Saturday, December 05, 2009

Expanding Earth Model and Pre-Cambrian Era: Part II

I have divided the discussion of Expanding Earth model for pre-Cambrian period in short postings: 5 altogether. In the first posting I discussedd the notions of super-continents, super-oceans, and glaciations during Neoproterozoic period with short coments inspired by R/2 scenario. In this posting I will discuss the dominating Snowball model for the climate during Neoproterozoic period in the same spirit. Third posting discussed TGD Expanding Earth view about super-continents, super-oceans and glaciations and compares it with Snowball Earth scenario. Fourth posting discusses paleomagnetic tests for R/2 scenario. Fifth posting discusses the Expanding Earth scenario about how life escaped to the underground lakes and seas created during the expansion and returned back in the beginning of Cambrian era.

Snowball Earth model for the glaciation during pre-Cambrian era

Snowball Earth (see this, this, and this) is recently the leading model for the glaciations (see this) during Proterozoic era. The term is actually somewhat misleading: Iceball Earth would more to the point. Slushball earth (see this) is a variant of Snowball Earth which does not assume total freezing near equator.

The history behind the Snowball Earth concept is roughly following (see this).

  1. Mawson studied the Neoproterozoic stratigraphy of South Australia and identified extensive glacial sediments and speculated with the possibility of global glaciation. He did not know anything about continental drift hypothesis and plate tectonic theory and thought that the ancient position of Australia was the same as it is today. Continent drifting hypothesis however explained the finding as sediments deposited at the higher latitudes the hypothesis was forgotten.

  2. Later Harland suggested on basis of geomagnetic data that glacial tillites (see this) in Svalbard and Greenland were deposited at tropical latitudes. In TGD framework with with R® R/2 these tillites would have been at higher latitudes towards North Pole.

  3. The facts are that Sun was 6 per cent fainter at that time and glaciations are known to occur. The question is whether they were global and long-lasting or a sequence of short-lasting possibly local glaciations. The Russian climatologist Budyko constructed a model based on energy balance and found that it is possible to have a global glaciation if the ice sheets proceeded enough from polar regions (to about 30 degree latitude). The model was based on the increased reflectiveness (albedo) of the Earth's surface due to the ice covering giving rise to positive feedback loop. Budyko did not believe that global glaciation had occurred since the model offered no way to escape eternal glaciation.

  4. Kirschwink introduced the term Snowball Earth, which is actually misleading. Iceball Earth would be more to to the point. He found that the so called banded iron formations are consistent with a global glaciation. He also proposed a mechanism for melting the snowball. The accumulation of CO2 from volcanoes would have caused ultra-greenhouse effect causing warming of the atmosphere and melting of the ice.

  5. Slushball Earth (see this) differs from Snowball Earth in that that only a thin ice cover or even its absence near equator is assumed. The model allows to explain various findings in conflict with Snowball Earth, such as the evidence for the presence of melt-water basins.

  6. Zipper rift model (see this) assumes that there was a sequence of glaciations rather similar to the glaciations that have occurred later. The model assumes that the rifts (see this) of the super-continent Rodinia occurred simultaneously with glaciations. The associated tectonic uplift led to the formation of high plateaus hosting the glaciers. The iron band formation can be be assigned with inland seas allowing complex chemistries and anoxicity near the sea floor.

1. The basic ideas of the Snowball Earth model

Snowball Earth (see this, this and this) differs from ordinary glaciations in that only oceans are frozen whereas in the ordinary glaciation land mass is covered by ice. The basic ideas of the snowball Earth relate to the mechanism initiating the global freezing and melting.

  1. The glaciation would have been initiated by some event, say a creation of super-volcano. Also astrophysical mechanism might be involved. Somewhat paradoxically, tropical continents during cryogenian period (see this) are needed for the initiation because they reflect the solar radiation more effectively than tropical oceans.

  2. The positive ice-albedo feedback is an essential concept: the more ice the larger the fraction of the radiation reflected back so that the more ice is generated. If the glaciation proceeds over a critical latitude about 30 degrees positive feedback forces a global glaciation.

  3. The problem of the model is how to get rid of the glaciation. The proposal of Kirschvink was that the accumulation of CO2 from volcanoes could have led to a global super-warming. The time scale for CO2 emissions is measured in millions of years. The needed atmospheric concentration of CO2 is by a factor 350 higher than the recent concentration. Due the ice cover the CO2 could not be absorbed to the siliceous rocks and concentration would increase. The melting of the ice meant higher absorbtion of heat by uncovered land. Positive feedback loop was at work again but in different direction.

2. Evidence for and objections against Snowball Earth

Wikipedia article about Snowball Earth (see this) discusses both evidence for and objections against Snowball Earth. Low latitude sediments at tropical latitudes and tropical tillites at Equatorial latitudes provide strong piece of evidence for Snowball Earth. Calcium carbonate deposits having 13C signature (per cent for the depletion of 13 isotope and large for organic material) consistent with that for mantle meaning abiotic origin is second evidence. Iridium anomaly located at the base of Calcium Carbonate deposits is third piece of evidence. The evidence for Snowball Earth will be discussed in more detail later since it is convenient to relate the evidence to R/2 model for glaciations.

  1. Paleomagnetic data (see this) used to the dating of sediments assuming tectonic plane theory and super-continent drifting might be misleading. No pole wandering maps exist and the polarity of the magnetic field must be deduced by statistical methods. The primary magnetization could have been reset and the orientation of the magnetic minerals could have changed from the original one. It is also possible that magnetic field patterns were not dipolar. Also the assumption of hypothetical super-continents and oceans brings in uncertainties. In R/2 model of course the determination of the positions changes completely.

  2. Carbon isotope ratios are not what they should be. There are rapid variations of 12C/13C ratio with organic origin. Suggests that freezing and melting followed each other in rapid succession. In standard framework this would suggest Slushball Earth meaning ice-free and ice-thin regions around the equator and hydrological cycles. In R/2 model the regions at Equator are near North Pole and the explanation would be in terms of ordinary glaciations.

  3. The distribution of isotopes of element Boron suggest variations of pH of oceans. The explanation is in terms of buildup of carbon dioxide in atmosphere dissolved into oceans/seas. In R/2 model a sequence of glaciations would explain the findings.

  4. Banded iron formations providing support for the model are actually rather rare and absent during Marinoan glaciation.

  5. Wave-formed ripples, far-traveled ice-rafted debris and indicators of photosynthetic activity, can be found throughout sediments dating from the 'Snowball Earth' periods. This serves a evidence open-water deposits. In snow-ball model these could be 'oases' of melt-water but computer simulations suggest that large areas of oceans would have left ice-free. in R/2 model these would be signatures of ordinary glaciations.

  6. Paleomagnetic data have led to the conclusion that Australia was at Equator. In R/2 model it would have been near North Pole. Namibia was also thought to be near Equator (see this). Indirect arguments forced the conclusion that it at 75 degree Southern latitude. In R/2 model this corresponds to 60 degrees Southern latitude and ordinary glaciation proceeding from South Pole is a natural explanation and ordinary glaciation would be in question in both cases.

  7. There is evidence for the continental ice cover does not fit with Snowball Earth predicts that there should be no continental ice-cover. The reason is that freezing of the ocean means that there is no evaporation from oceans and no water circulation so that ice-cover cannot develop on continents. There is considerable evidence that continents were covered by thick ice (see this). This suggests ordinary glaciations possible in R/2 model.

For details see the new chapter Expanding Earth Model and Pre-Cambrian Evolution of Continents, Climate, and Life of "Genes and Memes".


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