Geologic Carbon Cycle
The geologic cycle is primarily driven by the movement of the Earth's tectonic plates and associated geologic processes such as chemical weathering. Geologic processes operate very slowly. For example, the North American and European Tectonic Plates are moving away from each other at a speed of a couple of centimeters a year - about as fast as fingernails grow. The geologic carbon cycle has been operating continuously throughout geologic time - over 4.5 billion years. Before life on earth, it was the only carbon cycle. Once green plants began using atmospheric CO2 for photosynthesis, the much faster biological cycle had a greater impact on levels of atmospheric CO2 than the slow geologic cycle. However, the geologic cycle is still important because it operates continuously, and is also the primary source of natural atmospheric CO2.
Carbon dioxide is released to the atmosphere from volcanic activity such as eruptions of active volcanoes and CO2 gas seeps. Some of this CO2 is absorbed by the oceans through direct gas exchange at the surface, a process controlled by sea surface temperatures. Cold water absorbs CO2 from the atmosphere, whereas warm water may actually give off CO2.
Additional atmospheric CO2 becomes part of the chemical weathering cycle, combining with rainwater to form carbonic acid. Although carbonic acid is a very weak acid, it slowly dissolves many rocks, especially those which contain calcium carbonate, such as limestone and marble. This is the same the process which slowly removes the lettering on marble headstones.
During periods of mountain-building, such as the uplift of the Himalayas, the weathering cycle uses up huge amounts of atmospheric CO2 in the process of eroding the newly formed mountains. The carbon, in the form of carbonic acid, combines with minerals at the surface to form insoluble carbonate compounds. These are washed into the ocean by erosion and eventually form into limestone. The formation of limestone rock sequesters carbon for millions of years until it is eventually exposed at the earth's surface to be dissolved in another weathering cycle.
In the past, there were periods when volcanoes were much more active and vast quantities of CO2 were vented to the atmosphere. Levels of atmospheric CO2 sometimes reached levels that were over 100 times higher than today's CO2 levels. Before green plants, the excess CO2 was eventually absorbed by the slow action of the geologic cycle, a process that could take tens of millions of years. Once the biological carbon cycle began to operate, excess atmospheric CO2 from volcanic activity was removed much faster.