Limestone is the largest reservoir of geologic carbon storage in the Earth's crust. Laid down over hundreds of millions of years in ancient oceans, limestone is also a regulator of Earth's climate over geologic time scales.
Atmospheric carbon dioxide dissolves into water, including ocean and rain water, producing carbonic acid. Although carbonic acid can quickly etch limestone rock, fizzing on contact if a strongly acidic solution is applied, carbonic acid also etches hard silicate rock, albeit slowly. Rainfall slowly corrodes the granite rock of the continents.
As continental silicate rock erodes by the action of carbonic acid, slowly over millions of years, this chemical form of weathering carries dissolved mountain minerals like calcium by river to the sea. That calcium helps to fertilize the growth of coral-building animals, mollusks, and plankton that form calcium-carbonate shells around themselves, in the oceans of the world.
Organisms that make calcium carbonate shells pull both calcium and dissolved carbonic acid (mostly as bicarbonate) out of seawater to build solid mineral armor. Over time, dead shells of animals and plankton cells fall to the seafloor and build up as carbonate sediment, eventually limestone.
Over immense stretches of time CO2 from the atmosphere becomes slowly sequestered, bit by bit, into geologic strata as limestone. But the atmosphere today still contains CO2, and plants continue to grow. Limestone formation pulls CO2 from the air slowly, so air should be empty of CO2 in only about a million years by that process. What gives? How is CO2 replenished?
Volcanoes. Earth's thousands of surface and seafloor volcanoes constantly emit CO2 back into the atmosphere, much from subduction-related volcanoes where dead, old ocean crust rides under continental crust and melts, driving up magma filled with CO2 from heat-destroyed organic and carbonate sediments on top of the subducting ocean crust. Volcanic release of CO2 is slow and small worldwide on an annual basis, but over the ages their grinding, unstoppable contributions add up.
Across geologic time, the balance of limestone formation and volcanic return of CO2 to the atmosphere creates a planet-wide thermostat: the carbonate-silicate geochemical cycle.
When the Earth warms from some other source, like the Sun brightening for example, warmer air evaporates more water from the ocean and drives more intense precipitation, generally. More precipitation leads to faster chemical weathering of continental rock, which leads to faster delivery of calcium to the oceans, which draws more CO2 out of the air and cools the planet incrementally as more limestone is formed.
If the world cools for an extended geological time-period because of external factors, like the Sun dimming or something like that, the carbonate-silicate cycle will tend to drive CO2 up slowly, because a cooler world has slower rock-weathering processes and makes limestone more slowly, allowing volcanic CO2 to build up and warm the world.
Across the eons this process will eventually bring about the end of the Earth's biosphere. As the Sun ages it expands slowly and brightens slowly, and as it does so it delivers progressively more heat to Earth. The carbonate-silicate geochemical cycle responds over geologic timescales by slowly driving air CO2 downward as the Earth's climate is largely homeostated within a fairly narrow temperature range, while the Sun relentlessly brightens. This has been the case on our planet for billions of years.
Eventually this process must come to an end. The Sun will continue to brighten and air CO2 will continue to diminish over the oncoming geologic ages.
This has nothing to do with modern concerns of anthropogenic climate change, by the way. The carbonate-silicate geochemical cycle responds over timescales longer than the human species has existed; millions to hundreds of millions of years.
Even so, eventually air CO2 will be driven by planetary processes to near-zero, and plants will fail to thrive, and life on our world will diminish, and the biosphere as we know it now will slowly wind down. This will not happen for at least two-hundred to five-hundred million years from now, so this fate is not our near concern. Yet still this knowledge does teach us much of how planets operate, here and across the universe.
Learning about this fact of our world happened only because people were studying how climate behaves and is changed, mostly out of concerns for current anthropogenic climate heating. Science works like that. In searching for one question many other questions can be posed or answered.
Musical credits:
Opening/closing theme: "Air Hockey Saloon" by Chris Zabriskie. Check out his amazing music at freemusicarchive.org/music/Chr...
Volcano theme: "Tannhauser Gate" by Tri-Tachyon, whose unbelievable music can be located at freemusicarchive.org/music/Tri...