Page 78 - Earth's Climate Past and Future

54 PART II • Tectonic-Scale Climate Change

BOX 3-1 LOOKING DEEPER INTO CLIMATE SCIENCE
Organic Carbon Subcycle

early 20% of the carbon that cycles among Earth’s organic matter that consumes the remaining oxygen and
Ncarbon reservoirs today does so in organic form. allows organic debris to escape oxidation. These condi-
Photosynthesis is critical to the organic carbon subcycle, tions produce fine-grained, carbon-rich muds that even-
mainly because land plants extract CO from the atmos- tually turn into mudstones and then into harder rocks
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phere, and also because ocean plankton extract CO from called shales.
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inorganic carbon dissolved in the surface ocean. Most of The carbon buried in sediments and then rocks repre-
the organic carbon fixed and temporarily stored in land sents a net loss of CO from the interactive carbon reser-
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vegetation and ocean plankton is recycled and quickly voirs in the ocean, atmosphere, soil, and vegetation. Once
returned to the ocean-atmosphere system by means of buried, organic carbon stays in the rocks until tectonic
oxidation, which uses available oxygen in water or air to processes return it to the surface by slow-acting processes:
convert organic carbon back to inorganic form. (1) weathering (and oxidation) of carbon-bearing rocks
On land, oxidation consumes organic carbon just after at Earth’s surface and (2) thermal breakdown of organic
the seasonal fall of leaves or die-back of green vegetation carbon in rocks deep in Earth’s interior, with release of
and after the death of the woody tissue of trees. In the liberated CO through volcanoes.
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oceans, oxidation slowly consumes organic debris sinking Because this organic carbon subcycle carries one-
out of the sunlit surface layers where photosynthesis fifth of the carbon moving between Earth’s rocks and its
occurs. surface reservoirs, it has the potential to have substantial
Only a small fraction of the organic carbon formed by effects on the global carbon balance and on atmospheric
these processes is buried in the geologic record. Carbon CO over long (tectonic-scale) time intervals. Also, under
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from the land and carbon from the oceans contribute conditions that cause the onset of high productivity
roughly equal amounts to this total. Burial of organic car- and carbon burial in the ocean, large amounts of organic
bon is favored in water-saturated environments (marine carbon can be quickly extracted from the atmosphere,
or terrestrial) characterized by (1) low oxygen levels causing rapid reductions of CO levels and rapid climatic
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that minimize oxidation and (2) rapid production of cooling.

CO
2
Rapid
terrestrial Photosynthesis
C cycling
Organic carbon subcycle About 20%
of the carbon that shifts between
Earth’s surface reservoirs (air, water,
Burning and vegetation) and its deep rock
reservoirs moves in the organic carbon
Oxidation
Slow subcycle. Photosynthesis on land and
C cycling in the surface ocean turns inorganic
Photosynthesis C burial Burial carbon into organic carbon, most of
Upwelling Rapid Sinking Erosion which is quickly returned to the
atmosphere or surface ocean. A small
marine fraction of this organic carbon is
C cycling buried in continental and oceanic
CO 2
Slow release in sediments that slowly turn into rock.
C cycling volcanoes This carbon is eventually returned to
Oxidation
Burial
the atmosphere as CO , either by
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erosion of continental rocks or by
melting and volcanic emissions.