The implication of the oxygen isotope records in coexisting cherts and phosphates

Abstract

δ¹⁸O values have been determined for 53 coexisting chert-phosphate pairs from the Phanerozoic and Precambrian. In addition, δ¹⁸O values were also determined for 18 biogenic phosphates from the Phanerozoic. The oxygen isotope data for the chert-phosphate pairs permit the calculation of the temperatures of formation of the chert-phosphate pairs as well as the δ^(18)O of the waters in which they equilibrated. In turn the calculated δ¹⁸O of the waters permit the calculation of temperatures from the isotopic data of the biogenic phosphates. The temperatures for the Tertiary and Mesozoic samples are compatible with the literature values determined from oxygen isotope data for well-preserved calcareous marine skeletal remains. The temperatures calculated from the chert-phosphate pairs of the Paleozoic are usually higher and vary in cycles of about a 150 million year period. The early Precambrian temperatures are as high as 80°C and decrease with time. The use of chert-phosphate pairs for temperature determinations are sufficiently promising to warrant further investigations of this type on well preserved chert-phosphate or carbonate-phosphate pairs.

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