The Role of the Lithosphere in the Cycling of Carbon

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll be learning what the lithosphere is and its role in the carbon cycle for cycling both inorganic and organic carbon. We'll also examine how climate change is altering the lithosphere.

What Is the Lithosphere?

If you walk outside, what's holding you up under your feet? You might suggest the concrete, soil, or even the layers of rock beneath the Earth. These things are all part of the lithosphere. The lithosphere is the rocky, upper layer of the Earth's crust. Rocks in the lithosphere are rigid and form large slabs that make up tectonic plates. The lithosphere extends from the viscous core of the Earth, the asthenosphere, all the way to the gaseous atmosphere.

Beyond holding us upright, what are the other roles of the lithosphere? Although the rocky layers of the lithosphere might seem very different from the gaseous atmosphere, they actually are related through the carbon cycle.

Carbon is an element found in all parts of the Earth and makes up every living thing. Carbon exists in the atmosphere in its well known form of carbon dioxide. Trees take in carbon dioxide through photosynthesis, converting it to carbon based sugars that they use for food and growth. Animals eat the plants and also act as carbon sinks, storing carbon.

When living things die, their bodies decompose and carbon is returned back to the soil. Over millions of years the carbon is compressed into rock and occasionally fossil fuels like coal and oil in the lithosphere. The ocean also absorbs carbon dioxide from the atmosphere, which is incorporated into the shells of animals as calcium carbonate. Over time, these animals die and their shells become part of the ocean floor, again returning to the lithosphere over time.

Carbon Cycling in the Lithosphere

Carbon moves through the lithosphere in two forms, inorganic and organic carbon. Inorganic carbon is carbon that originates from non-living things, such as rocks. Organic carbon on the other hand is derived from living organisms, such as plants and animals.

Inorganic Carbon

Rocks might look pretty solid on the surface, but they are actually vulnerable to wear and tear just like everything else on Earth. The process of wearing away rocks is called erosion, and it starts with rainfall. Water vapor in the atmosphere combines with carbon dioxide to form carbonic acid. This slightly acidic rainfall now is capable of stripping ions from the rock, like calcium, potassium, or sodium. These ions are washed into the ocean.

The ocean also absorbs carbon dioxide, which combines with water to make carbonic acid. The calcium ions combine with carbonic acid in the ocean to make calcium carbonate, a key component of shelled organisms. When shellfish die, their calcium carbonate shells accumulate on the sea floor. Over millions of years, layers and layers of shells build up and are compressed into rock called limestone. The calcium from the shells is locked away in the limestone of the lithosphere where they are stored for millions of years.

This is the process that returns carbon to the Earth that we have put in the atmosphere through various human activities. It happens slowly because the process depends on ion availability from weathering to form the calcium carbonate for the shells. Limestone and other sedimentary rocks are essentially the long term storage of the carbon cycle.

Limestone cliffs are formed from inorganic carbon in the ocean
limestone

Organic Carbon

Organic carbon comes from living or once living organisms. When organisms die in the natural ecosystem, their bodies are first picked off by scavengers. After that decomposers like worms, maggots, bacteria, and fungi come in to break down the remains. The remaining material is recycled back into the soil. Over years, the material builds up on the forest floor, forming new layers of soil.

Under some conditions, the detritus and soil can be compressed further into fossil fuels, like coal and oil. The coal that exists today was formed hundreds of millions of years ago in pre-historic swamps. Plant matter called peat decayed and littered the swamp floor. Over time, it was compressed by other dead plants, soil, and water.

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