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ScienceWeek
EARTH SCIENCES: ON THE ASSEMBLY OF SUPERCONTINENTS
The following points are made by J.B. Murphy and R.D. Nance (American Scientist 2004 92:324):
1) Geological data indicate with considerable certainty that between 300 and 200 million years ago all of the Earth's continental land masses were assembled into a supercontinent, which has been named "Pangea" (meaning "all lands"), surrounded by a superocean known as "Panthalassa" (meaning "all seas"). Indeed, the evolution of the Earth over the past 200 million years has clearly been dominated by the breakup of Pangea and the resulting formation of new oceans, such as the Atlantic, between the dispersing continental fragments.
2) For the past 20 years, however, evidence has been amassing that Pangea itself was only the latest in a series of supercontinents that have assembled and dispersed over 3 billion years. Although the mechanisms responsible are controversial, many geoscientists agree that repeated cycles of supercontinent amalgamation and dispersal have not just taken place, but also have had a profound effect on the evolution of the Earth's crust, atmosphere, climate and life over billions of years.
3) The amalgamation of Pangea appears to have been preceded by that of Pannotia about 650 to 550 million years ago, and, although its configuration is debated, there is general acceptance of the existence of the supercontinent Rodinia about one billion years ago. Another supercontinent, variously termed Nuna or Columbia, is thought to have amalgamated about 1.8 billion years ago. Two others, Kenorland and Ur, are believed to have assembled 2.5 and 3.0 billion years ago, respectively.
4) Since the expression "the past is the key to the present" is one of the basic tenets of geology, a strong probability exists that another supercontinent will form in the future. But how would such a supercontinent form, and what would it look like? There are two competing models: One has the continents drift apart and back together again like an accordion; the other proposes that the continents break apart and march all the way around the Earth to reunite on the other side.
5) Plate tectonics is the theory that revolutionized our understanding of the Earth by providing a comprehensive explanation of the forces that shape it. According to the theory of plate tectonics, the Earth has a rigid outer layer, known as the "lithosphere", which is generally 100 to 150 kilometers thick and rides atop a hot plastic layer in the Earth's mantle called the "asthenosphere". Like a cracked eggshell, the lithosphere is broken up into a mosaic of about 20 slab-like fragments, or plates, which move relative to one another at rates that are typically less than 10 centimeters per year. As they move, the plates interact along their boundaries, where they may converge and collide, diverge and separate, or slide past one another. Over millions of years, such interactions have caused mountains to rise where plates collide and continents to break apart where plates diverge.(1-5)
References (abridged):
1. Gurnis, M., 1988, Large-scale mantle convection and the aggregation and dispersal of supercontinents: Nature 322:695-699
2. Hoffman, P. F. 1991. Did the breakout of Laurentia turn Gondwana inside out? Science 252:1409-1412
3. Murphy, J. B., and R. D. Nance. 2003. Do supercontinents introvert or extrovert?: Sm-Nd isotopic evidence: Geology 31:873-876
4. Murphy, J. B., and R. D. Nance. 1992. Supercontinents and the origin of mountain belts. Scientific American 266(4):84-91
5. Nance, R. D., T. R. Worsley and J. B. Moody. 1988. The supercontinent cycle, Scientific American, 259(1):72-79
American Scientist http://www.americanscientist.org
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EARTH SCIENCE: CONTINENTAL BREAK-UP AND "SNOWBALL EARTH"
The following points are made by Y. Donnadieu et al (Nature 2004 428:303):
1) Geological and palaeomagnetic studies indicate that ice sheets may have reached the Equator at the end of the Proterozoic eon, 800 to 550 million years ago(1,2), leading to the suggestion of a fully ice-covered "snowball Earth"(3,4). Climate model simulations indicate that such a snowball state for the Earth depends on anomalously low atmospheric carbon dioxide concentrations(5), in addition to the Sun being 6 per cent fainter than it is today. However, the mechanisms producing such low carbon dioxide concentrations remain controversial.
2) Long-term (10^(6) yr) evolution of the partial pressure of atmospheric CO2 (pCO2) is controlled by the relative importance of degassing through volcanic and mid-ocean-ridge processes and the consumption of CO2 through continental silicate weathering. Any long-term decrease in atmospheric CO2 can be induced either by a decrease in solid Earth degassing rate, or by an increase in the weathering of continental surfaces. Little is known about the evolution of the degassing rate over the Neoproterozoic era, so linking the global Neoproterozoic cold climate to low degassing rate would be extremely speculative.
