Coragyps writes:
Partly because, IIRC, calcium carbonate snowing down from the photosynthetic zone near-surface largely redissolves before it ever gets to a cold. deep ocean floor. Calcium carbonate is a bit odd in that respect - it's less soluble in cold water than warm. That's why it's the hot-water heater and pipes in your house that scale up if you have hard water.
Jesus Christ I'm dense! I just had a water softener installed a month ago, read all about this effect, but never put 2 and 2 together.
So let's imagine an area of very deep ocean near the equator with an abundance of water life, including algae, near the top of the water column, and with cold, dark water a couple miles below. The rain of particulate calcium carbonate skeletons from above dissolves before it reaches the sea floor. This process goes on for millenia, and one could argue that eventually the cold water at the bottom of the water column will supersaturate with dissolved calcium carbonate and that it would just precipitate out on the sea floor at a steady rate after that.
But once dissolved the calcium carbonate is much more subject to diffusion than to gravity, and so since we can assume marine geologists have in general not found significant formation of limestone layers in such regions, it must diffuse away to an extent sufficient to prevent supersaturation.
If a combination of diffusion and water currents carried such calcium carbonate rich water to shallow regions where it could warm, then the calcium carbonate would precipitate out, and this could be another factor in micrite formation.
Are we really sure that calcium carbonate is more soluble in cold water than warm? Poking about on the web definitely indicates that it is solubility that is the factor, but this makes no sense to me. I've found a couple mentions that when CO
2 concentrations in water drop, which happens with higher temperature, that CaCO
3 tends to precipitate out, so I don't think solubility is the whole story, but I couldn't find any details.
How does algae build its calcium carbonate shells? Does it just use calcium carbonate that's already dissolved in ocean water? Or does it create calcium carbonate? I've seen hints that they remove calcium from the water to create the shells, but no details. The world is so full of calcium carbonate that we dump it on our lawns, and there does not seem to be any large-scale inorganic source, so I expect the answer is that they create the calcium carbonate from calcium in the sea water.
--Percy