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ScienceWeek
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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GEOPHYSICS: ON THE SNOWBALL EARTH HYPOTHESIS
The following points are made by David Archer (Science 2003 302:791):
1) The geological record of Earth's climate resembles Arthur Conan Doyle's curious incident of the dog that didn't bark in the night (1). The Sun grows hotter with time, yet the temperature at Earth's surface does not leave the narrow constraints of the melting and boiling points of water -- at least not for long. But it seems that the dog did whimper during a period known as Snowball Earth, when much of Earth's surface appears to have been frozen. This episode occurred in the Neoproterozoic (1000 to 540 million years ago), just before complex fossils emerged in the geological record (2).
2) It is not difficult to explain the overall stability of the climate record. The problem is explaining why the dog barked at all. Ridgwell et al (3) argue that calcium carbonate (CaCO3) precipitation played a key role. The present-day carbon cycle is stabilized by plankton that precipitate CaCO3 in the open ocean. These organisms, coccolithophoridae and foraminifera, had not yet evolved in the Snowball days, and hence most CaCO3 deposition in the ocean took place in coastal waters. This difference may have been crucial in the events that drove Earth into the Snowball state.
3) The overall stability of Earth's climate is generally attributed to a balance between degassing of CO2 from deep within Earth, and consumption of CO2 by weathering reactions at Earth's surface. Urey (4) wrote the reaction as
metamorphosis CaSiO3 + CO2 <--> CaCO3 + SiO2 weathering
where the left-hand side is favored at the high temperatures of Earth's interior and the right-hand side is favored in the cool, wet conditions at Earth's surface. Walker et al. (5) proposed that the rate of weathering should depend on temperature and the intensity of the hydrological cycle, which in turn depend on the partial pressure of CO2 in the atmosphere (pCO2).The latter adjusts such that the CO2 sources and sinks balance. If the Sun warms up, weathering accelerates, consuming more CO2 until Earth's surface cools back down. The time scale of the Walker et al thermostat is approximately 500,000 years. This thermostat appears to have broken down during Snowball Earth. The Snowball Earth hypothesis is based on geological evidence of multiple glaciations at sea level in low latitudes. The glaciation deposits are accompanied by "banded iron formations", which appear to mark the oxidation of an iron-rich anoxic ocean. They are overlain by caps of mineralogically peculiar CaCO3 deposits that resemble abiotic precipitates from a highly supersaturated ocean.
4) The leading explanation for the Snowball is a runaway ice-albedo feedback. When ice sheets reach some critical latitude, they reflect so much solar energy back into space that the entire planet freezes over. In the frozen world, weathering stops. Hydrothermal iron becomes more abundant than weathering sulfur in the anoxic ocean, generating the first banded iron formations on Earth in 1000 million years (2). Ultimately, the Walker et al thermostat overcomes the ice albedo, because CO2 degassing from Earth's interior drives atmospheric pCO2 upward. The ice melts abruptly, transforming Earth into a hothouse, which the thermostat eventually ameliorates. In the process, weathering consumes large amounts of CO2, generating the cap carbonates.
References (abridged):
1. A. C. Doyle, Silver Blaze, in Strand Mag. (December 1892).
2. P. F. Hoffman, D. P. Schrag, Terra Nova 14, 29 (2002) [Abstract].
3. A. Ridgwell, M. J. Kennedy, K. Caldeira, Science 302, 859 (2003).
4. H. C. Urey, The Planets, Their Origin and Development (Yale Univ. Press, New Haven, CT, 1952).
5. J. C. G. Walker, P. B. Hays, J. F. Kasting, J. Geophys. Res. 86, 9776 (1981) [ADS].
Science http://www.sciencemag.org
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SNOWBALL VS. SLUSHBALL EARTH AND PALEOBIOLOGY
The Proterozoic era (also called the Algonkian) is the time-frame 2600 million years ago to 600 million years ago, and within that time-frame, the Neoproterozoic era is the time-frame approximately 800 million years ago to 600 million years ago.
The Earth's magnetic field has been dynamic throughout its history, and paleomagnetics is the study of the direction and intensity of the Earth's magnetic field throughout geological time. Paleomagnetic data suggest that the Earth was glaciated at low latitudes during the Paleoproterozoic Era (approximately 2.4 to 2.2 billion years ago), and also during the Neoproterozoic Era, although there is evidently some dispute concerning the Neoproterozoic data.
The "obliquity" of the Earth is the angle between the plane of the equator and the plane of the Earth's orbit, and it is quite important in determining climate belts around the Earth's sphere. Any departure of the axial tilt from its present posture will cause a change in the world climatic zones, and a consequent change in the seasonal swing of climate, and the climatic conditions associated with different obliquities have long been a matter of conjecture. Paleomagnetic data suggest that the Neoproterozoic glaciation involved ice sheets in low latitudes, and this has been explained by either a global glaciation ("snowball Earth hypothesis") or by a large obliquity during that era. Since the same era was a critical time in the evolution of multicellular animals on Earth, an important question is how early life survived under such environmental stress.
Recently, an alternative to the snowball Earth hypothesis has been proposed (W.T. Hyde et al [2000]; see related background material below), a so-called "slushball Earth hypothesis", the essential feature of which is that Neoproterozoic glaciation was not global but quasi-global, leaving large areas of open water at Equatorial latitudes.
