2nd August 2025

Notes in preparation of a writeup for my TOI blog on CO2 sequestration in limestone

(a placeholder post; I might add info even after the TOI piece is published)

According to NASA (the web page on “the carbon cycle”)

Most of Earth’s carbon—about 65,500 billion metric tons—is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels.

According to NASA (the web page on “slow carbon cycle”)

The movement of carbon from the atmosphere to the lithosphere (rocks) begins with rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—that falls to the surface in rain. The acid dissolves rocks—a process called chemical weathering—and releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean.

In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids and the chalky white substance that dries on your faucet if you live in an area with hard water. In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera). After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone—limestone and its derivatives.

Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale.

According to British Geological Survey

More than 99 per cent of the carbon in the carbon cycle is found in the Earth’s crust. Most of this has a biological origin, deposited on the ocean floor from the remains of the many marine creatures that use calcium carbonate in their skeletons and shells. After consolidation, these deposits may form a rock known as limestone.

According to Ian Plimer (Heaven and Earth)

1. We remove CO2 from the atmosphere into limestone, a rock that was very rare before 2500 Ma and a rock that has greatly increased in volume over time. This is why the atmospheric CO2 has been decreasing over time and will continue to decrease.

2. Most of the planet’s CO2 is held in rocks and the smallest amount of planetary CO2 is currently held in the atmosphere.

3. The oceans continually remove dissolved CO2 by shell formation, limestone formation and chemical reactions with rocks and sediments. The more CO2 dissolves in the oceans, the more CO2 is removed.

4. [Oceans] are rich in floating photosynthetic microorganisms that remove CO2 from the atmosphere and water. Floating organisms in the oceans also remove calcium carbonate to build shells. Shells accumulate as fossil-bearing sediments and limestones. Chemical reactions between seawater and submerged rocks remove CO2 from the oceans. This is why the oceans stay alkaline.

5. The world’s oceans contain about 39,000 billion tonnes of carbon; soils, vegetation and humus contain about 2000 billion tonnes of carbon, and carbonate rocks such as limestone contain 65,000,000 billion tonnes of carbon. There is more carbon in soil than the total amount of carbon in the atmosphere and living matter. Arctic soils especially are a huge sink of carbon. The atmosphere contains only 0.001% of the total carbon present in the atmosphere-ocean-upper crust system. This figure is probably an underestimate.

6. Over the last 4567 million years, the Earth has degassed about half of its estimated CO2 by geological processes. This CO2 has not been lost to space, it is stored in rocks (such as limestone) and life.

7. Burial of carbon compounds was accelerated at about 400 Ma after the evolution of terrestrial vascular plants. Forests grew quickly, there was a removal of CO2 from the atmosphere and carbon was not recycled as atmospheric CO2 because it was buried as coals, carbonaceous sediments, limey sediments and limestone reefs. There were times, such as the Carboniferous, when there was an explosion of plant life on Earth. There was a massive removal of CO2 from the atmosphere, further oxygenation and storage of recycled carbon in Northern Hemisphere coals.

Ian Plimer’s article in The Australian 2009

The original source of atmospheric CO2 is volcanoes. The Earth’s early atmosphere had a thousand times the CO2 of today’s atmosphere. This CO2 was recycled through rocks, life and the oceans.

Through time, this CO2 has been sequestered into plants, coal, petroleum, minerals and carbonate rocks, resulting in a decrease in atmospheric CO2.

The atmosphere now contains 800 billion tonnes of carbon as CO2. Soils and plants contain 2000 billion tonnes, oceans 39,000 billion tonnes and limestone 65,000,000 billion tonnes. The atmosphere contains only 0.001 per cent of the total carbon in the top few kilometres of the Earth.

From Geology: A Complete Introduction by David Rothery

calcium carbonate is the main rock-forming mineral in many sedimentary environments. The main reason for its ubiquity is biological, rather than physical or chemical, processes. A rock made mostly of calcium carbonate is called limestone, which is much the most abundant non-silicate rock type. Deposition of calcium carbonate is an important part of the rock cycle – calcium that was dissolved during weathering and the bicarbonate by-product of hydrolysis come out of solution in tandem, maybe thousands of kilometres from where the calcium was dissolved. Calcium carbonate is most commonly found as the mineral known as calcite, but there is also a polymorph called aragonite.

