Tuesday, 1 May 2012

Just a little time ago...

It's just a little time ago, when sea life was bursting. The long-lasting “Proterozoic” had came to its end, after about two billion ( years. Modern “Phanerozoic” times had started – just about five-hundred-forty million (540.000.000) years ago, and marine life made a giant leap. Previously, sea life already had left its traces on planet earth. Now more complex life forms, plants and animals, got ready to proliferate, first in the sea and then on land.

For the record, some dramatic events had happened: marine alga/bacteria-like life had oxygenated the atmosphere, several global glaciations have passed freezing the sea, and evolution of abundant soft-bodied multicellular organisms had taken place in the sea – but no fish, no shell-fish, no corals, no whales, just vigorously living jelly-stuff.

 Wayne Ranney  embraces the Great Unconformity
in Blacktail Canyon,
From blog: written-in-stone-seen-through-my-lens

Then, at the transition between the "Proterozoic" and "Phanerozoic", diversification of multicellular animals happens. Acquisition of mineralized skeletons, which get preserved more easily in the sediments, mark that step. The geological record shows a transition, the “Great Unconformity”. First trilobites and reef building animals such as corals appear; first appearance of a complex feeding burrows; first appearance of small, armored 'shelly fauna'. At their base crystalline rocks; the “Great Unconformity” represent a unique physical environmental transition.  

Global seawater chemistry changed during a time of profound expansion of shallow marine habitats. The marine sediments [1] record both an expansion of shallow continental shelf seas and a different pattern of chemical sedimentation.

Beast from Cambrian Sea
Oceanic alkalinity had increase and chemical weathering of continental crust had enhanced. These geochemical changes reflect a wast period of extensive continental erosion and physical reworking of soil and basement rock. A continental-scale marine transgression is observed. Increase input of silicates from the continents and lower solubility of calcareous minerals in the sea are favourable for the evolution of shells and skeletons. Massive marine sedimentary deposits form in shallow seas, bursting of new forms of life; the Cambrian explosion [2].

Pauline Lim - The Great Unconformity
(with her permission [4])
The stratigraphic surface, which separates (often) continental crystalline rock from much younger Cambrian shallow marine sedimentary deposits, is known as the Great Unconformity; its formation “may have been an environmental trigger for the evolution of bio-mineralization and the ‘Cambrian explosion’ of ecologic and biodiversity following the... emergence of animals" [3].


p.s. Seeing the article [3] by Peters and Gaines in NATURE caused the desire to share it in context of Maris Mundus, to illustate an other aspects of the enormous treasure the sea is. My text is built on their abstract. I hope that simplifying the matter did not distort their idea. For some related reading, also about bio-mineralisation see: http://io9.com/multicellular-organism/
[1] deposited approximately 540–480 Million years ago

[2] from Wikipedia: The Cambrian explosion or Cambrian radiation was the relatively rapid appearance (over a period of many millions of years), around 530 million years ago, of most major animal phyla, as demonstrated in the fossil record, accompanied by major diversification of organisms including animals, phytoplankton, and calcimicrobes. Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years the rate of evolution accelerated by an order of magnitude (as defined in terms of the extinction and origination rate of species) and the diversity of life began to resemble that of today.

[3] Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion; Shanan E. Peters & Robert R. Gaines http://www.nature.com/nature/journal/v484/n7394/full/nature10969.html; for further reading: http://www.physicstoday.org/daily_edition/down_to_earth/mind_the_gap or http://phys.org/news/2012-04-great-unconformity-evidence-geologic-trigger.html

[4] http://www.paulinelim.net/071102Pages/GreatUnconformity.html

No comments:

Post a Comment