87
Composition of the Major Components of Seawater
compared to deep Atlantic values since the waters are older and oxidation enriches the
waters with 18 O.
The vapor pressure of HDO is lower than that of H 2 O, which means that during evaporation, H 2 O enters the vapor phase more readily than does HDO, with the result that
D becomes preferentially concentrated in the residual water. This accounts for its higher
concentrations in surface waters near the equator. In the precipitation process, the reverse
is true. The residual phase vapor becomes depleted of HDO, which is preferentially precipitated. As the raindrop falls, further isotopic enrichment can take place because of evaporation, especially in dry climates.
Deuterium analyses of coexisting ice and arctic waters showed that the ice contained 2%
more D than waters from which it was formed. In general, 18 O follows the same pattern as
D; however, since the relative masses between 18 O and 16 O are smaller, the effects are also
smaller. The 18 O/ 16 O ratio in snow is lower than in rain or sea water.
In general, the two isotopic ratios are quite variable in sea water. Both heavier isotopes are
concentrated in surface water relative to deep water, but for different reasons. Evaporation
causes the effect for D/ H, while dilution by sinking polar waters produces a decrease of
18 O/ 16 O in deep water. Since organisms prefer 16 O, older waters have a larger 18 O/ 16 O ratio
than younger waters.
2.7.4 isotopes of Sulfur
Sulfur has four stable isotopes with atomic weights 32, 33, 34, and 36. The heaviest is the
least abundantly occurring isotope. The 32 S/ 33 S ratio is 123.4 for sea water collected in
the Pacific, Atlantic, and Arctic and North and Baltic Seas. This ratio is lower than that
found in rainwater, suggesting that the SO 4
2– sulfur in precipitation is not derived from
the salt spray. The most important agents in isotopic fractionation of S were SO 4
2– -reducing
bacteria:
H 34 S – + 32 SO 4
2– = H 32 S – + 34 SO 4
2–
(2.40)
When SO 4
2– is reduced to HS – , the equilibrium will produce H 2 S that is deficient in the
heavier S isotope. This will occur only in sediments or anaerobic water, and the effect will
not alter the physical properties of sea water significantly. It is significant that SO 4
2– minerals occurring in marine evaporite deposits have the same 32 S/ 33 S ratio as sea water. The
SO 4
2– minerals associated with precipitation in anaerobic conditions have a higher ratio
(as high as 23.2 for some shales).
References and Further Reading
Aston, S.R., Estuarine chemistry, Chapter 41, Chemical Oceanography, Vol. 7, 2nd ed., Riley, J.P., and
Chester, R., Eds., Academic Press, New York, 361–440 (1978).
Borchert, H., Principles of oceanic salt deposition and metamorphism, Chapter 19, Chemical
Oceanography, Vol. 2, Riley, J.P., and Skirrow, G., Eds., Academic Press, New York, 205–276 (1965).
Brewer, P., Minor elements in sea water, Chapter 7, Chemical Oceanography, Vol. 1, 2nd ed., Riley, J.P.,
and Skirrow, G., Eds., Academic Press, New York, 416–496 (1975).
Composition of the Major Components of Seawater
compared to deep Atlantic values since the waters are older and oxidation enriches the
waters with 18 O.
The vapor pressure of HDO is lower than that of H 2 O, which means that during evaporation, H 2 O enters the vapor phase more readily than does HDO, with the result that
D becomes preferentially concentrated in the residual water. This accounts for its higher
concentrations in surface waters near the equator. In the precipitation process, the reverse
is true. The residual phase vapor becomes depleted of HDO, which is preferentially precipitated. As the raindrop falls, further isotopic enrichment can take place because of evaporation, especially in dry climates.
Deuterium analyses of coexisting ice and arctic waters showed that the ice contained 2%
more D than waters from which it was formed. In general, 18 O follows the same pattern as
D; however, since the relative masses between 18 O and 16 O are smaller, the effects are also
smaller. The 18 O/ 16 O ratio in snow is lower than in rain or sea water.
In general, the two isotopic ratios are quite variable in sea water. Both heavier isotopes are
concentrated in surface water relative to deep water, but for different reasons. Evaporation
causes the effect for D/ H, while dilution by sinking polar waters produces a decrease of
18 O/ 16 O in deep water. Since organisms prefer 16 O, older waters have a larger 18 O/ 16 O ratio
than younger waters.
2.7.4 isotopes of Sulfur
Sulfur has four stable isotopes with atomic weights 32, 33, 34, and 36. The heaviest is the
least abundantly occurring isotope. The 32 S/ 33 S ratio is 123.4 for sea water collected in
the Pacific, Atlantic, and Arctic and North and Baltic Seas. This ratio is lower than that
found in rainwater, suggesting that the SO 4
2– sulfur in precipitation is not derived from
the salt spray. The most important agents in isotopic fractionation of S were SO 4
2– -reducing
bacteria:
H 34 S – + 32 SO 4
2– = H 32 S – + 34 SO 4
2–
(2.40)
When SO 4
2– is reduced to HS – , the equilibrium will produce H 2 S that is deficient in the
heavier S isotope. This will occur only in sediments or anaerobic water, and the effect will
not alter the physical properties of sea water significantly. It is significant that SO 4
2– minerals occurring in marine evaporite deposits have the same 32 S/ 33 S ratio as sea water. The
SO 4
2– minerals associated with precipitation in anaerobic conditions have a higher ratio
(as high as 23.2 for some shales).
References and Further Reading
Aston, S.R., Estuarine chemistry, Chapter 41, Chemical Oceanography, Vol. 7, 2nd ed., Riley, J.P., and
Chester, R., Eds., Academic Press, New York, 361–440 (1978).
Borchert, H., Principles of oceanic salt deposition and metamorphism, Chapter 19, Chemical
Oceanography, Vol. 2, Riley, J.P., and Skirrow, G., Eds., Academic Press, New York, 205–276 (1965).
Brewer, P., Minor elements in sea water, Chapter 7, Chemical Oceanography, Vol. 1, 2nd ed., Riley, J.P.,
and Skirrow, G., Eds., Academic Press, New York, 416–496 (1975).