical climate models of energy balances, in an attempt to understand the intensity of the event and the mechanism that caused it. He found that the increase in the ratio between the light reflected from the Earth's surface and the sunlight hitting it (a ratio known as albedo), due to being covered by glaciers, may cause instability in the Earth's climate. White snow and ice have a high reflection (about 0.8 of the total radiation, meaning about 80%), and therefore snow/ice surfaces cause a lot of light reflection from the Earth. On the other hand, sea water has a low return (about 0.1), and therefore the oceans are a body that absorbs energy. Continents that are not covered by snow/ice are characterized by medium return. The greater the reflection of light from the earth's surface, the lower the average temperature on the earth's surface. Boydiko showed that the closer the area covered by glaciers gets to the equator, the greater the return of light/energy from the Earth, and this is because the ratio of the areas between ice and ocean and continents that are not covered by ice at every degree of latitude - increases. When the glaciers extend to a critical latitude, which is at approximately 30 degrees, the return of energy will be so great that temperatures will drop. Under these conditions, an ice sheet can cover the entire planet. It is important to note that the oceans freeze to an average depth of only one kilometer because of the heat emitted from the Earth's interior, which prevents their complete freezing.
According to paleomagnetist Joseph Kirschvink, who coined the term Snowball Earth in 1992, it was the movement of the Earth's plates that led to the concentration of the continents around the equator in the Neoproterozoic. The movement of the plates also involved volcanic activity, during which various gases were emitted into the atmosphere, including the main greenhouse gas on Earth - carbon dioxide. The assumption is that the intensity of the sun's radiation 600-700 million years ago was six percent lower than today. This is because the density of the sun increases linearly as a result of fusion processes and the creation of helium, and therefore, in 4 billion years, the temperature increased by 25%. Since there is evidence of snow melting during the warming periods, the concentration of carbon dioxide in the atmosphere during this period had to be 350 times greater than its concentration in today's atmosphere, and this in order to eliminate the effect of the high albedo (radiation reflection) during this period.
Reinforcement for the claim that the continents in this period were concentrated around the equator was obtained from the research of Hoffman and Schrag, who claimed that the breakup of the Rodinia supercontinent about 870 million years ago created about ten smaller continents that were located around the equator, between latitudes 30 south and 30 north.
However, not everyone agrees with this explanation. George Williams (Williams) and his colleagues claimed (1997), that the appearance of glaciers at the equator and the drop in temperatures resulted from a change in the angle of inclination of the Earth's rotation axis in relation to its orbit around the Sun. The meaning of the change is that compared to today, the radiation at the poles was stronger and in the lower latitudes - weaker. According to this argument, glaciers existed only at the equator, not at the poles. This explanation therefore rules out the Snowball Earth phenomenon.
Based on the age of glacial deposits, and based on paleomagnetic measurements (box 2) which indicate that these glacial deposits were concentrated in low latitudes (0-10 degrees), and that the distribution of most of the glacial deposits from the Neoproterozoic does not pass north and south of latitude 60, claims David Evans that there were four ice ages: 600-570 million years (ms) before the present; 600-630 ms before the present; 750-700 ms before the present; and 770-740 ms before the present.
These periods are of great importance in terms of the development of life on Earth. One of the fascinating key questions in the evolution of animals is the cause of the seemingly sudden and rapid transition between the algae and unicellular organisms that dominated the animal world for about two billion years to about 11 families of relatively developed unicellular and multicellular organisms (see Figure 3). This change occurred at the transition from the Neoproterozoic to the Cambrian period. Brian Harland and Martin Rudwick (1964) claimed (XNUMX) that this development is also indirectly related to the glacial event we are dealing with, and more on that later.
