Question

Can you explain how carbon is cycled between the hydrosphere and geosphere?

Answer 1

Recall that the hydrosphere entails all forms of water on the Earth and the geosphere entails soil, rocks and minerals on, above or below the surface of the Earth. We know that atmospheric $CO_2$ is dissolved in water droplets that fall to the surface of the Earth in the form of precipitation. In such a case, determine the compound that forms as a result of such dissolution, and how it reacts when incident on land as well as surface water bodies, and how such compounds disintegrate after some time to release $CO_2$ back into the atmosphere.

Complete answer:
Let us begin by first understanding what the terms carbon cycle, hydrosphere and geosphere mean.
Carbon cycle is essentially a biogeochemical cycle through which carbon is exchanged between the biosphere, pedosphere, geosphere, hydrosphere, and the atmosphere of the Earth. It consists of a sequence of events that describe the movement of carbon and its compounds as they are recycled and reused throughout the biosphere, and occurs cyclically with time.
Now, the hydrosphere, as the name suggests, is the combined mass of water found on, under or above the surface of the Earth, and includes surface water, ice, groundwater and water vapour. The geosphere on the other hand, refers to the solid earth containing soil, rocks (lithosphere) and minerals. From the interior to the exterior, the solid earth consists of the interior and exterior core, the mantle and the crust. The thin layer at the top of the crust is called the pedosphere.
Now, the carbon cycle consists of exchange of carbon between the various systems of the earth, but we focus only on the cycling of carbon between the hydrosphere and the geosphere.
Precipitation or rain water falling from the atmosphere has some atmospheric carbon dioxide molecules dissolved in its water droplets. Some of the carbon dioxide molecules in the rain react with the water droplets to form carbonic acid that falls with the rain. The carbonic acid that falls to the surface of the Earth dissolves rocks by the process of chemical weathering, where it releases calcium, magnesium, potassium or sodium ions. These ions are carried by rivers to the ocean. The carbonic acid that falls directly into surface water bodies reacts with the water to form bicarbonate ions.
Now, the oceans have an influx of calcium and bicarbonate ions from the above two mechanisms. The calcium ions combine with bicarbonate ions to form calcium carbonate (limestone) that settles or sediments over time into rocks. However, most of the calcium carbonate in the oceans these days is made from shell-building organisms and planktons. After such organisms die, they sink to the seafloor overtime and the layers of skeletons, shells and sediments get cemented together and turn into rocks that form the geosphere, storing carbon.
This carbon is slowly returned from the geosphere to the atmosphere and the hydrosphere. Since rocks melt under extreme heat and pressure during volcanic eruptions caused by collision between moving crustal plates that consist of the land and ocean surfaces, the heated rocks react with silicate rocks to release carbon dioxide back into the atmosphere. However, the carbon is returned from rocks into the hydrosphere when the carbonic acid from the rain dissolves carbonate rocks, releasing carbon dioxide.
This is how carbon gets cycled between the hydrosphere and the geosphere.

Note:
The chemical reaction by which carbon dioxide gives carbonates, bicarbonates and carbonic acid is called as carbonation, whereas the process by which carbon and its compounds are liberated from rocks (or the geosphere) is called degassing, and may occur directly during volcanism or indirectly by weathering of sedimentary rocks. Thus, carbonation and degassing form the essential processes constituting the cycling of carbon between the atmosphere, hydrosphere and geosphere.