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Tuesday, January 02, 2007

More interesting stuff from 2006

Over the holidays, I get a chance to catch up on the literature. One of the more intriguing papers I read was by Squire et al. (Earth and Planetary Science Letters, v. 250, 116-133). The paper posits that the building of the Gondwana supercontinent during Neoproterozoic to Cambrian time (650-490 Ma) provided much of the geochemical influx necessary for the Cambrian explosion. The story is relatively simple. The Gondwana supercontinent (composed of South America, Africa, India, Madagascar, Australia and Antarctica and some smaller blocks) was assembled in a polyphase manner during this time interval. The authors focus on what is thought to be the main collision between East (India, Madagascar, Australia and Antarctica) with West (South America and Africa).
Although there are some errors in the description of the collision (it is viewed rather simplistically in the paper), the overall conclusion is that it resulted in an enormous mountain chain. It is referred to as the "Transgondwanan Supermountain". Because the landscape was barren during this interval, physical and chemical weathering of the supermountain resulted in a tremendous flux of Phosphorus, iron, strontium and calcium into the ocean.
There is isotopic evidence for the erosion of this mountain chain (especially with respect to Strontium isotopes). Radioactive strontium (87Sr) is concentrated in the continental crust whereas stable strontium (86Sr) is used as a basis for comparison. A low 87/86 strontium ratio can indicate little erosion of continental crust (or an increased mantle input of oceanic crust which is low in 87Sr). A high 87/86 ratio indicates significant input of radiogenic strontium typically via erosion of 87Sr rich crust. The highest 87/86 ratios in the last 543 million years occurs during the Cambrian suggesting large input of continentally derived material.
The authors then go on to describe the fact that phosphorus and calcium were also added to seawater resulting in nutrient rich seawater (from which organisms can build calcium-rich or phosphorus-rich skeletons). The first skeletons were composed primarily of phosphate and later organisms used calcium carbonate for the building of shells. In addition to the supply of nutrients, the Gondwana continent was located in a favorable climatic zone needed to supply warm shallow waters that are conducive to life.
The real question is whether or not these events are merely coincidental or causative. Of course the authors argue that these events triggered the explosion of animals, but it largely ignores biological explanations (such as Hox genes, arms races etc). What is clear is that the timing of the main pulse of radiation is coincident with the final stages of Gondwana assembly.
I suspect that we will hear more about Gondwana and the Cambrian explosion in 2007.


Joe Meert


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