Characteristics that distinguish the Neoproterozoic period
We can name several phenomena unique to the Neoproterozoic period, related to Snowball Earth. First, the fission of the Rodinia supercontinent and the dispersal of the ten continents that were formed at low latitudes, and their location near the equator. This location of the continents was a critical condition for the development of the ice ages, as we will argue later. Second, sharp changes in the concentration of carbon dioxide in the atmosphere: carbon dioxide is a gas released into the atmosphere during volcanic activity. This gas is also the main greenhouse gas on Earth. As the concentration of carbon dioxide in the atmosphere increases, the emission of energy from the earth decreases, and the average temperature increases. If it weren't for balancing processes - fixation of CO2 from the atmosphere in the process of photosynthesis and deposition of carbonates, mainly chalk and dolomite - very high temperatures would prevail on Earth, similar to the planet Venus. During an ice age, when the rains are few, the processes of chemical weathering are reduced and following them - the deposition of carbonates. Therefore, there is an increase in the concentration of carbon dioxide in the atmosphere, which leads to the rise in temperatures and the melting of glaciers. Indeed, in the transition from each of the ice ages to the warming period (the interglacial period) that followed, an increase in the concentration of CO2 in the atmosphere occurred, up to concentrations 350 times higher than its concentration today.
Rocks representing the glacial period (the term "glacial period" refers to all the ice periods) in the Neoproterozoic have been described mainly from Namibia, the western United States, Australia and Mauritania, and they are divided into two main types: glacial sedimentary rocks, which represent the glacial periods, and carbonate cover rocks, which represent the warming periods. The latter are exposed in a sharp pass above the glacial rocks. The glacial sedimentary rocks are conglomerates whose composition of pebbles is varied, and they are formed at the base of the glacial drift on land or near it, or as a result of the slides of the sediments of the melted glacier on slopes. Conglomerates are also formed when the glacier melts in the ocean, and the rock fragments that drift at the base of the glacier sink into a clayey medium. The "trailing" rock fragments at the base of the glacier are characterized by a groove, which was created due to the erosion of the pebble against the bedrock under the glacier. The carbonate cap rocks include well-bedded dolomites or chalks containing stromatolites. (Stromatolites are fossilized algae, their seven growths form a thin layered structure.) The thickness of the layers in the glacial sediment rocks is usually a few meters, however, the rocks represent periods of up to ten million years. In contrast, the thickness of the carbonate cover rocks may reach tens of meters, although they represent relatively short periods of warming, of only tens of thousands of years.
Lying iron deposits
Iron-enriched and overlying sedimentary rocks appear mainly in periods earlier than the Neoproterozoic period - in the Archaic, about 3.5 to 2 billion years ago. The accepted explanation for the enrichment in iron is that in this early period the oceans and atmosphere contained only little oxygen, and the iron appeared as divalent iron ions (recycled iron) dissolved in the ocean water. With the passage of time, as a result of biological activity, the concentration of oxygen in the atmosphere and ocean waters increased. The divalent iron oxidized, and sank to the bottom of the oceans as layers rich in trivalent iron. Such bedded iron deposits were also found in the Neoproterozoic, and they appear mainly in Namibia, the western United States, Australia and Mauritania.
Snowball Earth as an explanation for these phenomena
How do these phenomena fit into an overall explanation that supports the theory that during the Neoproterozoic period the entire Earth was covered by an ice sheet? The strength of the theory of the earth as a snowball is that it offers explanations for most of the unique geological features of that period.
As Evans pointed out, as well as Schrag and Hoffman, the continents at this time were centered around the equator. Hence, at that time, a tropical climate prevailed in most of the continents, characterized among other things by a large amount of precipitation. The large amounts of precipitation caused rapid weathering processes, during which the carbon dioxide was consumed and its concentration in the atmosphere decreased. As mentioned, carbon dioxide is the main greenhouse gas in the Earth's atmosphere, and the decrease in its concentration in the atmosphere, therefore, increased the albedo, resulted in a large return of energy from the face of the Earth and caused cooling and the beginning of baldness.
During the ice ages, when the oceans froze for millions of years and were, in effect, separated from the atmosphere, recirculating conditions were created in the oceans, and the iron was melted. However, with the transition to a period of warming, in which the mixing of ocean water became possible and a transition to oxidizing conditions, the dissolved iron oxidized and sank as layers rich in iron. This process explains the presence of iron-rich overlying sedimentary rocks in the Neoproterozoic.