Seashells found on the beach are made of calcium carbonate, and some limestones are made of accumulations of such shells, or of shell fragments. More important globally are microscopic (generally single-celled) plants and animals that live in the sea. Most drift around near the sea surface where there is plenty of sunlight. These are referred to as plankton. Less abundant forms live on the sea floor. The most important animal forms are foraminiferans, which are simple organisms similar to the amoeba except for having multi-chambered calcareous shells, commonly up to about a millimetre across. The most important plant variety is the coccolithophores. These are smaller and, rather than having a simple shell, they are covered in an array of disc-like plates called coccoliths, typically only 20 micrometres in diameter, that separate after death.

When microscopic plankton like these die, their shelly remains sink slowly towards the sea floor. If conditions are gentle enough, they can settle to form extensive deposits. England’s famous White Cliffs of Dover are cut into a 70-million-year-old accumulation of coccoliths that was deposited on the floor of a calm, shallow sea that had no major rivers discharging muddy sediments into it (otherwise the cliffs would not be so white). This rather special kind of limestone is called chalk. Deposits of fine-grained calcium carbonate derived from the shells of marine plankton cover much of the ocean floor in places far from land where there is little other sediment supply.

… Coral reefs are familiar to many people through marine wildlife television documentaries. If the substrate they have colonized is subsiding, they can build upwards so as to keep pace and thereby maintain the living part of the reef close to sea level. Charles Darwin realized this in his explanation for coral atolls, which are rings of reef made of robust, wave-resistant coral sheltering a shallow lagoon that hosts more delicate corals (Figure 9.13). Drilling has shown that the volcanic basement to some atolls has sunk to more than 1000 m below sea level. Mururoa atoll in the Pacific (which achieved notoriety in the mid-1990s as the site of French underground nuclear testing) is of this type.

From Geology by James Geikie (2022)

Molluscs, crustaceans, corals, and the like, secrete from the ocean the carbonate of lime of which their hard shells and skeletons are composed, and these hard parts go to the formation of limestone. The most remarkable masses of modern limestone occur within intertropical regions. These are the coral reefs of the Pacific and Indian Oceans.

From Encyclopedia of Geography (Barney Warf, ed. 2010)

Fossiliferous limestone is formed from fossil shells and coral and is the most common
biochemical sedimentary rock.

2. The slow precipitation of limestone (CaCO3) on the ocean floor provides the only long-term storage for carbon and forms the largest pool of carbon on earth.

3. Today, [CO2] makes up more than 98% of the atmospheres on Mars and Venus but only 0.03% of that of Earth. The missing atmosphere is found sequestered into rocks, such as the limestone rocks that cover an eighth of the planet’s land surface, and in all the other organic rocks and sedimentary layers. Most limestone rocks are constructed of small shells. It is life that has buried Earth’s excess CO2.

From Geological History of Earth by Jaheim Hackett

The high eustatic sea level and warm climate of the Cretaceous meant a large area of the
continents was covered by warm shallow seas. The Cretaceous was named for the
extensive chalk deposits of this age in Europe, but in many parts of the world, the
Cretaceous system consists for a major part of marine limestone, a rock type that is
formed under warm, shallow marine circumstances. Due to the high sea level there was
extensive accommodation space for sedimentation so that thick deposits could form.
Because of the relatively young age and great thickness of the system, Cretaceous rocks
crop out in many areas worldwide.

Youtube video

Typically on an annual basis only about 0.03 gigatonnes of carbon is extracted from the atmosphere and goes into limestone which goes into long-term geologic storage. It adds up there and has ended up to be the largest carbon reservoir of all on our planet. But day to day year by year it’s a very small drawdown. Nonetheless numbers add up even when they’re small and even at that slow rate the drawdown of CO2 from our atmosphere by shell building organisms … would completely exhaust the atmosphere of CO2 in less than a million years.

Peter Ward et al

In their 2002 book, The life and death of planet Earth, Peter Ward and Donald Brownlee showed how the overall biological productivity of the planet was higher in the past and will come to an end in 200-500 million years from now.

WHY? BECAUSE OF THE END OF CO2 FROM THE ATMOSPHERE.