<p>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 (<a href="https://earthscience.stackexchange.com/a/8490/6272">I wonder why do you give the leverage to he people who chronically cannot understand a question even after it was answered</a>). You do not need animals for the CaO to react with CO<sub>2</sub>. Actually, there was much more CO<sub>2</sub> 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?</p>

CC BY-SA 3.0
2016-08-01T13:29:57.923Z
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.
Gimelist
CC BY-SA 3.0
2016-08-01T13:33:23.640Z
@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?
Little Alien
CC BY-SA 3.0
2016-08-01T13:35:19.733Z
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.
Gimelist
CC BY-SA 3.0
2016-08-01T13:47:23.593Z
@Michael You previously answered that we have CaO preexisting. CaO + CO2 combines naturally. It does not need any biology to bypass the equilibrium and saying that there was some primordial CaCO3 does not extricate this mystery.
Little Alien
CC BY-SA 3.0
2016-08-01T13:48:42.690Z
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.
Gimelist
CC BY-SA 3.0
2016-08-04T05:48:39.653Z
@Friddy Do you mean that animals use oxygen CaO + O2 + C instead of the carbon dioxide CaO + CO2?
Little Alien
CC BY-SA 3.0
2016-08-06T22:13:21.030Z
@LittleAlien marine animals use (Ca)2+ and (CO3)2-, both of which are dissolved in the sea water. There is no CaO.
Gimelist
<earth-history><limestone><crust>
Why do we see the limestone on top of Cambrian layer?
Little Alien
# Why do we see the limestone on top of Cambrian layer?

(Asked by: username_0 on 2016-08-01)

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](https://earthscience.stackexchange.com/a/8490/6272)). 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?

**Tags:** earth-history, limestone, crust

> **Comments:**
> **username_1**: 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.  
> **username_0**: @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?  
> **username_1**: Who said it fixed most of the calcium? It fixed just a username_0 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.  
> **username_0**: @Michael You previously answered that we have CaO preexisting. CaO + CO2 combines naturally. It does not need any biology to bypass the equilibrium and saying that there was some primordial CaCO3 does not extricate this mystery.  
> **username_1**: 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.  
> **username_0**: @Friddy Do you mean that animals use oxygen CaO + O2 + C instead of the carbon dioxide CaO + CO2?  
> **username_1**: @username_0 marine animals use (Ca)2+ and (CO3)2-, both of which are dissolved in the sea water. There is no CaO.  

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## 0 answers