The sharp transition between the glacial sedimentary rocks and the carbonate cover rocks represents the rapid transition from an ice age to a warming period. A thick section of carbonate rocks, representing a relatively short period, means a rapid development of algae and bacteria in the oceans and a rapid supply of weathering products. This increase originates from a high concentration of CO2 in the atmosphere, which caused acid rain (carbon dioxide in water creates carbonic acid), which accelerated the chemical weathering processes. The mixing of the ocean water and the rapid weathering increased the concentration of nutrients in the sea water, and caused an increase in the fertility of the water and increased activity of algae and bacteria, which almost disappeared during the ice ages.
Another evidence of what happened during these periods is provided by the composition of the isotopes of oxygen and carbon in the rocks above and below the glacial sedimentary rocks (see box 2). The section of the carbonate rocks below the glacial rocks is characterized by a 1.5% enrichment of carbon-13 compared to the volcanic rocks. This fact shows that about half of the carbon was subtracted from the oceans as organic material that did not have enough time to oxidize and decompose, because it was quickly buried by the weathering products. In the carbonate rocks, which are above the glacial sedimentary rocks, the ratios change with the increase in the section, that is, from an older age to a younger one, from a deficit in carbon-13 (5 to 6 thousandths (negative) to an enrichment of carbon-13 in relation to carbon-12 (around 5 thousandths positive ), as a result of the rapid release of buried organic matter and the renewal of biological activity. Although such changes in the ratio of carbon-13 to carbon-12 are not unique to a baldness event, the intensity of the changes and the rapid transition are unusual, and the Snowball Earth theory explains them well.
How might the Snowball Earth period have affected the flowering of multicellular species at the end of the Neoproterozoic and the Cambrian, between 600 and 500 million years before our time? During the ice ages, in fact, the activity of life on the surface of the earth ceased, except in warm areas in the oceans, close to centers of volcanic activity. Life activity is possible in areas where the temperatures are extremely high, up to 400 degrees Celsius, as found in present-day colonies discovered in smokestacks on the seabed west of California. The difficult living conditions typical of these habitats, as well as their isolation from each other, created strong selection pressures, which in turn led to increased diversity and an increase in the variety of species. After the ice ages, between 575-555 million years before our time, more developed multicellular species appeared for the first time in global distribution, including sponges, molluscs, arthropods and more (see Figure 3).
The strength of the theory of the Earth as a snowball is that it offers explanations for most of the unique geological features of that period.
6.6.2006
And in our region - the East African orogen
In our area, the Neoproterozoic is represented in the rocks of the East African Orogen. An orogen is a mountain chain that develops in subduction zones (that is, areas where one tectonic plate is subducted under another). When the Rodinia supercontinent split into several continents, about a billion years ago, a rift was created in our region where the Mozambican Ocean developed, the size of which was similar to the size of the Pacific Ocean today. The Mozambique Ocean existed from about 850 to 630 million years before the present, and at its margin the subduction zone of the East African Orogen developed. This orogen was of the order of magnitude of the Alpine or Cordillera system, and the mountains in it rose to heights similar to those of the Himalayas.
In the northern part of the East African orogen is an area consisting of Neoproterozoic rocks, known as the Arabian-Nubian massif (block) (Figure 4). This group includes Arabia, Jordan, southern Israel, Sinai and areas in Egypt, Sudan, Ethiopia and Eritrea. The fiber consists entirely of a primary crust, created by a magma that came directly from the Earth's interior. Its northern, exposed end is the Neoproterozoic rocks of Eilat and Timana.
The East African Orogen made a very important contribution to the formation of the continental crust on the surface of the Earth. This orogen extended from Israel in the north to South Africa and Mozambique, and probably to Madagascar, India and Antarctica. The north of the orogen differs from its south mainly in the structure of the primary crust of the Arabian-Nubian massif.
