Geoscience Reference
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found in particular in the Late Precambrian of China, are chemical precipitates and seem
to be associated with episodes of global glaciation. In low-temperature waters, phosphates
form numerous complexes and, as indicated by the dissociation reactions above, speciation
is pH-dependent. Phosphorus concentration in seawater and river water is limited by the
very low solubility of apatite. Phosphate radicals often attach themselves to the surface of
iron oxyhydroxide colloids when they precipitate in estuaries.
Phosphorus occurs principally in the center of oxygen tetrahedra. In seawater, P is one
of the essential nutrients: in the Krebs cycle, adenosine triphosphate (ATP) is the major
energy repository for cells, while ribose phosphates form the building blocks of nucleic
acids DNA and RNA. Phosphate is brought to seawater by rivers and is removed, though
only after many cycles across the thermocline, with the hard parts of biogenic debris. It is
severely depleted in surface water but is regenerated during the dissolution of falling debris
in deep water. It is depleted in young North Atlantic Deep Water with respect to the older
Antarctic Bottom Water. Diagenetic dissolution, transport, and reprecipitation of apatite is
common in sediments.
13.10 Carbon
Carbon is not an extremely abundant component of the Earth. It is both siderophile (in its
reduced form) and atmophile (in its oxidized form). Substantial amounts of this element
may be dissolved in the core. In the mantle, carbon occurs as graphite and, at depth in
excess of about 120 km, as diamond when the conditions are reducing. In oxidizing condi-
tions, carbon occurs as carbon dioxide, which, at depths of about 70-100 km, reacts with
mantle silicates to form carbonates, e.g:
Mg 2 SiO 4 +
CO 2
+
MgCO 3
MgSiO 3
(13.1)
(olivine)
(magnesite)
(pyroxene)
Carbon dioxide solubility in magmas rapidly changes with pressure and therefore depth.
Carbon dioxide outgassing from mantle-derived magmas starts at a depth of approximately
60 km and quickly strips the magma of many volatile species, such as rare gases, well
before eruption. Carbon dioxide is found as fluid inclusions in olivine phenocrysts (the
prime target for He isotope measurements) and makes up a very important component of
the gas phases in mid-ocean ridge basalts and ocean island basalts.
Oxidized carbon in the crust occurs as sedimentary carbonates, mostly calcite which is
two to three times more abundant than dolomite. The reduced forms are countless, from
crystalline graphite to amorphous organic varieties (coal, oil, kerogen, methane). Carbon
dioxide is an important component of metamorphic gases and, in particular, is the dominant
species in fluids from the granulite facies. In ground-, river-, and sea-water, carbon occurs
as the carbonate oxyanions CO 2 3 and HCO 3 , but traces of soluble organic components
(humic and fulvic acids) are ubiquitous. Carbon dioxide makes up about 350 ppm per
volume of the atmospheric gases.
 
 
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