A study reveals cyclic changes in the rate of burial of biogenic calcium carbonate at the Pacific ocean floor 43 million to 33 million years ago, as Earth exited a warm 'greenhouse' state to become an ice-capped planet. See Article p.609
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Stoll, H. Ancient burial at sea. Nature 488, 596–597 (2012). https://doi.org/10.1038/488596a
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DOI: https://doi.org/10.1038/488596a
Peter Gibson
Stoll/ Pälike et al appear to be discussing a density gradient or partition produced by ions in the Pacific, and presumably in other seas. Such barriers have existed through geological time and are seen today in the form of thermoclines. The evidence used by Pälike et al is sea bed cores. These surely can only be of use over a very limited period: in their case the Cainozoic. Longer cores presumably cannot exist due to the movement of the sea bed due to plate tectonics and ultimately subduction. This is not a criticism of their study but would appear to limit it application.
I and others have suggested that the evolution of invertebrates occurred via larval-like organisms prior to the Precambrian explosion above a physical barrier: this may have been a thermocline. At that time there no were benthic invertebrates other than possibly platyhelminths because the sea bed was anoxic. The photic zone would have formed in effect a cell where nutrients were recycled. The organisms would have diversified giving rise to the major invertebrate groups similar to the larvae of present day invertebrates. The thermocline would have broken up due to increased carbon dioxide levels, global warming, greater cloud cover and ultimately cooling due to reflection of the sun?s rays. As with present day winter cooling turbulence in the water column was set up and the thermocline destroyed. This would presumably have been cyclic. The destruction of the barrier led to planktonic organisms sinking to the sea bed to feed of the accumulated nutrients dropping from the photic zone. The cyclic nature of the process would have led to a series of migrations giving rise to the various extant invertebrate phyla.
A feature of planktonic organism is that the mouth is either at the side of body or at the lower pole. Such positions are suitable for benthic feeding. On the sea bed the organism would have become larger since this would have increased their fecundity. There would be more internal space for the storage of eggs: they would have started in effect as eggs sacks. This is obvious in polychaetes which bud from what remains of the planktonic organism. This bears the sense organs and is the origin of the gonads, chaetae and so on.
This scenario would of course help explain the removal of carbon dioxide from the sea water as it became bound up in the shells of molluscs and arthropods and has since contributed to the terrestrial geology. Therefore the events suggested by Pälike et al may have far wider implications.