Understanding the development and schedule of the East African orogen is a necessary condition for integrating the elements that may indicate the existence of global glaciation in our region. Most of the areas where the Snowball Earth theory has been studied so far have been continents where orogenic activity of creating mountain chains did not occur during the Neoproterozoic period; On the other hand, in our region it is an orogen, an area of folding, where it is more difficult to document geological evidence. In recent years, Israeli researchers and their colleagues have conducted many studies in southern Israel, Sinai, Eritrea, and Ethiopia, which contributed to the refinement of the general scheme of the development of the orogen proposed by Robert Stern in 1994. Thus, for example, the age of the Mozambican oceanic crust was determined to be 810 million years, and the end of a process The folding of the orogen is determined to be 630 million years before the present. Determining the periods of removal (erosion, physical weathering) at the end of the orogen is extremely important, since it is possible that glacial activity is responsible for the erosion. Two main erosion periods have been determined: the first and main one 600 million years ago, and is represented by conglomerates and sandstones; and the second, 532 million years ago, which created the removal surface (Peneplain) on the roof of the Neoproterozoic rocks.
The Snowball Earth and the East African Orogen
If the ice ages at the end of the Neoproterozoic were indeed a worldwide phenomenon, evidence for this should also be found in the East African Orogen.
In the late 1972s and early 800s, Bate (1) conducted geological mapping in Tigra in northern Ethiopia. The basement rocks from the Neoproterozoic were divided into two groups: the lower one, which is about 5 million years old, consists of metamorphic volcanic rocks6 thousands of meters thick, and indicates the subduction zone of the Mozambican ocean margin. The upper group consists of units of metamorphic sedimentary rocks, bedded dolomites and stromatolitic chalks, which were deposited in the Mozambican Ocean and folded during the development of the orogen (Figures 7, XNUMX, XNUMX). In the roof of the upper group, above the dolomites, conglomerates were found which are glacial sediments.
1Metamorphic rocks are rocks whose structure has changed due to heat, pressure or both.
In preliminary works carried out by Nathan Miller (Miller) and his colleagues (2003) and Bait and his colleagues (2003) it was found that there is a high probability that the findings indicate the occurrence of one or more glacial events. The carbonate rocks contain the typical structures of the carbonate cover rocks, and are also characterized by the appropriate ratio between carbon-13 and carbon-12. Despite the characteristics of the carbonate cover rocks so far, no clear evidence has been found that there are sedimentary rocks of glacial origin beneath them. As of today, we are faced with more questions than answers, but the rock types characteristic of the glacial period or, according to the theory discussed here, the period of the Earth as a snowball appear in the area, and these justify the continuation of research in the area.
Bentor and Garfunkel have already raised the possibility that unique conglomerates from this period, extending from the south of Israel to the interior of the Arabian Peninsula and the Penna Plain (a flat surface, extending over distances of thousands of kilometers, from Israel to Ethiopia in the south and Alge Yar in the west), were formed by weathering processes associated with glaciers. Evidence of chemical weathering of the upper part of the surface due to acid rain that washed the area during the interglacial warming periods was also found.
Contemporary studiesIn order to further deepen the insights regarding the Snowball Earth theory, several researchers from the Israel Geological Survey and the Hebrew University teamed up with scientists from the University of Texas at Dallas and Ethiopian geologists from Tigrah. The extensive experience and knowledge acquired by the researchers gave them the necessary tools to examine the Snowball Earth theory in northern Ethiopia.
The preliminary results show that in the work area in Tigra in Ethiopia there are granite rocks that penetrated sequences of metamorphic sedimentary rocks (Figure 6), and that the age of the granite rocks is about 610 million years. It was also found that the age of the glacial deposits and sands in East and West Tigra is older than 700 million years. This suggests the possibility that the glacial event in Tigra belongs to the first event and perhaps the second of the four events we mentioned.
