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Wednesday, 21 January 2009 22:30

You had to be a good swimmer on Earth 2.5 billion years ago

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According to a mathematical model developed by French and Australian scientists, the Earth was almost completely covered with water about 2.5 billion years ago, with only about two to three percent of land sticking above the oceans. Talk about your pricey ocean-front property!


Their paper “A case for late-Archaean continental emergence from thermal evolution models and hypsometry” was published on October 10, 2008, in the journal Earth and Planetary Science Letters (DOI: 10.1016/j.epsl.2008.08.029).

The French-Australian authors include Nicolas Flament and Nicolas Coltice, of the Université de Lyon, Laboratoire de Sciences de la Terre, France, and Patrice F. Rey (and Flament), of the EarthByte Group, School of Geosciences, The University of Sydney, Sydney, NSW, Australia.

The modern surface of the Earth is composed of about 28% water and 72% land. However, approximately 2.5 billion years ago, those percentages were far different.

Scientists are not sure of the exact evolution of the Earth from then and to now, but they do know that the amount of land (continental crust) increased quite drastically as the waters subsided.

They state in the abstract to their paper, “The secular cooling of the Earth's mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans, in the oceanic bathymetry and in the area of emerged continental crust. The evolution of these variables is of fundamental importance to the geochemical coupling of mantle, continental crust, atmosphere and ocean.”

To determine the amount of water and land on Earth several billion years ago, the team decided to develop a model that calculates the area of emerged continental crust based on the temperature of the Earth’s mantle, along with continental area and hypsometry (the study of land elevation with respect to sea level).

Page two describes their mathematical model, along with comments from the scientists.




The French and Australian researchers state, “… we investigate the continental freeboard predicted using different models for the cooling of the Earth.”

Continental freeboard is defined as the average level of the surface of the sea relative to the continents.

In fact, they show through their mathematical models that the mantle of the Earth was 110 to 210 degrees (Celsius) hotter than what it is today, primarily due to more abundant radioactive elements being present.

Radioactive elements, such as uranium, decay over time, and in the process produce heat. At that time, heat flow from the mantle to the crust was about three times higher than it is today.

The extra heat would have made the continental crust hotter than today and, thus, more thick than it is today.

With the continental crust laying the foundation of the Earth’s oceans, a thicker crust would have meant the bottom of the oceans were higher then than they are now.

The researchers state, “We show that constancy of the continental freeboard (± 200 m) is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110–210  C hotter than present. Such numbers imply either a very limited cooling of the planet or, most likely, a change in continental freeboard since the Archaean.”

Page three defines the Archaeon period, along with stating what happened to Earth after land began to appear from beneath the oceans far away.




The Archean, or Archean, period is a geologic eon that occurred before the Proterozoic and Paleoproterozoic. It occurred before about 2.5 billion years ago.

They add, “During the Archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere, thus limiting crustal thickening due to mountain building processes and the maximum elevation in the Earth's topography …. Taking this into account, we show that the continents were mostly flooded until the end of the Archaean and that only 2–3% of the Earth's area consisted of emerged continental crust by around 2.5 Ga.”

Thus, as the mantle cooled, the crust became less thick, which lowered the height of the oceans and raised the land above the waters.

Also within their abstract, they state, “These results are consistent with widespread Archaean submarine continental flood basalts, and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from [about] 2.5 Ga."

And, "The progressive emergence of the continents as shown by our models from the late-Archaean onward had major implications for the Earth's environment, particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the Earth's deep and surface reservoirs.”

The New Scientist article When Earth really as the blue planet” states, “The team believe[s] that this transition may help to explain why levels of oxygen in the atmosphere rose around this time. During the water-world period, any oxygen produced by photosynthesizing bacteria would have been quickly used up through reactions with decaying organic matter in the oceans.”

Further, “When the newly emerged land eroded, it produced sediment that; once washed into the oceans, would have buried the organic matter, preventing any further reactions with oxygen, and so allowing it to build up in the atmosphere.” [January 3-9, 2009, page 8]

Thus, oxygen built up in the atmosphere, which allowed organisms that breathed in oxygen to flourish. Due in part to this period of Earth’s early history, we are now a living, breathing species, like the other simple and complex multitudes of creatures living on, above, and underneath the surface of planet Earth.


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