IB Environmental systems and societies 2.3C Carbon in Lithosphere Notes

The Lithosphere: Earth's Largest and Slowest Carbon Store

1. The lithosphere is the solid outer Earth: the crust, upper mantle, and soils.
2. It is the largest long-term carbon reservoir in the global carbon cycle.
3. It holds carbon in both organic and inorganic forms, often for hundreds of millions of years.

Carbon Is Locked Away as Fossil Fuels and Carbonate Rocks

Lithospheric carbon takes two main forms: one organic, one inorganic.

Fossil Fuels as Carbon Stores

1. Fossil fuels (coal, oil, and natural gas) are carbon-rich deposits formed from the remains of ancient plants and animals.
2. They act as long-term carbon stores, holding their carbon for hundreds of millions of years.
3. Burning them rapidly releases that carbon into the atmosphere as CO₂.

Carbonate Rocks

1. Rocks such as limestone and dolomite store carbon as calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃).
2. They form through biological processes, such as coral reef deposition, and through chemical precipitation in oceans.

Residence Time: Why Lithospheric Carbon Stays Put

1. Carbon in the atmosphere and biomass has short residence times, from years to decades.
2. Carbon in limestone and fossil fuels has extremely long residence times, hundreds of millions of years.
3. This slow cycling stabilises the global carbon balance, but burning fossil fuels releases ancient carbon rapidly and disturbs that balance.

Marine Organisms Build the World's Largest Carbon Store

1. Limestone is the largest store of carbon on Earth, trapping it as calcium carbonate.
2. Reef-building corals and molluscs build this store, one shell and skeleton at a time.
3. The process works in stages:
   1. Ion absorption: organisms take up Ca²⁺ and HCO₃⁻ ions from seawater.
   2. Calcification: they build CaCO₃ skeletons and shells.
   3. Sedimentation: when they die, their remains settle on the seafloor.
   4. Lithification: compaction and cementation over millions of years form limestone.
4. Not all limestone is biological: some forms by chemical precipitation or evaporation in shallow seas and lakes.

Formation of Fossil Fuels

1. In past geological eras, organic matter from plants and marine organisms was fossilised into coal, oil, and natural gas.
2. This took tens of millions of years and created the carbon stores we now extract and burn.

Coal: From Swamp Plants

1. Coal originates from terrestrial plants (ferns, mosses, trees) in swampy, low-oxygen environments.
2. Organic matter builds up in peat bogs, is buried by sediments, and is exposed to pressure and heat.
3. Over millions of years it converts into lignite, then bituminous, then anthracite coal.

Oil and Natural Gas: From Marine Life

1. Oil and natural gas form from marine organisms such as plankton and algae that settled on the sea floor.
2. In the Mesozoic era (252 to 66 million years ago), this organic material accumulated and slowly turned into oil and gas.
3. The oil and gas collect in porous rocks such as sandstone or limestone.

Time Scale and Irreversibility

1. Fossil fuel formation is extremely slow, taking tens to hundreds of millions of years.
2. This makes them non-renewable on human timescales, because we extract them far faster than they form.