Stromatolites (fossilized algae mats, Fig. 7) from the Neoproterozoic were found in both the dolomite rocks and the limestone rocks at the site. In the lower stromatolitic chalk, negative values were found in the ratio of carbon-13 to carbon-12 (-4 to -1 thousandths) and positive values (+7 to +4 thousandths) in the upper black chalk. The difference in the ratio of the amounts between the isotopes in the lower and upper part of the carbonate section characterizes carbonate cover rocks that are above glacial deposits.
In conclusion, the theory of the Earth as a snowball is still controversial among scientists, but there is no dispute that in the period in question there were several balding events, during which an ice sheet was formed that extended over large areas and at low latitudes. The main question that remains open is, did the ice cover the entire surface of the earth and the oceans froze to a depth of about a kilometer on average for periods of millions of years, or during these periods did areas remain without ice cover. Evidence from the rock sequence in southern Israel and Ethiopia, which represents the period in question, may contribute to the clarification of this question.
ThanksWe thank the Israel-United States Binational Foundation for funding the research. Our American partners in this research are Prof. Robert Stern and Dr. Nathan Miller from the University of Texas at Dallas. We also thank Dr. Judith Harlevan and Prof. Ruth Bate-Marom, who invested a lot of effort in editing the article.
The science of geology and climate change
The history of the development of the earth, from its formation as part of the solar system until today, is the science of geology - the theory of the earth. Geology is based on the study of the sequence of rocks (stratigraphy) and the properties of rocks (petrography). It uses chemical (geochemistry), physical (geophysics) and biological methods (paleontology - the animal world as fossilized and preserved in rocks).
The Earth, unlike most of the known planets, is an active planet. Two main sources of energy drive the processes taking place in it. One is nuclear fission that occurs inside the Earth, in the crust and mantle, and creates high thermal energy. This energy is responsible for the magmatic-volcanic activity and the creation of a new crust in the mid-oceanic ridges, and is also the engine for the movement of the plates and the migration of the continents. The earth's crust is made up of "plates" that migrate, that is, change their position, during the history of the earth.
The second source of energy is the sun, which activates processes that occur before the earth: differential heating of the oceans, the atmosphere and the continents.
There is much evidence of climatic changes that took place during the development of the Earth in the last 700 million years, and especially in periods younger than 20,000 years. The duration of the ice ages did not exceed more than tens of thousands of years each, during which there were drops of up to 150 meters in the ocean levels. Evidence of drops in levels of these magnitudes also exist on the coasts of Israel. However, the most interesting fact is that for most of the known geological history of the Earth, ice did not cover the poles, and sea levels were higher than today. For example, about 100 million years ago, a warm and shallow sea covered large parts of the continents, as evidenced by the limestone and dolomite rocks, which are also common in our country. During this period, the level of the ocean relative to the continents was affected by the addition of water resulting from the melting of glaciers, on the one hand, and by the uplift of the continents due to the release of the load of the glaciers, on the other hand.
What is the connection between continental drift and climatic changes? The state of the plates and continents determines the regime of currents in the oceans, and these affect the Earth's climate. In the regime of an equatorial current, the kind that dominated the Earth about 100 million years ago, during the Cretaceous period, when the Tethys Ocean surrounded the globe in the equatorial region, the Earth was warmer, the temperatures on its surface were relatively uniform, and the continents were more rainy. On the other hand, in a regime of oceans with currents around the poles, like 25 million years ago, when the Tethys Ocean closed and the poles were isolated, the Earth was cold, the temperature drop from the poles to the equator was large, and glaciers began to appear at the poles.
The movement of the plates is, therefore, the main cause of the long-term climatic changes during the history of the earth. A set of factors affecting the short-term changes, including the increase in the concentration of greenhouse gases, such as carbon dioxide, methane and water vapor in the atmosphere; Volcanic eruptions on a large scale, causing a "nuclear winter"; collision of asteroids with the Earth, as in the event of the extinction of the dinosaurs; Or slight changes in the angle of the Earth's axis in relation to the Sun, which cause cyclical climatic changes. It was the climate researcher Milotin Milankovic who proposed that slight and cyclical changes in the Earth's orbit around the sun cause cycles of ice ages: "long" cycles that last about 100,000 years, and "short" cycles that last 20,000-40,000 years.
