酸素同位体比法による古水温の推定

抄録

The oxygen isotope technique introduced since last two decades, has become more extensively used than any other geochemical technique for paleotemperature determination. In this paper, representative studies using this technique and the principle of the method are briefly presented and problems posed directly and indirectly from this technique are discussed.
In 1947, H. C. UREY found that the ratio of stable oxygen isotopes in calcium carbonate varies with temperature at which this carbonate is produced from water. He suggested that the variation in isotope concentration can be used as a geological thermometer. UREY et al. (1951) first published a paper on paleotemperature determination. Since then many paleotemperature measurements have been published by various workers who used calcium carbonate-bearing fossil as basic materials.
Few paleotemperature data from the Paleozoic Group are available at present. Those of Mesozoic paleotemperature are given by BOWEN (1966, et al.) and many other workers. As the result of the values of the Mesozoic temperature based on the belemnite shells collected from various localities in the world, the paleotemperature trend in Europe differs from those in Australia. The Tertiary temperature is known only in Australia (DORMAN, 1966), New Zealand (DEVEREUX, 1967), and Northwest Pacific (DOUGLAS et al., 1971). The paleotemperature change in the Quaternary was discussed by many workers. Among others, EMILIANI (1966) showed the generalized climatic curve during these 450, 000 years. The middle Pleistocene temperature in the Boso Peninsula is calculated by the writer and his colleagues based on the oxygen isotopic composition of the pelecypod shell. The results give the temperature ranging from 6.5°to 19.5°C.
The carbonate-water temperature scale were formulated very accurately by CRAIG (1965), and HORIBE and OBA (1972). No problem arises in regard with the analytical techniques and the measurements of the oxygen isotope composition in order to calculate the paleotemperature.
Three problems the paleotemperature technique is now facing are : the question of isotopic preservation, “vital effect”, and the uncertainty of the isotopic composition of ancient sea water in which the fossil organisms lived.
The unaltered shells must be used as the samples of the paleotemperature measurement, because it is known that the primary oxygen isotope ratio changed during the post depositional and diagenetic processes. A few studies dealing with the problems of the original isotopic preservation are also presented.
It may be considered that the calcium carbonate as shell constituents precipitates in equilibrium with the environmental water in which the shell is growing. However, oxygen isotopic compositions are different among different genera which lived in a same condition, as reported by KEITH et al. (1964) and others. Because of highly selective physiological absorption of certain oxygen isotopes, an equilibrium between precipitated calcium carbonate in organisms and surrounding water is not possibly maintained. This disharmonious phenomenon is called “vital effect”. The values of δO¹⁸ of Glycymeris are about 0.5% lower than those of Mactra collected from the same horizons. This difference may be only due to the differences in time and rate of the precipitation of calcium carbonate between the two genera. In most cases, the “vital effect” could be explained by the inorganic process of equilibrium fractionation effect for the oxygen isotope.
The oxygen isotope ratio of marine shells is affected both by the temperature and salinity of the sea water. The oxygen isotopic composition of ancient water in which the fossil lived can not be known using the carbonate thermometer.