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

Question

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?

Answer 1

The short answer is yes: significant amounts of limestone could exist on an Earth-like planet without carbon-based life. While it is true that most limestone on Earth today is biogenic (formed from the skeletal remains of marine organisms), limestone can also form through purely abiogenic (chemical) precipitation.

In an Earth-like environment with liquid water and a CO₂-rich atmosphere, limestone formation would occur through the chemical weathering of silicate rocks. This process releases calcium ions (Ca²⁺) into solution. When these ions combine with dissolved bicarbonate (HCO₃⁻), calcium carbonate (CaCO₃) precipitates according to the reaction:

Ca²⁺ + 2HCO₃⁻ → CaCO₃ + CO₂ + H₂O

In the absence of life to accelerate this process through biological mechanisms such as shell formation, the planet would rely entirely on inorganic chemical precipitation. This is analogous to natural processes observed on Earth, such as stalactite formation in caves or the development of oolitic limestones in warm, supersaturated marine environments.

On an abiotic Earth-like planet, one would likely expect the accumulation of carbonate deposits such as tufa, travertine, or micritic limestones, particularly in shallow evaporative basins or near hydrothermal systems. In fact, during the Archean Eon of early Earth, significant carbonate platforms formed prior to the widespread evolution of shell-building organisms.

Therefore, although the texture, structure, and fossil content of such limestones would differ from those formed biologically, the underlying geochemical processes on an Earth-like planet strongly favor the formation of limestone as an important carbon sink.