3) On the other hand, the sink of CO2 via continental silicate weathering depends on a variety of parameters, such as the air temperature, continental runoff, vegetation, and mechanical weathering. The long-term evolution of some of these parameters can be evaluated within the particular context of the Neoproterozoic.
4) The tectonic environment is indeed characterized by the dispersal of continental plates through the break-up of the Rodinia supercontinent between 800 and 700 Myr ago. This break-up may have had two major effects on the sink of CO2 through continental silicate weathering. First, the break-up of Rodinia is heralded by, and accompanied by, the eruption of large basaltic provinces, resulting in an increase of the weatherability of the continental surface and consumption of atmospheric CO2 on the 10^(6)-yr timescale. More importantly, the break-up of a supercontinent into several smaller plates will result in an increase of precipitation and runoff over the continental masses, owing to an increase in the sources of moisture along continental borders. This process can boost continental silicate weathering and consume atmospheric CO2. Hence, precise quantitative evaluation of changes in atmospheric pCO2 due to palaeogeographic changes requires a sophisticated approach in which the weathering rates are spatially resolved.
5) In summary: The authors assess the effect of the palaeogeographic changes preceding the Sturtian glacial period, 750 million years ago, on the long-term evolution of atmospheric carbon dioxide levels using the coupled climate-geochemical model GEOCLIM. In their simulation, the continental break-up of Rodinia leads to an increase in runoff and hence consumption of carbon dioxide through continental weathering that decreases atmospheric carbon dioxide concentrations by 1320 ppm. This indicates that tectonic changes could have triggered a progressive transition from a "greenhouse" to an "icehouse" climate during the Neoproterozoic era. When these results are combined with the concomitant weathering effect of the voluminous basaltic traps erupted throughout the break-up of Rodinia, the simulation results in a snowball glaciation.
References (abridged):
1. Evans, D. Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox. Am. J. Sci. 300, 347-433 (2000)
2. Sohl, L. E., Christie-Blick, N. & Kent, D. V. Paleomagnetic polarity reversals in Marinoan (ca 600 Ma) glacial deposits of Australia: Implications for the duration of low-latitude glaciation in Neoproterozoic time. Geol. Soc. Am. Bull. 111, 1120-1139 (1999)
3. Kirschvink, J. L. in The Proterozoic Biosphere: A Multidisciplinary Study (eds Schopf, J. W. & Klein, C. C.) 51-52 (Cambridge Univ. Press, Cambridge, 1992)
4. Hoffman, P. F. & Schrag, D. P. The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14, 129-155 (2002)
5. Hyde, W. T., Crowley, T. J., Baum, S. K. & Peltier, R. W. Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model. Nature 405, 425-429 (2000)
Nature http://www.nature.com/nature
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GEOLOGY: ON THE SUPERCONTINENT RODINIA
The following points are made by Trond H. Torsvik (Science 2003 300:1379):
1) Earth's surface is divided into a dozen tectonic plates that either drift apart, creating new oceanic crust, or collide, generating mountain belts such as the Himalayas. In the past, continents have coalesced into single supercontinents, which had dramatic effects on both surface and deep Earth processes. But while much is known about Pangaea (the most recent supercontinent on Earth), the earlier Rodinia supercontinent remains shrouded in mystery.
2) Pangaea started to form approximately 330 million years ago and reached its maximum extent in the Late Permian (250 million years ago). Not all continents coalesced simultaneously; some were added along Pangaea's margins just as others rifted off. The supercontinent changed the distribution of land and sea areas and brought about unusual climatic and biological conditions. Increased mantle temperatures and continental bulging in the interior of Pangaea may also have occurred as a result of long-term shielding of large parts of the underlying mantle. The ultimate breakup of Pangaea approximately 175 million years ago was preceded by and associated with widespread magmatic activity.
3) There is some evidence that supercontinents have formed periodically during Earth's history. The existence of a supercontinent in the Precambrian (before 544 million years ago) was proposed in the 1970s, when many geologists noted a large number of mountain belts with similar ages (1300 to 1000 million years old) that are today located on different continents. In the early 1990s, the name Rodinia was adopted for this supercontinent.
4) Most Rodinia models have sought to match the 1300- to 1000-million-year-old mountain belts. In these models, Laurentia forms the core of the supercontinent, with Australia-East Antarctica situated along its present-day western margin and Baltica-Amazonia along the eastern margin.
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