74
C. Variations of Stable Isotope Ratios in Nature
1969; PICCIOTTO et aI., 1960; DANSGAARD and TAUBER, 1969; ALDAZ
and DEUTSCH, 1967; LORIUS et al., 1969 and others). The measurement of
D and 18 0 is commonly used to distinguish the annual layers of ice.
DANSGAARD et al. (1969) have used the 180-concentration in a Greenland ice sheet to deduce the climatic changes during the past
100000 years. The mean isotopic composition of a section of an ice core
reflects the average climatic conditions existing at the surface during the
period of time when the original deposit was formed (increasing 15 18 0
along the core corresponds to warming climate and decreasing 15 18 0
corresponds to cooling climate). Gow and EpSTEIN (1972) demonstrated
that different ice types (e.g. glacial ice and fresh-water ice formed from
desalinated sea water) can be distinguished by stable isotope analysis.
Fractionations due to freezing-point differences playa minor role in
producing variations on a geographic scale, because the differences
produced are not so great compared with differences due to vapor pressure differences, and because ice is relatively immobile and usually melts
not far from the place in which it forms.
FRIEDMAN et al. (1964) discussed changes in the isotope concentration of water undergoing any change of state. The fractionation factor
for deuterium between ice and water has been estimated and determined
experimentally by several authors (e.g., O'NEIL, 1968; ARNASON, 1969).
From these results we may deduce that ice will contain 2% more deuterium than the water from which it is freezing.
1. Ocean Water
Ocean water exhibits a very narrow range in isotopic composition,
less than 1 % for JD and 1%0 for 15 18 0. However, evaporation processes
strongly affect the isotope composition, since they cause a preferential
depletion in the lighter isotopes, which become enriched in the vapor
phase. Consequently, the remaining water will be heavier, i.e., highly
saline waters generally have the greatest D and 18 0 content. Low salinities, which are caused by a dilution of fresh waters, correlate with low D
and 18 0 concentrations (EpSTEIN and MAYEDA, 1953; REDFIELD and
FRIEDMAN, 1965). There is one interesting feature that is difficult to
explain: Deep-water masses originating in the Northern hemisphere,
definitely contain a slightly larger amount of deuterium than those of
Antarctic origin (REDFIELD and FRIEDMAN, 1965). All factors governing
isotopic variations in ocean water have been discussed extensively by
CRAIG and GORDON (1965).
Secular changes in the isotopic composition of the oceans have occurred as a result of the variable extent of continental glaciation. The
maximum effect during the Pleistocene age has been estimated by var-
C. Variations of Stable Isotope Ratios in Nature
1969; PICCIOTTO et aI., 1960; DANSGAARD and TAUBER, 1969; ALDAZ
and DEUTSCH, 1967; LORIUS et al., 1969 and others). The measurement of
D and 18 0 is commonly used to distinguish the annual layers of ice.
DANSGAARD et al. (1969) have used the 180-concentration in a Greenland ice sheet to deduce the climatic changes during the past
100000 years. The mean isotopic composition of a section of an ice core
reflects the average climatic conditions existing at the surface during the
period of time when the original deposit was formed (increasing 15 18 0
along the core corresponds to warming climate and decreasing 15 18 0
corresponds to cooling climate). Gow and EpSTEIN (1972) demonstrated
that different ice types (e.g. glacial ice and fresh-water ice formed from
desalinated sea water) can be distinguished by stable isotope analysis.
Fractionations due to freezing-point differences playa minor role in
producing variations on a geographic scale, because the differences
produced are not so great compared with differences due to vapor pressure differences, and because ice is relatively immobile and usually melts
not far from the place in which it forms.
FRIEDMAN et al. (1964) discussed changes in the isotope concentration of water undergoing any change of state. The fractionation factor
for deuterium between ice and water has been estimated and determined
experimentally by several authors (e.g., O'NEIL, 1968; ARNASON, 1969).
From these results we may deduce that ice will contain 2% more deuterium than the water from which it is freezing.
1. Ocean Water
Ocean water exhibits a very narrow range in isotopic composition,
less than 1 % for JD and 1%0 for 15 18 0. However, evaporation processes
strongly affect the isotope composition, since they cause a preferential
depletion in the lighter isotopes, which become enriched in the vapor
phase. Consequently, the remaining water will be heavier, i.e., highly
saline waters generally have the greatest D and 18 0 content. Low salinities, which are caused by a dilution of fresh waters, correlate with low D
and 18 0 concentrations (EpSTEIN and MAYEDA, 1953; REDFIELD and
FRIEDMAN, 1965). There is one interesting feature that is difficult to
explain: Deep-water masses originating in the Northern hemisphere,
definitely contain a slightly larger amount of deuterium than those of
Antarctic origin (REDFIELD and FRIEDMAN, 1965). All factors governing
isotopic variations in ocean water have been discussed extensively by
CRAIG and GORDON (1965).
Secular changes in the isotopic composition of the oceans have occurred as a result of the variable extent of continental glaciation. The
maximum effect during the Pleistocene age has been estimated by var-