Carbon Stores in the Lithosphere
- The lithosphere is the solid outer layer of the Earth composed of the crust, upper mantle, and soils.
- It is one of the most significant long-term carbon reservoirs in the global carbon cycle.
- It stores carbon in both organic and inorganic forms
- These stores hold carbon for long periods, often hundreds of millions of years.
1. Fossil Fuels as Carbon Stores
- Fossil fuels (coal, oil, and natural gas) are carbon-rich deposits formed from the remains of ancient plants and animals.
- Over millions of years, organic material buried under heat and pressure transformed into these fuels, which now store large amounts of carbon.
- When coal is burned for energy, the carbon that was stored in the Earth's lithosphere is quickly released, contributing to global carbon emissions.
- The carbon in fossil fuels is locked away for hundreds of millions of years.
- Fossil fuels act as long-term carbon stores until they are extracted and burned, which releases the stored carbon back into the atmosphere as CO₂.
2. Carbonate rocks
- Rocks such as limestone and dolomite store carbon in the form of calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃).
- These rocks are formed through biological processes (e.g., coral reef deposition) and chemical precipitation in oceans.
In some geological events, such as when limestone is subjected to intense heat during mountain building, it can release CO₂ through processes like calcination, contributing to the carbon cycle over much longer time scales.
NoteThe carbon in limestone can remain stored for hundreds of millions of years.
Carbon Residence Time
Residence time in carbon cycle
Residence time is the average duration that a carbon atom remains in a particular store before moving to another part of the carbon cycle.
- Carbon in the atmosphere and biomass has short residence times (years–decades).
- Carbon in limestone and fossil fuels has extremely long residence times (hundreds of millions of years).
This slow cycling of carbon in the lithosphere helps stabilize the global carbon balance over geological time, but human exploitation of fossil fuels releases this ancient carbon rapidly, disturbing the natural equilibrium.
Biological Deposition of Calcium Carbonate
- Marine organisms such as reef-building corals and molluscs contribute to long-term carbon storage through the formation of calcium carbonate structures.
- Their process involves:
- Ion absorption: organisms take in Ca²⁺ and HCO₃⁻ ions from seawater.
- Calcification reaction: formation of CaCO₃ skeletons or shells.
- Sedimentation: upon death, remains settle on the seafloor.
- Lithification: compaction and cementation over millions of years form limestone.
Limestone as the Largest Carbon Store
Limestone
Limestone is a sedimentary rock composed primarily of CaCO₃, formed through both biological and chemical precipitation processes.
