carbon cycle
44
and the pH to increase. In the surface ocean the concentrations of COl- and
Ca 2 + are sufficiently high for supersaturation of CaC0 3 as crystalline calcite
or aragonite (-7carbonate, mineral), allowing organisms to form calcareous
shells and platelets. Abiotic precipitation of CaC0 3 does not occur. Eventually,
the skeletal material settles to the deep-sea floor, where sometimes the
ambient concentration of COl- is sufficiently low again for CaC0 3 to be
undersaturated and the skeletons of first aragonite and then calcite dissolve
again.
carbon cycle the global transport of carbon atoms (C) between the oceans, the
atmosphere, the continents, and several other reservoirs. The overall cycling
is very complex, being driven by combinations of physical, biological, chemical, and geological forces. There exist many overlaps with the similar
cycling of other chemical elements, such as nitrogen (N), phosphorus (P),
sulfur (S), etc., notably with respect to biological processes. Major inorganic
forms of carbon are the carbon dioxide in the atmosphere and dissolved in the
oceans, as well as huge sedimentary deposits of biogenic calcium carbonate
PnotosynlNSIS
AlO"e
Green planlS
Cyanol>aC1.".
==~
--C H . -----.....::::=:~
I'~
/ Methanogenoc
I bacten.
:~~~
\ compounds)
\ \
\ "- "- "- ..........
---Carbon cycle. (After Brock and Madigan 1988)
ReSPIratiOn
"'ants
A.nuna!s
MICroorganIsms
A.,obic
Anaerobtc resptt&uon
fer"""'tlItJOn
Anaeroboe
mICroorganisms
tndud>ng
phototrnpl1ic
baCt&na