100
C. Variations of Stable Isotope Ratios in Nature
the dolomite was deposited as metastable protodolomite, the protodolomite-water fractionation should be considerably smaller than the dolomite-water fractionation (FRITZ and SMITH, 1970; TARUTANI et aI.,
1969).
However, the question of isotopic equilibrium might not be relevant
in this connection. If we assume that dolomite is always formed by the
replacement of calcium carbonate, then the oxygen isotope composition
of the dolomitizing solutions should be of great importance. Dolomites
formed at an early diagenetic stage should be similar or slightly enriched
in 18 0 relative to the precursor calcite, since during early diagenesis pore
solutions should be similar to ocean water. Late diagenetic solutions
should be more variable in isotopic composition; therefore, the late
diagenetic dolomites should be more variable, too (BAUSCH and HOEFS,
1972).
Appreciable variations in I3C/l2C ratios have also been found in dolomites (from +21 to -64%0, MURATA et at., 1967; DEUSER, 1970). The
light values are directly related to the oxidation of organic matter; the
heavy ones could be indirectly related (through equilibration of light
methane with heavy carbon dioxide). SPOTTS and SILVERMAN (1966) were
able to demonstrate the production of small dolomite crystals around oil
droplets, which were formed during the oxidation of organic matter.
3. Phosphates
As has already been pointed out by UREY et al. (1951), the development of another temperature scale using calcium carbonate and another
oxygen compound precipitated by organisms, i.e., phosphates, would
permit a temperature scale independent of the oxygen isotopic composition of ocean waters.
LONGINELLI (1965, 1966) analyzed co-precipitated carbonate and
phosphate and used the carbonate calibration to obtain a tentative calibration of the phosphate-water paleothermometer. LONGINELLI and
NUTI (1968a) suggested that fossil marine organisms containing phosphatic material preserved the original isotopic composition moderately
well and that the phosphatic material was deposited under isotopic equilibrium conditions. However, in this connection it must be considered
that phosphorous is a key element in all biological processes. The phosphate metabolic cycle is so complicated that it seems impossible to
exclude an isotope exchange due to "life processes".
LONGINELLI and NUTI (1968b) measured the l80rO ratios of phosphorites from marine formations of different geological ages. They interpreted their results as showing that marine phosphorites of Upper Tertiary age were probably formed under isotopic equilibrium conditions.
C. Variations of Stable Isotope Ratios in Nature
the dolomite was deposited as metastable protodolomite, the protodolomite-water fractionation should be considerably smaller than the dolomite-water fractionation (FRITZ and SMITH, 1970; TARUTANI et aI.,
1969).
However, the question of isotopic equilibrium might not be relevant
in this connection. If we assume that dolomite is always formed by the
replacement of calcium carbonate, then the oxygen isotope composition
of the dolomitizing solutions should be of great importance. Dolomites
formed at an early diagenetic stage should be similar or slightly enriched
in 18 0 relative to the precursor calcite, since during early diagenesis pore
solutions should be similar to ocean water. Late diagenetic solutions
should be more variable in isotopic composition; therefore, the late
diagenetic dolomites should be more variable, too (BAUSCH and HOEFS,
1972).
Appreciable variations in I3C/l2C ratios have also been found in dolomites (from +21 to -64%0, MURATA et at., 1967; DEUSER, 1970). The
light values are directly related to the oxidation of organic matter; the
heavy ones could be indirectly related (through equilibration of light
methane with heavy carbon dioxide). SPOTTS and SILVERMAN (1966) were
able to demonstrate the production of small dolomite crystals around oil
droplets, which were formed during the oxidation of organic matter.
3. Phosphates
As has already been pointed out by UREY et al. (1951), the development of another temperature scale using calcium carbonate and another
oxygen compound precipitated by organisms, i.e., phosphates, would
permit a temperature scale independent of the oxygen isotopic composition of ocean waters.
LONGINELLI (1965, 1966) analyzed co-precipitated carbonate and
phosphate and used the carbonate calibration to obtain a tentative calibration of the phosphate-water paleothermometer. LONGINELLI and
NUTI (1968a) suggested that fossil marine organisms containing phosphatic material preserved the original isotopic composition moderately
well and that the phosphatic material was deposited under isotopic equilibrium conditions. However, in this connection it must be considered
that phosphorous is a key element in all biological processes. The phosphate metabolic cycle is so complicated that it seems impossible to
exclude an isotope exchange due to "life processes".
LONGINELLI and NUTI (1968b) measured the l80rO ratios of phosphorites from marine formations of different geological ages. They interpreted their results as showing that marine phosphorites of Upper Tertiary age were probably formed under isotopic equilibrium conditions.