86
Chemical Oceanography, 4th Edition
to local tap water. This was unsatisfactory since differences occur in the D content of tap
waters. Some workers, in their 18 O/ 16 O work on natural waters, used the average of their
isotopic data for a number of deep- water samples taken from the Atlantic, Pacific, and
Indian Oceans as their standard. This, however, is not completely satisfactory. Harmon
Craig pointed out the desirability of a standard reference for both D and 18 O analyses. He
determined the D/ H ratio for a set of ocean waters similar to the ones chosen by earlier
workers for their 18 O data. He suggested that both D and 18 O data could be specified in
terms of the National Bureau of Standards (NBS) isotopic reference samples No. 1 and
No. 1-A. These are distilled water samples of large volumes, intended as a cross- check calibration to mass spectrophotometry labs. After consultation, Craig (1961) defined standard
mean ocean water (SMOW) in terms of the NBS reference sample:
D/ H(SMOW) = 1.050 D/ H(NBS) –1
(2.37)
18 O/ 16 O(SMOW) = 1.008 18 O/ 16 O(NBS)
(2.38)
It was suggested that isotopic data for both isotopes be reported as parts per mille.
Enrichment δ relative to defined SMOW is given by
δ = [(R SAMPLE / R SMOW ) – 1]1000
(2.39)
where the values of R are isotopic ratios. SMOW is thus defined in terms of an actual water
reference standard NBS-1 and provides a convenient and consistent zero reference level
for reporting enrichment.
2.7.2 Deuterium
Ocean waters have widely different D/ H ratios. Equatorial waters are richer in D than
waters in high latitudes. D follows the same trend with depth as 18 O; that is, surface waters
contain relatively more than deep waters. D/ H varies from 6410 to 6536 for the Atlantic,
Pacific, and Indian Oceans. In summary, high- latitude waters tend to contain less D than
equatorial waters, and deep waters contain less D than surface waters.
2.7.3 Oxygen-18
The 18 O/ 16 O ratios of a number of freshwaters derived from snow, rain, lakes, and rivers as
well as the oceans have been examined. The 18 O of freshwaters varies considerably; thus,
18 O in the oceans varies depending on how much freshwater is in the source. Melted snow
can have values of 18 O 3.5 times greater than the waters from the Mississippi. For waters
collected in the Alaska and California currents, the low salinity and high 18 O content are
due to the melting of snow and ice.
Deep waters contain less 18 O than surface samples. The differences are greater than
would be expected from evaporation. The deep water is diluted by sinking polar waters,
with a consequential lowering of 18 O. Mediterranean waters (surface and deep) showed a
much smaller difference because the high evaporation rate causes less fractionation of the
isotopes, which in turn implies that freshwaters entering the ocean are less depleted in 18 O
than in the other cases discussed. The deep waters of the Pacific have higher 18 O values
Chemical Oceanography, 4th Edition
to local tap water. This was unsatisfactory since differences occur in the D content of tap
waters. Some workers, in their 18 O/ 16 O work on natural waters, used the average of their
isotopic data for a number of deep- water samples taken from the Atlantic, Pacific, and
Indian Oceans as their standard. This, however, is not completely satisfactory. Harmon
Craig pointed out the desirability of a standard reference for both D and 18 O analyses. He
determined the D/ H ratio for a set of ocean waters similar to the ones chosen by earlier
workers for their 18 O data. He suggested that both D and 18 O data could be specified in
terms of the National Bureau of Standards (NBS) isotopic reference samples No. 1 and
No. 1-A. These are distilled water samples of large volumes, intended as a cross- check calibration to mass spectrophotometry labs. After consultation, Craig (1961) defined standard
mean ocean water (SMOW) in terms of the NBS reference sample:
D/ H(SMOW) = 1.050 D/ H(NBS) –1
(2.37)
18 O/ 16 O(SMOW) = 1.008 18 O/ 16 O(NBS)
(2.38)
It was suggested that isotopic data for both isotopes be reported as parts per mille.
Enrichment δ relative to defined SMOW is given by
δ = [(R SAMPLE / R SMOW ) – 1]1000
(2.39)
where the values of R are isotopic ratios. SMOW is thus defined in terms of an actual water
reference standard NBS-1 and provides a convenient and consistent zero reference level
for reporting enrichment.
2.7.2 Deuterium
Ocean waters have widely different D/ H ratios. Equatorial waters are richer in D than
waters in high latitudes. D follows the same trend with depth as 18 O; that is, surface waters
contain relatively more than deep waters. D/ H varies from 6410 to 6536 for the Atlantic,
Pacific, and Indian Oceans. In summary, high- latitude waters tend to contain less D than
equatorial waters, and deep waters contain less D than surface waters.
2.7.3 Oxygen-18
The 18 O/ 16 O ratios of a number of freshwaters derived from snow, rain, lakes, and rivers as
well as the oceans have been examined. The 18 O of freshwaters varies considerably; thus,
18 O in the oceans varies depending on how much freshwater is in the source. Melted snow
can have values of 18 O 3.5 times greater than the waters from the Mississippi. For waters
collected in the Alaska and California currents, the low salinity and high 18 O content are
due to the melting of snow and ice.
Deep waters contain less 18 O than surface samples. The differences are greater than
would be expected from evaporation. The deep water is diluted by sinking polar waters,
with a consequential lowering of 18 O. Mediterranean waters (surface and deep) showed a
much smaller difference because the high evaporation rate causes less fractionation of the
isotopes, which in turn implies that freshwaters entering the ocean are less depleted in 18 O
than in the other cases discussed. The deep waters of the Pacific have higher 18 O values