The official story says like Cambrian Ordovician transition was caused/accompanied by animals learned how to build the Ca-based skeletons. These skeletons and shells started to fall upon the sea bed causing the rich and durable limestone formations on top of earth crust. I wonder didn't limestone form itself in pre-Cambrian epochs? You already had the CaO in the primordial crust (I wonder why do you give the leverage to he people who chronically cannot understand a question even after it was answered). You do not need animals for the CaO to react with CO2. Actually, there was much more CO2 even before Cambrian, billions of years ago. But we do not see any limestone formations at that time. So, why do we see the limestone on the top layer sediments 460-60 mln years ago rather than at the very bottom of the crust?
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2$\begingroup$ Carbonates existed before the Cambrian. There are Precambrian limestones and dolomites, which formed by chemical precipitation from seawater, not by accumulation of fossils. Such rocks even exist today, it's just that you get much more limestone from fossils. $\endgroup$– GimelistAug 1, 2016 at 13:29
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$\begingroup$ @Michael If that limestone fixed most of the Ca from the crust, we would not have any from the animals and postcembrian formations. Why could animals find that CaO while the inanimate nature could not? $\endgroup$– Little AlienAug 1, 2016 at 13:33
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1$\begingroup$ Who said it fixed most of the calcium? It fixed just a little bit. Not much. Enough to make some limestones, not enough to deplete all of the calcium from the oceans. As to why animals can do it and nature not, it's up to biology. Animals produce a whole bunch of organic material that nature does not. Biology is a non-equilibrium process. $\endgroup$– GimelistAug 1, 2016 at 13:35
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2$\begingroup$ It does combine naturally, but not all of it. Obviously, if you look at karst caves, the reverse process can also happen. Anyway, you were asking about limestones from the precambrian, saying they do not exist. They do. $\endgroup$– GimelistAug 1, 2016 at 13:48
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1$\begingroup$ CO2 can provide the carbon in CaCO3 but it also acidifies water allowing it to keep more Ca in solution. The Ca leached and corroded from bedrock minerals (silicates) would have precipitated out during the Precambrian epoch but this precipitation would have been more diffuse, (Many sedimentary rocks contain CaCO3). On the other hand organisms would have concentrated the CaCO3 in to more localized locations, such as marine shelves. These deposits in some cases are nearly pure CaCO3 and formed much of the limestone we see today. $\endgroup$– user824Aug 3, 2016 at 22:42
1 Answer
No. CaO will not exist in most environments for any significant time in the presence of CO2, it will react to form CaCO3. I doubt CaO is the primary source of Ca in our world. Silicates are likely the original source of Ca in our oceans https://en.wikipedia.org/wiki/Silicate_minerals. CaCO3 will partially dissolve or corrode in water to form an aqueous solution of Ca +2 + 2 HCO3 -. The dissolution of the CaCO3 in water is an equilibrium reaction based on the acidity and temperature of the water, this is why the acidification of the oceans is a concern for our marine life. I am not a chemist or a biologist but organisms will extract the aqueous Ca +2 ions found in the water to form shells etc. Because these organisms are not evenly distributed in the ocean their shells and skeletons will and have accumulated the CaCO3 in distinct areas.