In the ocean, over half the carbonate rock is created as coral reef. Coral reefs are definitely not a net carbon sink. Even though they do remove carbon from the ocean, their growth actually releases carbon to the atmosphere. The chemistry of this is well known to carbonate petrologists, but it is a source of a persistent popular myth.
The ocean is a pH buffered system. Conservation of electrical charge dictates that for every molecule of bicarbonate in seawater that is converted to carbonate upon conversion to reef rock, another molecule of bicarbonate decomposes to carbonic acid (ocean acidification!), but then must be released to the atmosphere as CO2 to balance the charge of the reaction. The balanced equation says that for every atom of carbon that is sequestered in carbonate rock, one is released to the atmosphere, and the ocean is incrementally acidified.
Note that this natural source of atmospheric carbon is fifty times smaller than fossil fuel inputs for a unit time in the modern era.