The following points are made by D.P. Schrag and P.F. Hoffman (Nature 2001 409:306):
1) The authors state they do not believe the slushball Earth hypothesis is consistent with the most striking geological and paleomagnetic observations explained by the snowball Earth hypothesis. Paleomagnetic and geological data from Neoproterozoic glacial deposits indicate that glaciations were long-lived (lasting for millions of years) and locally associated with iron formation. The glacial deposits are covered by extraordinary sequences of carbonate sediments ("cap carbonates"). The authors suggest the snowball Earth hypothesis can explain these and other observations, whereas a semi-frozen (slushball) Earth does not.
2) The authors suggest that interest in a semi-frozen Earth explaining the geological observations stems from concern for the survival of *eukaryotic life in such extreme and extended glaciations. The critical feature is the survival of groups of photosynthetic algae that evolved before the glaciations. The survival of multicellular animals (metazoans) is less of a problem because such organisms (if they existed) could live wherever primary producers (photosynthetic or chemosynthetic biological systems) were still active. The authors suggest photosynthetic algae could survive a series of glaciations in refuges near volcanic islands, such as Iceland or Hawaii, or beneath thin equatorial ice cover. The authors suggest evolution might well be stimulated by this prolonged genetic isolation, and by perturbations of biogeochemical cycles during the *post-glacial ultragreenhouse climate. This is consistent not merely with the survival of eukaryotic life, but also with the coincident *evolutionary radiation of metazoa and other groups.
In a reply to the above criticism, the following points are made by W.T. Hyde et al ((Nature 2001 409:306):
1) The authors state they are not convinced that the data discussed by Schrag and Hoffman can be interpreted in only one way. Concerning the ability of metazoans to survive under the extreme conditions of a hard snowball Earth, the authors state they are "not as sanguine as Schrag and Hoffman. Whether life could survive on a few scattered volcanic islands is a matter of conjecture." A "thin-ice" scenario is not consistent with results indicating that such regions have temperatures substantially colder than those referred to by Schrag and Hoffman. Although evidence for life extends almost to the oldest rocks, 3 billion years transpired before the appearance of metazoans, and this vast time interval suggests that the environmental tolerance of metazoans is much narrower than that of simpler and hardier biological systems. If deep waters were anoxic, metazoans could not survive on deep-sea chemosynthetic communities either, as these organisms still require free oxygen.
2) The authors (Hyde et al) conclude: "Future data may call for the reassessment of our open-water scenario, but we consider that the hard-snowball scenario is not yet proven. We believe that the open water solution is much more favorable for the survival of metazoans, allowing their remote progeny to continue this discussion."
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
eukaryotic: In general, refers to cells which contain a nucleus.
post-glacial ultragreenhouse climate: Advocates of the snowball Earth hypothesis believe that an extremely large and sudden increase in atmospheric carbon dioxide (and a consequent "greenhouse effect") terminated Neoproterozoic glaciation.
evolutionary radiation: In this context, the term "radiation" refers to the spread of a group of biological entities into new environments with consequent diversification.
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A NEOPROTEROZOIC "SNOWBALL EARTH" SIMULATION
The following points are made by W.T. Hyde et al ((Nature 25 May 00 405:425):
1) Extant data indicate that some of the most dramatic events in the history of Earth occurred during the Neoproterozoic era, including the formation of the supercontinent called "Rodinia", and the later breakup of this land mass and eventual reassembly into a different configuration. There were also major changes in strontium, sulfur, and carbon isotopes, together with the most extensive glaciation of the past billion years. At least two main phases of ice advance occurred, with glaciers apparently extending to the Equator at sea level.
2) The late Proterozoic also marked the first appearance of multicellular animals (metazoans), perhaps as early as 1000 to 700 million years ago, and extensive glaciation may have exerted a significant stress on living forms during a critical interval in their evolution. Recent work has focused attention on the Neoproterozoic by interpreting new carbon isotope data to indicate that biological productivity of the oceans virtually ceased for perhaps millions of years during the glacial era, and from this work and from other evidence it has been concluded that the planet entered a "snowball Earth" state, in which it was completely covered by ice until carbon dioxide outgassing produced a sufficiently large greenhouse effect to melt the ice. In this scenario, the sudden warming caused a rapid precipitation of calcium carbonate, producing certain types of carbonate rocks often observed in strata of this era. The repetition of such formations suggests this sequence of events occurred at least twice in the Neoproterozoic era.
3) The authors report that to simulate a snowball Earth in their computer simulations, they used only a reduction in the solar luminosity (solar constant) compared to present-day conditions and kept atmospheric carbon dioxide concentrations near present levels. Their results indicate rapid transitions into and out of full glaciation that are consistent with the geologic evidence. When they combine their results with a general circulation model, some of the simulations result in an equatorial belt of open water that may have provided a refuge for multicellular animals.
4) The authors conclude: "Although there is clearly a need for more climate modeling and additional geological data... our results indicate that inclusion of explicit ice-sheet physics significantly closes the gap between models and data for the largest glaciation of the past billion years and for one of the most critical intervals of evolution in Earth history."
Nature http://www.nature.com/nature
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