The Slow and Fast Carbon Cycles

Last updated over 3 years ago

10 questions

Objective: Students can describe the cycling of carbon among the hydrosphere, atmosphere, geosphere, and biosphere and explain how the burning of fossil fuels impacts the carbon cycle.

Carbon is the backbone of life on Earth. We are made of carbon, we eat carbon, and our civilizations—our economies, our homes, our means of transport—are built on carbon. We need carbon, but that need is also entwined with one of the most serious problems facing us today: global climate change.
Forged in the heart of aging stars, carbon is the fourth most abundant element in the Universe. Most of Earth’s carbon—about 65,500 billion metric tons—is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels.
Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth.
Over the long term, the carbon cycle seems to maintain a balance that prevents all of Earth’s carbon from entering the atmosphere (as is the case on Venus) or from being stored entirely in rocks. This balance helps keep Earth’s temperature relatively stable, like a thermostat.
This thermostat works over a few hundred thousand years, as part of the slow carbon cycle. This means that for shorter time periods—tens to a hundred thousand years—the temperature of Earth can vary. And, in fact, Earth swings between ice ages and warmer interglacial periods on these time scales. Parts of the carbon cycle may even amplify these short-term temperature changes.

How does the carbon cycle impact Earth’s temperature?

Has Earth’s temperature stayed relatively stable since it's formation or has it changed? Explain.

The Slow Carbon Cycle

Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On average, 1,013 to 1,014 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, human emissions of carbon to the atmosphere are on the order of 1,015 grams, whereas the fast carbon cycle moves 1,016 to 1,017 grams of carbon per year.

The movement of carbon from the atmosphere to the geosphere begins with rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—that falls to the surface in rain. The acid dissolves rocks—a process called chemical weathering—and releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean.  
You observed this process in our weathering and erosion lab! 

In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids.  In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms such as corals and plankton.  After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone—limestone and its derivatives.

Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale.

Which aspect of the carbon cycles moves the most carbon around? Slow carbon cycle, fast carbon cycle or anthropogenic (human) activities? Use evidence from the reading to support your answer.

Fossil fuels are coal, oil, and natural gas. Because they are made of dead things, they have a lot of carbon in them. Why do you think we call them fossil fuels?

What role do volcanoes play in the carbon cycle?

The Slow Carbon Cycle (Continued) 

The slow cycle returns carbon to the atmosphere through volcanoes. Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide.
When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.

The slow carbon cycle also contains a slightly faster component: the ocean. At the surface, where air meets water, carbon dioxide gas dissolves in and ventilates out of the ocean in a steady exchange with the atmosphere. Once in the ocean, carbon dioxide gas reacts with water molecules to release hydrogen, making the ocean more acidic.
Before the industrial age, the ocean vented carbon dioxide to the atmosphere in balance with the carbon the ocean received during rock weathering. However, since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Over millennia, the ocean will absorb up to 85 percent of the extra carbon people have put into the atmosphere by burning fossil fuels, but the process is slow because it is tied to the movement of water from the ocean’s surface to its depths

Which process adds more carbon dioxide to the atmosphere: volcanic eruptions or anthropogenic (human) emissions?

Draw a model of the slow carbon cycle.

In your model, be sure to include carbon moving from the biosphere, geosphere, atmosphere, hydrosphere

Use the reading to help you. Things your model should also probably include to help you-
ocean
plate tectonics
volcanoes
decay of living things into fossil fuels
formation of limestone at the bottom of the ocean acid rain

The Fast Carbon Cycle

The time it takes carbon to move through the fast carbon cycle is measured in a lifespan. The fast carbon cycle is largely the movement of carbon through life forms on Earth, or the biosphere. Between 1015 and 1017 grams (1,000 to 100,000 million metric tons) of carbon move through the fast carbon cycle every year.

Carbon plays an essential role in biology because of its ability to form many bonds—up to four per atom—in a seemingly endless variety of complex organic molecules. Many organic molecules contain carbon atoms that have formed strong bonds to other carbon atoms, combining into long chains and rings. Such carbon chains and rings are the basis of living cells. For instance, DNA is made of two intertwined molecules built around a carbon chain.


The bonds in the long carbon chains contain a lot of energy. When the chains break apart, the stored energy is released. This energy makes carbon molecules an excellent source of fuel for all living things.

Plants and phytoplankton are the main components of the fast carbon cycle. Phytoplankton (microscopic organisms in the ocean) and plants take carbon dioxide from the atmosphere by absorbing it into their cells. Using energy from the Sun, both plants and plankton combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen.
Four things can happen to move carbon from a plant and return it to the atmosphere, but all involve the same chemical reaction:
Plants break down the sugar to get the energy they need to grow.
Animals (including people) eat the plants or plankton, and break down the plant sugar to get energy.
Plants and plankton die and decay (are eaten by bacteria) at the end of the growing season.
Fire consumes plants. In each case, oxygen combines with sugar to release water, carbon dioxide, and energy.

In all four processes, the carbon dioxide released in the reaction usually ends up in the atmosphere. The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the Earth is breathing.

Draw a model of the fast carbon cycle.

In your model, be sure to include carbon moving from the biosphere, geosphere, atmosphere and hydrosphere

Use the reading to help you. Things your model should probably include to help you:
photosynthesis in plants
animals eating plants/other animals
fire
seasonal changes in atmospheric carbon dioxide plants and animals dying decomposing

What connections can you make to what you learned about the carbon cycle here and the carbon cycle dice game we played in class? Did you spend more time in the fast or slow carbon cycle? Did you get to experience both?