"Indirect" evidence of climatic changes
Beyond the "direct" evidence of climatic changes throughout geological history - the cross section of the rocks - there are also "indirect" evidences, the importance of which increases the earlier the change event. It is easier to follow the development and retreat of the glaciers from 20,000 years before our time to the present, than after the glaciers in the Carboniferous period (about 300 million years before our time) and the Ordovician (about 400 million years before our time); And it is even more difficult to trace the ice ages of the end of the Precambrian-Neoproterozoic (about a billion years before our time).
We must, therefore, use indirect, geochemical methods, which rely on the ratio of quantities between stable isotopes, and especially on the ratio of quantities between the heavy isotope oxygen-18 and the light oxygen-16. The light isotope evaporates more easily than the heavy isotope, so the rainwater is enriched with the light oxygen. During interglacial periods, most of the rainwater returns as runoff to the oceans, and the isotopic ratio in the oceans remains constant. On the other hand, during the ice ages some of these sediments are trapped in glaciers, and the oceans become enriched in the heavy isotope. For every 10-meter drop in sea level, which results from water draining from the oceans and trapping them in glaciers, the heavy isotope oxygen-18 is enriched by 0.1 parts per million relative to light. The quantity ratios between the two isotopes of oxygen allow us, therefore, to follow the transition between glacial periods and interglacial periods.
The ratio of carbon-13 to carbon-12 indicates the burial of organic matter, and indirectly the decrease in the concentration of carbon dioxide in the atmosphere. Plants that produce organic matter by photosynthesis tend to use the light carbon. Therefore, the ratio of carbon-13 to carbon-12 in living things is lower than in the primary carbon dioxide, which is the product of volcanic activity. The initial ratio is 1% carbon-13 and 99% carbon-12. If the organic matter is buried, the ocean water becomes "heavy", as do the carbonates that sink from this water. Conversely, if the organic matter is oxidized, the water retains its initial value. In the case of oxidation of a large amount of buried organic material, the ratio can drop to negative values ("negative" means enrichment in the light isotope and "positive" - enrichment in the heavy isotope). Oxidation of a large amount of buried organic matter involves a decrease in the concentration of oxygen in the atmosphere.
In order to analyze the essence of climate change, it is important to examine the geographical location of the plates, and the continents within them, at the time of the event. The position is determined by the paleomagnetic method: when a magnetic mineral cools below the Curie point (500 degrees Celsius), its magnetic directions are "frozen" according to the magnetic field at that time. Since the direction of the Earth's magnetic field changes over time, and since we know what this direction was at different points in time, the direction of the magnetic field, as preserved in the rocks that solidified at a certain point in time, allows us to determine what the position of the tectonic plate was at that time.
for further reading:
www-eps.harvard.edu
Beyth, M., 1972, The Geology of Central-Western Tigre: University of Bonn, 159 pp.
Beyth, M., Avigad, D., Wetzel, HU, Matthews., A. and Berhe. SM, 2003, Crustal Exhumation and Indications for Snowball Earth in the East African Orogen: North Ethiopia and East Eritrea: Precambrian Research 123:187-201.
Evans, DAD, 2000, Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox: American Journal of Science 300;347-433.
Harland, WB and Rudwick, MJS, 1964, The great Infra-Cambrian ice age: Sci. Am., 211:28-36.
Hoffman, PF and Schrag, DP, 2000, Snowball Earth: Scientific America, pp. 50-57.
Stern, RJ, 1994, Arc assembly and continental collision in the Neoproterozoic East African Orogen: Implications for the consolidation of Gondwanaland: Ann. Rev. Earth Planet. Sci. 22: 319-351.
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<<Biographies>>
Dr. Michael Bate is a researcher at the Israel Geological Survey.
Dov Avigad is a professor at the Institute of Earth Sciences, the Hebrew University of Jerusalem.
