The slow carbon cycle involves five key stages in the movement of carbon around the cycle that takes place over many tens and hundreds of millions of years.
1. The transfer of carbon into the oceans from the atmosphere and land surface
Direct CO2 absorption as part of the atmosphere-ocean exchange is supplemented by the erosion of carbon-rich terrestrial surfaces as naturally-acidic rainfall dissolves surface rocks and transfers soluble bicarbonate compounds, via rivers, to the sea.
2. The deposition of carbon compounds on the ocean floor
Marine plants (including phytoplankton) absorb CO2 and marine creatures take in carbon to construct skeletons and shells. Phytoplankton are consumed by zooplankton and their carbon-rich excrement falls to the ocean floor. The skeletal and shell remains of marine creatures also fall the sea bed.
3. The conversion of ocean sediments into carbon-rich rock
On continental shelves carbon-rich accumulations of deposits may be converted into carbon-rich rocks (such as chalk and limestone) or become contained as concentrations within sandstones and shales to form organic deposits, some of which become fossil-fuel reserves in time. This process of sedimentary rock formation is called lithification.
4. The transfer of carbon rocks to tectonic margins
As sedimentary rocks are created by heat and pressure over millions of years, they are also moved in the direction their crustal plate is moving. If they eventually become a collision margin, they may be uplifted to become surface mountain ranges (as in the Himalayas). The carbon-rich strata may then be exposed to weathering and erosion to return to the ocean as eroded carbonate rocks.
5. The return of carbon compounds to the atmosphere in volcanic eruptions
At subduction zones, carbon-rich rocks may be ejected at the surface from volcanic eruptions, usually in the form of gaseous compounds into the atmosphere. Here, CO2 contributes to the formation of carbonic acid in clouds, which then begins the process of solution of surface rocks and a starting of the terrestrial component again, or being absorbed by ocean surfaces for the marine component.
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