# Implications of ocean basins filled with coral sand

Setting

Assume we have a planet with shallow oceans. A fast-growing coral (or any organism with calcium carbonate skeletons) fills the oceans from the bottom up, layer by layer. As the biomass of the coral grows, the ocean is slowly consumed, until at last, the coral skeletons have enveloped the entire ocean. The intense pressure of the skeletons above the lower skeletons causes the bottom of this coral mass to form limestone. Slowly, the upper layers are eroded away into coral sand.

Large plateaus, formed from what used to be the continents, rise a modest 100m or so above the desert. The increasing concentration of the ocean causes it to turn acidic, to the point it drains into the limestone and forms widespread and enormous aquifers. Soon, the world turns into a desert planet, like Arrakis or Tatooine. The ocean basins are filled with whitish sand, mixed with volcanic ash from hotspots.

Questions

• Would such a world even be possible? i.e. are there biological restraints I haven't thought about?
• What interesting scenarios could arise from such a world?
• Could anyone explain how one could have shallower oceans so the corals consume them faster?

Notes for clarification

In the beginning, the coral is chemosynthetic. In the middle regions of the ocean, the corals feed on small crustaceans. At the top, the corals evolve photosynthesis.

The corals grow in films, which stack on top of each other.

The corals were introduced during a bio-weapons test.

As the ocean gets shallower, eventually most of it evaporates.

At the top, a layer of sandstone is formed by grains of coral skeleton-type things.

• That's an awful lot of calcium carbonate... Where does it come from? – AlexP Apr 17 '20 at 18:46
• Hand-waving, i.e. "This planet is very rich in calcium/carbonate ions for reasons we have yet to determine because we're too lazy to do proper research. Yay." – user75058 Apr 17 '20 at 18:55
• The thing is, water is pretty much indestructible. It is the ultimate recyclable material. There is no way for anything to "fill up the ocean" -- all it can do it fill the depressions. The ocean will raise and spread out over the land. Eventually you will have a shallow ocean covering all the planet. – AlexP Apr 17 '20 at 18:58
• Couldn't it drain into the limestone? Like aquifers? – user75058 Apr 17 '20 at 18:58
• Would you say that in your world there would be no cratons and no plate tectonics? – Alexander Apr 17 '20 at 18:59

There is at least one study of plate tectonics which credits marine sediment deposition with both kickstarting and accelerating subduction; emphasis added for relevance:

According to new research, the transition to plate tectonics started with the help of lubricating sediments, scraped by glaciers from the slopes of Earth's first continents. As these sediments collected along the world's young coastlines, they helped to accelerate the motion of newly formed subduction faults, where a thinner oceanic plate dips beneath a thicker continental plate.

The new study, published June 6, 2019 in the journal Nature, is the first to suggest a role for sediments in the emergence and evolution of global plate tectonics. Michael Brown, a professor of geology at the University of Maryland, co-authored the research paper with Stephan Sobolev, a professor of geodynamics at the GFZ German Research Centre for Geosciences in Potsdam.

The findings suggest that sediment lubrication controls the rate at which Earth's crust grinds and churns. Sobolev and Brown found that two major periods of worldwide glaciation, which resulted in massive deposits of glacier-scrubbed sediment, each likely caused a subsequent boost in the global rate of plate tectonics.

The most recent such episode followed the "snowball Earth" that ended sometime around 635 million years ago, resulting in Earth's modern plate tectonic system.

This would suggest in a world such as yours in which coral or similar structures are practically omnipresent, tectonic activity should be at such a pace their sediments (sand) are consumed by the crust at a faster rate than our own and therefore faster than it can accumulate to such depths.

To avoid this scenario you would need either:

• A tectonically dead or near-dead world, perhaps smaller and/or older than Earth, or
• A crust that does not vary in density or composition as ours does, and so lacks the opportunity for subduction.

But another consideration is whether your oceans, which must have far higher concentrations of the building blocks of calcium carbonate (CaCO$$_3$$), would be habitable in the first place. Higher levels of CO would need to be matched by much higher levels of calcium to avoid fatal acidification. But on Earth calcium is just 400 ppm of seawater; approximately 1.2% of all dissolved minerals. A significantly higher concentration of calcium would necessarily mean much more alkaline seawater. I'm not sure this aspect of the question can be answered without knowing the concentration of calcium in the environment.

• I didn't mean habitable to humans but to corals. – rek Apr 18 '20 at 1:51