How would coral reefs thrive on this super-earth world?

Question

I have a super-earth world. The surface of the world is shallower due to extra gravity, so shorter mountain ranges and less ocean depths. The world orbits a binary star system at a distance of the furthest region of the habitable zone, both of the stars having masses of 0.93x that of our suns. The world is highly volcanic, approximately 15 to 16 times more volcanic than our planet. The lithosphere is 4x richer in calcium, magnesium and has much more other minerals than present on earth that is very useful for marine life such as boron and strontium.

Surface gravity: 1.35x of Earths.

Atmosphere thickness: 10x of Earths (a result of it being highly volcanic in comparison to earth.)

Atmospheric composition: Nitrogen 78%, 15% Oxygen, 5% Carbon dioxide, 2% Trace gases.

Temperature: rather hot, the tropical regions being 90 to 110 degrees Fahrenheit. (based on what I've tested on Universe Sandbox 2)

Land coverage: 75% water, and 25% land (but less water being present on this world as the oceans oceans are shallower mostly due to a lot of island formations)

Radius: 7750 kilometers

Day/Night length/full rotation: 28 hours.

Axial tilt: 53 degrees.

Orbital period: 1.15 years

Eccentricity: 0.025

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How could you make it possible for coral reefs to thrive in such a world based on the fact carbon dioxide makes the oceans acidify?

Answer 1

So, let’s be clear about the ocean acidification problem here: it’s happening really fast. As in, we’ve dropped the ocean’s pH by ~.1 pH units in the last 200 years, which is about a 30% increase in H⁺ ions because logarithms. That’s incredibly fast on both geologic and evolutionary timescales.

Corals are dying because they’re having a hard time making calcium carbonate (CaCO₃) shells/structures. That’s harder in an acidic environment because calcium carbonate is composed of calcium and carbonate, and there’s less carbonate in an acidic ocean because more of it is in the form of carbonic acid instead. You’ve actually taken the first step toward solving this problem by increasing the calcium reservoir of your planet - that alone will mitigate some effects.

However, barring any recent and dramatic atmospheric changes, your corals will already be capable of handling the atmosphere. There are some species of coral in the real world that are more acid-resistant, but on an evolutionary timescale they’re spending energy on overly-efficient enzymes to catalyze carbonate precipitation, and are largely outcompeted by less energy-intensive varieties. In your world, however, they’ll already have been selected for and will survive quite happily in the 5% CO₂ atmosphere.

An especially rainy world might have some additionally helpful effects on the health of corals, but all major influences would be limited to inland seas or areas very close to river inputs. Elsewhere, the solubility pump will work to offgas additional CO₂ from the river inputs.

Answer 2

I can think of two options:

If you want to keep lime based coral you need a higher "calcium flux", the calcium cycle on your world needs to be accelerated. This will make more calcium carbonate available to the oceans, some of that will offset the acidity from dissolving carbon dioxide but most of it will be taken up by biological processes. To do this you need to accelerate the carbonate rock cycle of the world. Luckily your atmosphere is already highly acidified due to abundant atmospheric carbon which will dissolve in rainwater to form carbonic acid; that will accelerate chemical erosion of the land and rocks of the continents. To balance that you need more geological activity, uplifting of existing coastlines and reefs above sea level and subduction of calcium rich sediments for volcanic recycling.

Alternatively you could change the chemistry a little, diatoms are a subset of oceanic plankton that use silica instead of calcium carbonate to make their hard shells, if you were to apply a similar mechanism to coral formation then far from being an issue the acidity of the ocean becomes an advantage as it will mobilise more silicon for biological uptake. Rather than limestone reefs and reef deposits now resemble quartzite.