Friday, June 21, 2013

The Leaky Earth the Driver of the Cambrian Explosion?

Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion


1. S. Maruyama (b)
2. Y. Sawaki (a)
3. T. Ebisuzaki (c)
4. M. Ikoma (d)
5. S. Omori (e)
6. T. Komabayashi (a)


a. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

b. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

c. RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

d. Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

e. Open University of Japan, 2-11, Wakaba, Mihama-ku, Chiba, 261-8586, Japan


For life to have dramatically evolved and diversified during the so-called Cambrian explosion, there must have been significant changes in the environmental conditions of Earth. A rapid increase in atmospheric oxygen, which has been discussed as the key factor in the evolution of life, cannot by itself explain such an explosion, since life requires more than oxygen to flourish let alone survive. The supply of nutrients must have played a more critical role in the explosion, including an increase in phosphorus (P) and potassium (K) which are key elements for metabolisms to function. So, what happened at the onset of the Cambrian to bring about changes in environmental conditions and nutrient supply and ultimately evolution of life?

An ultimate trigger for the Cambrian explosion is proposed here. The geotherm along subduction zones of a cooling Earth finally became cool enough around 600 Ma to allow slabs to be hydrated. The subduction of these hydrated slabs transferred voluminous water from the ocean to the mantle, resulting in a lowering of the sea level and an associated exceptional exposure of nutrient-enriched continental crust, along with an increase in atmospheric oxygen. This loss of water at the surface of the Earth and an associated increase in exposed landmass is referred to here as leaking Earth. Vast amounts of nutrients began to be carried through weathering, erosion, and transport of the landmass; rock fragments of the landmass would break down into ions during transport to the ocean through river, providing life forms (prokaryote) sufficient nutrients to live and evolve. Also, plume-driven dome-up beneath the continental crusts broadened the surface area providing a supply of nutrients an order magnitude greater than that produced through uplift of mountains by continental collision. Simultaneously, atmospheric oxygen began to increase rapidly due to the burial of dead organic matter by enhanced sedimentation from the emergence of a greater landmass, which ultimately inhibited oxidation of organic matter. Hence, oxygen began to accumulate in the atmosphere, which when coupled with a continuous supply of nutrients, resulted in an explosion of life, including an increase in the size. An enhanced oxygen supply in the atmosphere resulted in the formation of an ozone layer, providing life a shield from the UV radiation of the Sun; this enabled life to invade the land. In addition to a change in the supply of nutrients related to a leaking Earth, the evolution of life was accelerated through mass extinction events such as observed during Snowball Earth, possibly related to a starburst in our galaxy, as well as mutation in the genome due to radiogenic elements sourced from carbonatite magma (atomic bomb magma) in rift valley. There are two requirements to find a habitable planet: (1) the initial mass of an ocean and (2) the size of a planet. These two conditions determine the history of a planet, including planetary tectonics and the birth of life. This newfound perspective, which includes the importance of a leaking planet, provides a dawn of new planetary science and astrobiology.

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