76
C. Variations of Stable Isotope Ratios in Nature
PDB. DEUSER and HUNT (1969) report o 13 e-values of oceanic inorganic
carbon between - 1 and + 2%0 and find a correlation below 200 meters
water depth between the concentration of dissolved oxygen and the o l3 e,
which is due to the oxidation of organic matter.
Fresh waters show a o l3 e-range for dissolved inorganic carbon of - 5
to -11%0 (SACKETT and MOORE, 1966). Brackish waters of estuaries
increase in l3e with salinity. Therefore, carbon isotope composition may
be used to characterize mixing of various water masses. A variable portion of bicarbonate in fresh waters is derived from biogenic sources in
the soil (VOGEL, 1959 and MUNNICH and VOGEL, 1962). Locally, ground
waters may be affected by dissolution of "heavy" limestones.
b) Sulfate in Ocean Water and Fresh Water
Sulfur Isotope Composition. Present ocean water with its large sulfate
reservoir has a fairly constant isotopic sulfur composition of + 20%0. An
interesting question is whether an isotope fractionation occurs in the
sulfate during evaporation of seawater. NIELSEN and RICKE (1964)
showed that late evaporites within the different evaporation cycles are
depleted in 34S by about 2%0 relative to the normal stages. NIELSEN (data
to be published), through experimental data, and THODE and MONSTER
(1965), through calculated data, supported this relationship. Nevertheless, the differences that might occur in the late stages may be neglected if
we consider the gypsum precipitate - brine relationship. If we assume
that evaporite sulfates preserve the 034S-values of the ancient oceans -
and there are reasonable arguments to do so - then we may conclude
that gypsum, anhydrite, and other sulfate-containing evaporite minerals
give us information about the isotopic composition of oceanic sulfate
during the geologic past.
The surprising fact is that the isotopic composition of oceanic sulfate
has not remained constant during the geologic past. The general trends
of evaporite 034S-evolution are (after NIELSEN, 1965 and THODE and
MONSTER, 1964): high o-values (+ 20 to + 30%0) in the early Paleozoic,
decreasing to + 11 %0 in Permian time, rapidly increasing in the early
Mesozoic, and later on slight oscillations around the present value of
+ 20%0. Figure 29 b shows a more detailed picture.
The mechanisms controlling the decrease in Permian time and the
abrupt increase at the Paleozoic-Mesozoic boundary are not understood. Some speculations are discussed at the end of this chapter.
The trend of o-values of sulfate evolution in the world's oceans is so
consistent, especially the very narrow range of Permian o-values, that it
has been used successfully to determine the age of unknown salt deposits
and the origin of sulfate-containing formation water (MULLER et al.,
1966, NIELSEN, 1968c).
C. Variations of Stable Isotope Ratios in Nature
PDB. DEUSER and HUNT (1969) report o 13 e-values of oceanic inorganic
carbon between - 1 and + 2%0 and find a correlation below 200 meters
water depth between the concentration of dissolved oxygen and the o l3 e,
which is due to the oxidation of organic matter.
Fresh waters show a o l3 e-range for dissolved inorganic carbon of - 5
to -11%0 (SACKETT and MOORE, 1966). Brackish waters of estuaries
increase in l3e with salinity. Therefore, carbon isotope composition may
be used to characterize mixing of various water masses. A variable portion of bicarbonate in fresh waters is derived from biogenic sources in
the soil (VOGEL, 1959 and MUNNICH and VOGEL, 1962). Locally, ground
waters may be affected by dissolution of "heavy" limestones.
b) Sulfate in Ocean Water and Fresh Water
Sulfur Isotope Composition. Present ocean water with its large sulfate
reservoir has a fairly constant isotopic sulfur composition of + 20%0. An
interesting question is whether an isotope fractionation occurs in the
sulfate during evaporation of seawater. NIELSEN and RICKE (1964)
showed that late evaporites within the different evaporation cycles are
depleted in 34S by about 2%0 relative to the normal stages. NIELSEN (data
to be published), through experimental data, and THODE and MONSTER
(1965), through calculated data, supported this relationship. Nevertheless, the differences that might occur in the late stages may be neglected if
we consider the gypsum precipitate - brine relationship. If we assume
that evaporite sulfates preserve the 034S-values of the ancient oceans -
and there are reasonable arguments to do so - then we may conclude
that gypsum, anhydrite, and other sulfate-containing evaporite minerals
give us information about the isotopic composition of oceanic sulfate
during the geologic past.
The surprising fact is that the isotopic composition of oceanic sulfate
has not remained constant during the geologic past. The general trends
of evaporite 034S-evolution are (after NIELSEN, 1965 and THODE and
MONSTER, 1964): high o-values (+ 20 to + 30%0) in the early Paleozoic,
decreasing to + 11 %0 in Permian time, rapidly increasing in the early
Mesozoic, and later on slight oscillations around the present value of
+ 20%0. Figure 29 b shows a more detailed picture.
The mechanisms controlling the decrease in Permian time and the
abrupt increase at the Paleozoic-Mesozoic boundary are not understood. Some speculations are discussed at the end of this chapter.
The trend of o-values of sulfate evolution in the world's oceans is so
consistent, especially the very narrow range of Permian o-values, that it
has been used successfully to determine the age of unknown salt deposits
and the origin of sulfate-containing formation water (MULLER et al.,
1966, NIELSEN, 1968c).