<blockquote>
  <p>In an Earth-like planet with no history of carbon-based life forms, would there be limestone?</p>
</blockquote>

<p>Short answer: <strong>Maybe</strong>.</p>

<p>The form of carbon in limestone is oxidised carbon: carbonate: CO<sub>3</sub><sup>2-</sup>. This is different from organic carbon, which is mostly composed of carbon-hydrogen ("hydrocarbons") molecules and compounds.</p>

<p>Limestone is a sedimentary rock composed mostly of calcite or aragonite - calcium carbonate. As you mentioned - that calcium carbonate is coming from the shells of dead marine animals. I would note that if you have a planet with carbon-based lifeforms, but they do not form shells from calcium carbonate so this process of limestone formation would not exist.</p>

<p>Limestones can also form chemically. There is calcium carbonate dissolved in oceans and lakes. If you evaporate and dry them, that calcium carbonate will precipitate. The solubility of calcium carbonate depends on pH. If those water bodies become less acidic (rising pH), you will precipitate chemical deposits of calcium carbonate.</p>

<p>You know that white scale in your tea kettle? This is calcium carbonate. Limestone. So this can form without any life, just by changing the temperature of the water, this changing the solubility of the calcium carbonate. Examples of chemically precipitated "limestones" are <a href="https://en.wikipedia.org/wiki/Tufa" rel="nofollow noreferrer">tufa</a> and <a href="https://en.wikipedia.org/wiki/Travertine" rel="nofollow noreferrer">travertine</a>.</p>

<p>If we venture out of the "sedimentary limestones", there are also carbonatites, which are igneous rocks crystallised out of a carbonate rich magma. These are rare on Earth (we know of only <a href="https://en.wikipedia.org/wiki/Ol_Doinyo_Lengai" rel="nofollow noreferrer">one volcano</a> that erupts this stuff), but could be more common on other planets. Weathering of these rocks can produce proper sedimentary limestones.</p>

CC BY-SA 4.0
2018-06-06T14:48:31.307Z
Wouldn't the bodies of water with calcium carbonate become more acidic due to the carbonic acid reactions, rather than less acidic?
TylerH
CC BY-SA 4.0
2018-06-07T00:12:13.013Z
@TylerH it's a hypothetical planet. You can make up any chemical reaction you want (within reason) to buffer the oceans so they're not acidic enough to dissolve away the calcium carbonate.
Gimelist
CC BY-SA 4.0
2018-06-06T00:46:35.250Z
2018-06-06T00:46:35.250Z
Gimelist
<p>I'd have to say, yes. Once life evolved in the world's oceans, the vast majority of limestone formation has been through the organic process. However, before life did evolve, limestone formed through an inorganic process where rain water falling through a CO<sub>2</sub>-rich atmosphere reacted to form a weak carbonic acid solution that then reacted with calcium-containing minerals to form calcites. This then washed into the world's oceans to form limestone deposits.<br><br>From <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Geophys/limestone.html" rel="noreferrer">gsu.edu</a>:</p>

<blockquote>
  <p>Limestone may form inorganically or by biochemical processes.<br><br>
  Another form of limestone of inorganic origin is oolitic limestone, limestone formed from small spherical grains called ooids.</p>
</blockquote>

<p>In an paper from <a href="https://news.stanford.edu/news/2004/june2/lowegeo-62.html" rel="noreferrer">Stanford.edu</a>:</p>

<blockquote>
  <p>Carbonic acid dissociated to form hydrogen ions, which found their way into the structures of weathering minerals, and bicarbonate, which was carried down rivers and streams to be deposited as limestone and other minerals in ocean sediments.</p>
</blockquote>

<p>So even in a world devoid of carbon-based life, limestone could still form if it had a CO<sub>2</sub>-rich atmosphere, water (and thus rain), and calcium-containing crustal deposits. Though the paper from Stanford is theoretical, note that this does not mean hypothetical and is (I believe) based on solid science.</p>

CC BY-SA 4.0
2018-06-05T15:54:26.657Z
@BillDOe Interesting, the atmosphere was rich (enough) in carbon dioxide before life arose on Earth for that process to happen?
TylerH
CC BY-SA 4.0
2018-06-05T00:55:53.357Z
2018-06-05T00:55:53.357Z
BillDOe
<p>The extent of my understanding of limestone is about what Wikipedia says at the very top of the page:</p>

<blockquote>
  <p>Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).</p>
</blockquote>

<p>What catches my curiosity is the word "mainly" there... I can't tell if it means "the main part of <em>all</em> limestone" is [skeletal fragments of organisms]" or if it means "part of limestone (in the usual cases) is [skeletal fragments of organisms]".</p>

<p>Phrased in the most general and interesting way (to me):</p>

<p>Given an Earth-like planet with all the right ingredients (e.g. large bodies of liquid water, iron-nickel partially-liquid core, etc.) with one exception&mdash;no history of carbon-based life forms&mdash;would we find significant amounts of limestone like we have on Earth today?</p>

CC BY-SA 4.0
2018-06-04T21:16:57.857Z
This may be strolling toward the theoretical, so I apologize if this is off-topic here.
TylerH
<geology><planetary-science><sedimentology><petrology><carbon>
In an Earth-like planet with no history of carbon-based life forms, would there be limestone?
TylerH
# In an Earth-like planet with no history of carbon-based life forms, would there be limestone?

