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?

  • $\begingroup$ This may be strolling toward the theoretical, so I apologize if this is off-topic here. $\endgroup$ – TylerH Jun 4 '18 at 21:16
  • $\begingroup$ Significants amounts surely no. Ocean limestones are all comming from organisms. CaCO3 is also formed on some lakes, I don't know if a non-life rocky planet could have the necessary chemical conditions on atmosphere and lake/river. $\endgroup$ – Universal_learner Jun 4 '18 at 21:32
  • $\begingroup$ There is also magmatic carbonatites with CaCO3 -note limestone is only for sedimentary rocks-. At early stages of earth's magmatism. $\endgroup$ – Universal_learner Jun 5 '18 at 2:45

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:

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:

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.

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    $\begingroup$ Algae are sugested to be envolved at oolites formation. $\endgroup$ – Universal_learner Jun 5 '18 at 2:43
  • $\begingroup$ the paper is realy interesting, as always for ancient Archaic processes. It migth formed little deposits when the Earth was 95% ocean from weathering of that starting continental crust. My answer is more for "significant amounts". The universe is huge, maybe at some rocky planet those kind of deposits are frequent and formed large deposits. $\endgroup$ – Universal_learner Jun 5 '18 at 11:21
  • $\begingroup$ @BillDOe Interesting, the atmosphere was rich (enough) in carbon dioxide before life arose on Earth for that process to happen? $\endgroup$ – TylerH Jun 5 '18 at 15:54
  • $\begingroup$ The fact is the temperature was so huge that CO2 precipitated. Yes. at early stages of Earth there was CO2 at atmosphere. Stanford geologist sugests CO2 was stored quickly because of this procces to the sea bed. As continental crust was not yet formed, no subduction etc so CO2 was stored some my. $\endgroup$ – Universal_learner Jun 5 '18 at 19:55
  • $\begingroup$ My guess is first continental crust on Earth should be rich in Fe and Mg from calcium, and CaCO3 not the main storing mollecule. At another rocky planet? Who knows? Universe is huge $\endgroup$ – Universal_learner Jun 5 '18 at 19:58

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 and 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 that erupts this stuff), but could be more common on other planets. Weathering of these rocks can produce proper sedimentary limestones.

  • $\begingroup$ Wouldn't the bodies of water with calcium carbonate become more acidic due to the carbonic acid reactions, rather than less acidic? $\endgroup$ – TylerH Jun 6 '18 at 14:48
  • $\begingroup$ @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. $\endgroup$ – Gimelist Jun 7 '18 at 0:12

The sedimentary register shows on Earth firsts significant amounts of limestones came from stromatolites and foraminifera at the end of Precambrian. After life explosion, two main carbonated deposits are registered:

- Powerful basin deposits at the coast with coral reefs.

- Big benthic deposits with foraminifera and coccolithophoridae at seabed

A limestone uses to be those skeletal fragments, that are cemented by CaCo3 at diagenesis, but not always. At calm environments, mudstone can be formed, primarily from algae. Another exception to this are crystalline limestones that have changed their texture at diagenesis and skelets are not recognizable, but that doesn't mean origin of CaCo3 wasn't organic.

enter image description here

Dunham classification of carbonates. Image from: commons.wikimedia.org

There is another exception, as noticed: oolites. Oolites are inorganical concretions around a bioclast or clast. What I have read is this structures are formed at the beaches thanks too to algaes.

Anyhow, a non-life rocky planet can produce carbonates at his first magmatic phases. The rock is called carbonatite, wich is a magmatic rock, not a sedimentary limestone. You could then have some scattered sedimentary deposits where cement is CaCo3. I find it unlikely, but erosion of carbonatites could lead to true limestones.

There are too terrestrial limestones formed on lakes and rivers called tufa.

"Recently it has been demonstrated that microbially induced precipitation may be more important than physico-chemical precipitation. Pedley et al. (2009) showed with flume experiments that precipitation does not occur unless a biofilm is present, despite supersaturation."

source: wikipedia (see quoted paper)

There is no evidence of such deposits on Mars neither. Carbonates are found , but being magnesite (MgCo3) the main carbonate with little percentages of calcite.

So, I can be wrong, but my answer to "significant amounts" is no, knowing little deposits of CaCO3 could thermodinamicaly been formed on a non-life rocky planet.


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