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Geological Background

Carbon dioxide and the rock cycle

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Cartoon of global carbon cycle with caption reproduced from Schubert et al., 2005. (German Advisory Council on Global Change)

Before humans began changing their environment through fossil fuel use, slow processes like rock weathering, mineral and organic matter deposition, and volcanic activity controlled the concentrations of carbonate system ions in the oceans over thousands of years.  As a result, the ocean was slightly basic (Figure 2 on home page; Ridgwell and Zeebe 2005).

These slow geological processes also created deep underground deposits of petroleum and coal.  When humans began burning these materials for energy about a century ago, they essentially short-circuited the geological system to provide energy, thereby venting large amounts of CO2 into the atmosphere (Figure 1 on home page).  Currently, the CO2 released by fossil fuel use is accumulating in fast-reacting reservoirs of the atmosphere, the ocean, and vegetation (Figure 1 at right), which respond to CO2 changes over months to centuries. If the rate of carbonate sedimentation and rock formation were able to match the rate of fossil fuel consumption, the CO2 released to the atmosphere by humans would soon be cycled back into deep, long-term storage.

Because of aquatic CO2 chemistry and the ocean's size, the ocean can store a great deal of CO2.  In previous geological ages when atmospheric CO2 levels were high, the ocean absorbed CO2 and ocean pH did not change very much because rock weathering provided enough negative ions to keep pH within a certain window.  (see geological ph/CO2 figure) WHAT WERE CONSEQUENCES OF OA EVENTS IN PAST?

Today, CO2 is accumulating in the ocean and lowering ocean pH at a rate much faster than rock weathering can match. If we stopped all carbon emissions tomorrow, the ocean would still take ___ years to equilibrate with the atmosphere and stabilize ocean pH. 

Last updated: January 26, 2009

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