96
C. Variations of Stable Isotope Ratios in Nature
pods, for instance, seems to grow primarily in the warm temperature
range, whereas the gastropods show winter as well summer growth.
Some forms retain shell growths at low temperatures.
3) The third point seems to be the most serious one. The isotopic
composition of oxygen in an aragonite or calcite shell will remain unchanged until the shell material dissolves and recrystallizes during diagenesis. Some of the criteria by which unaltered samples might be recognized have been discussed by LOWENSTAM (1961). But the problem of
how to prove the preservation is still unsolved.
Paleotemperature techniques have been moderately successful when
applied in the climatic history of the Pleistocene. The Pleistocene epoch
appears to have been characterized by alternation between climates similar to those experienced today and colder climates.
EMILIANI (1955, 1958, 1966) has measured isotopic variations of foraminifera of ocean sediments and observed eight cyclic isotopic fluctuations in the foraminifera of cores representing about 425000 years. As
has already been mentioned, one of the chief unknown quantities in the
determination of past temperatures is the isotopic composition of the
ocean water in the geological past. SHACKLETON (1967, 1968, 1969) estimated a glacial-interglacial change in the isotopic composition of ocean
water of approximately 1.0 to 1.4%0. SHACKLETON (1968) assumed that
about one-third of the total isotopic variation observed in Caribbean
cores by EMILIANI should be attributed to temperature change and twothirds to isotopic change in the water.
b) Limestones
Isotopic data on several hundred limestone samples have been reported so far in the literature, the majority of which have a J 13 C-content
close to zero. KEITH and WEBER (1964) have stated that the average
carbon isotopic composition of marine limestones appears to be moderately constant from Cambrian through Tertiary time.
During diagenesis, marine carbonates generally become lighter in
carbon isotope composition (BAERTSCHI, 1957; GROSS, 1964). Submarine
alteration, however, may generate heavier cement than primary bioclastic carbonate (FRIEDMAN, 1964).
During early diagenesis and the following geological history most
limestones also re-equilibrate their 180-content with the surrounding
pore fluids, which are generally different from ocean water. Most carbonates become lighter with increasing age (DEGENS and EpSTEIN, 1964;
KEITH and WEBER, 1964). Variations with age in the isotopic composition of marine and freshwater limestones are shown in Fig. 35.
C. Variations of Stable Isotope Ratios in Nature
pods, for instance, seems to grow primarily in the warm temperature
range, whereas the gastropods show winter as well summer growth.
Some forms retain shell growths at low temperatures.
3) The third point seems to be the most serious one. The isotopic
composition of oxygen in an aragonite or calcite shell will remain unchanged until the shell material dissolves and recrystallizes during diagenesis. Some of the criteria by which unaltered samples might be recognized have been discussed by LOWENSTAM (1961). But the problem of
how to prove the preservation is still unsolved.
Paleotemperature techniques have been moderately successful when
applied in the climatic history of the Pleistocene. The Pleistocene epoch
appears to have been characterized by alternation between climates similar to those experienced today and colder climates.
EMILIANI (1955, 1958, 1966) has measured isotopic variations of foraminifera of ocean sediments and observed eight cyclic isotopic fluctuations in the foraminifera of cores representing about 425000 years. As
has already been mentioned, one of the chief unknown quantities in the
determination of past temperatures is the isotopic composition of the
ocean water in the geological past. SHACKLETON (1967, 1968, 1969) estimated a glacial-interglacial change in the isotopic composition of ocean
water of approximately 1.0 to 1.4%0. SHACKLETON (1968) assumed that
about one-third of the total isotopic variation observed in Caribbean
cores by EMILIANI should be attributed to temperature change and twothirds to isotopic change in the water.
b) Limestones
Isotopic data on several hundred limestone samples have been reported so far in the literature, the majority of which have a J 13 C-content
close to zero. KEITH and WEBER (1964) have stated that the average
carbon isotopic composition of marine limestones appears to be moderately constant from Cambrian through Tertiary time.
During diagenesis, marine carbonates generally become lighter in
carbon isotope composition (BAERTSCHI, 1957; GROSS, 1964). Submarine
alteration, however, may generate heavier cement than primary bioclastic carbonate (FRIEDMAN, 1964).
During early diagenesis and the following geological history most
limestones also re-equilibrate their 180-content with the surrounding
pore fluids, which are generally different from ocean water. Most carbonates become lighter with increasing age (DEGENS and EpSTEIN, 1964;
KEITH and WEBER, 1964). Variations with age in the isotopic composition of marine and freshwater limestones are shown in Fig. 35.