(Asked by: username_0 on 2018-06-04)

The extent of my understanding of limestone is about what Wikipedia says at the very top of the page:

> Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).

What catches my curiosity is the word "mainly" there... I can't tell if it means "the main part of *all* limestone" is [skeletal fragments of organisms]" or if it means "part of limestone (in the usual cases) is [skeletal fragments of organisms]".

Phrased in the most general and interesting way (to me):

Given an Earth-like planet with all the right ingredients (e.g. large bodies of liquid water, iron-nickel partially-liquid core, etc.) with one exception—no history of carbon-based life forms—would we find significant amounts of limestone like we have on Earth today?

**Tags:** geology, planetary-science, sedimentology, petrology, carbon

> **Comments:**
> **username_0**: This may be strolling toward the theoretical, so I apologize if this is off-topic here.  

---

## 2 answers

### Answer by username_2 on 2018-06-05. Score: 5 | Accepted answer

I'd have to say, yes. Once life evolved in the world's oceans, the vast majority of limestone formation has been through the organic process. However, before life did evolve, limestone formed through an inorganic process where rain water falling through a CO2-rich atmosphere reacted to form a weak carbonic acid solution that then reacted with calcium-containing minerals to form calcites. This then washed into the world's oceans to form limestone deposits.  
  
From [gsu.edu](http://hyperphysics.phy-astr.gsu.edu/hbase/Geophys/limestone.html):

> Limestone may form inorganically or by biochemical processes.  
>   
> Another form of limestone of inorganic origin is oolitic limestone, limestone formed from small spherical grains called ooids.

In an paper from [Stanford.edu](https://news.stanford.edu/news/2004/june2/lowegeo-62.html):

> Carbonic acid dissociated to form hydrogen ions, which found their way into the structures of weathering minerals, and bicarbonate, which was carried down rivers and streams to be deposited as limestone and other minerals in ocean sediments.

So even in a world devoid of carbon-based life, limestone could still form if it had a CO2-rich atmosphere, water (and thus rain), and calcium-containing crustal deposits. Though the paper from Stanford is theoretical, note that this does not mean hypothetical and is (I believe) based on solid science.

> **Comments:**
> **username_0**: @username_2 Interesting, the atmosphere was rich (enough) in carbon dioxide before life arose on Earth for that process to happen?  

---

### Answer by username_1 on 2018-06-06. Score: 3

> In an Earth-like planet with no history of carbon-based life forms, would there be limestone?

Short answer: **Maybe**.

The form of carbon in limestone is oxidised carbon: carbonate: CO32-. This is different from organic carbon, which is mostly composed of carbon-hydrogen ("hydrocarbons") molecules and compounds.

Limestone is a sedimentary rock composed mostly of calcite or aragonite - calcium carbonate. As you mentioned - that calcium carbonate is coming from the shells of dead marine animals. I would note that if you have a planet with carbon-based lifeforms, but they do not form shells from calcium carbonate so this process of limestone formation would not exist.

Limestones can also form chemically. There is calcium carbonate dissolved in oceans and lakes. If you evaporate and dry them, that calcium carbonate will precipitate. The solubility of calcium carbonate depends on pH. If those water bodies become less acidic (rising pH), you will precipitate chemical deposits of calcium carbonate.

You know that white scale in your tea kettle? This is calcium carbonate. Limestone. So this can form without any life, just by changing the temperature of the water, this changing the solubility of the calcium carbonate. Examples of chemically precipitated "limestones" are [tufa](https://en.wikipedia.org/wiki/Tufa) and [travertine](https://en.wikipedia.org/wiki/Travertine).

If we venture out of the "sedimentary limestones", there are also carbonatites, which are igneous rocks crystallised out of a carbonate rich magma. These are rare on Earth (we know of only [one volcano](https://en.wikipedia.org/wiki/Ol_Doinyo_Lengai) that erupts this stuff), but could be more common on other planets. Weathering of these rocks can produce proper sedimentary limestones.

> **Comments:**
> **username_0**: Wouldn't the bodies of water with calcium carbonate become more acidic due to the carbonic acid reactions, rather than less acidic?  
> **username_1**: @username_0 it's a hypothetical planet. You can make up any chemical reaction you want (within reason) to buffer the oceans so they're not acidic enough to dissolve away the calcium carbonate.  

---

