I wish to add to the inquiry by Ivan that where I live Devonian limestones are very common and it is characteristic that they display lamination of organic stain between deposition of more limestone. The organic stain is interpreted as a period of anoxic conditions. Therefore, the deposition of the limestone occurs during periods of oxygenated seawater. Just to say that mixing conditions were poor is not enough. Deposition of limestone stopped during the formation of the organic stain. Why? I am not looking for a geologic answer but a chemical one, though geology will certainly play a primary role in setting up the conditions for the chemistry to occur.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don