43
carbonate system
calcium carbonate derived from skeletons or in carbonate formed in situ in
the upper water layers. Both these effects cause the CaC0 3 saturation of seawater to decrease with increasing depth. Below a critical depth, CaC0 3 is
undersaturated and tends to dissolve. The depth at which calcium carbonate
shows a rapid acceleration in dissolution is termed the lysocline. Whether or
not calcium carbonate can be preserved at certain depths is a complex function of the rate of deposition versus the rate of dissolution. The depth below
which the rate of dissolution exceeds the rate of deposition is termed carbonate compensation depth (CCD). ~carbonate, mineral.
carbonate, mineral mineral compound characterized by a fundamental anionic
structure of C0 3 -2. The common rock-forming calcium carbonate (CaC0 3 )
exists as the mineral polymorphs calcite and aragonite. Both may form as
inorganic precipitates or as biological secretions in the hard parts of organisms. Many multicellular invertebrates, lime-secreting algae and foraminifers
produce hard parts of coverings and skeletal carbonate material which
eventually become part of ocean sediments or reefs. This process depends
only on the chemistry of body fluids or enzymes and may take place even
where surrounding waters are barely saturated or undersaturated. Calcite is
one of the most common minerals and the principal constituent of limestone; it also occurs in crystalline form in marble, loose and earthy in chalk
and stalactitic in cave deposits. The calcite mineral is usually white or colorless, with a perfect rhombohedral cleavage, a vitreous lustre, and a hardness of 3 on the Mohs scale. Aragonite is the high pressure equivalent of calcite; it has a greater density and hardness, and a less distinct cleavage. It is
metastable under earth's surface conditions and therefore less common than
calcite. Many shells are formed of aragonite, which in time undergoes
recrystallization to calcite. Aragonite occurs as a deposit from hot springs
and as a major constituent of some marine shells (pteropods), shallow marine
muds and the upper parts of coral reefs. The double carbonate dolomite,
CaMg(C0 3 )2' is mainly a diagenetic mineral formed by the alteration of calcium carbonate through the action of magnesium bearing water. Dolomite
is white, colorless or tinged yellow, brown, pink or gray; it has perfect rhombohedral cleavage and a pearly to vitreous luster.
carbonate system carbon dioxide is dissolved in seawater where it is equilibrated
with several other dissolved forms, the bicarbonate ion (HC0 3 -) being very
predominant. Equilibration with atmospheric carbon dioxide would also be
expected, but in fact the surface waters of the ocean are often either over- or
undersaturated relative to the atmosphere. The various forms of carbon
dioxide (gaseous COl' H l C0 3 , HC0 3 -, COl-) in seawater are related by wellknown chemical equilibria and the overall carbonate system is acting as a
buffer for the acidity of seawater, maintaining the pH at about the 8.2 level.
Photosynthetic uptake of COl (~photosynthesis) causes the system to shift
carbonate system
calcium carbonate derived from skeletons or in carbonate formed in situ in
the upper water layers. Both these effects cause the CaC0 3 saturation of seawater to decrease with increasing depth. Below a critical depth, CaC0 3 is
undersaturated and tends to dissolve. The depth at which calcium carbonate
shows a rapid acceleration in dissolution is termed the lysocline. Whether or
not calcium carbonate can be preserved at certain depths is a complex function of the rate of deposition versus the rate of dissolution. The depth below
which the rate of dissolution exceeds the rate of deposition is termed carbonate compensation depth (CCD). ~carbonate, mineral.
carbonate, mineral mineral compound characterized by a fundamental anionic
structure of C0 3 -2. The common rock-forming calcium carbonate (CaC0 3 )
exists as the mineral polymorphs calcite and aragonite. Both may form as
inorganic precipitates or as biological secretions in the hard parts of organisms. Many multicellular invertebrates, lime-secreting algae and foraminifers
produce hard parts of coverings and skeletal carbonate material which
eventually become part of ocean sediments or reefs. This process depends
only on the chemistry of body fluids or enzymes and may take place even
where surrounding waters are barely saturated or undersaturated. Calcite is
one of the most common minerals and the principal constituent of limestone; it also occurs in crystalline form in marble, loose and earthy in chalk
and stalactitic in cave deposits. The calcite mineral is usually white or colorless, with a perfect rhombohedral cleavage, a vitreous lustre, and a hardness of 3 on the Mohs scale. Aragonite is the high pressure equivalent of calcite; it has a greater density and hardness, and a less distinct cleavage. It is
metastable under earth's surface conditions and therefore less common than
calcite. Many shells are formed of aragonite, which in time undergoes
recrystallization to calcite. Aragonite occurs as a deposit from hot springs
and as a major constituent of some marine shells (pteropods), shallow marine
muds and the upper parts of coral reefs. The double carbonate dolomite,
CaMg(C0 3 )2' is mainly a diagenetic mineral formed by the alteration of calcium carbonate through the action of magnesium bearing water. Dolomite
is white, colorless or tinged yellow, brown, pink or gray; it has perfect rhombohedral cleavage and a pearly to vitreous luster.
carbonate system carbon dioxide is dissolved in seawater where it is equilibrated
with several other dissolved forms, the bicarbonate ion (HC0 3 -) being very
predominant. Equilibration with atmospheric carbon dioxide would also be
expected, but in fact the surface waters of the ocean are often either over- or
undersaturated relative to the atmosphere. The various forms of carbon
dioxide (gaseous COl' H l C0 3 , HC0 3 -, COl-) in seawater are related by wellknown chemical equilibria and the overall carbonate system is acting as a
buffer for the acidity of seawater, maintaining the pH at about the 8.2 level.
Photosynthetic uptake of COl (~photosynthesis) causes the system to shift