Oxygen isotope variation in bone phosphate

Abstract

Temperature dependent fractionation of O isotopes occurs between water and phosphate of fish bones, mammal bones and the phosphatic fraction of mollusc skeletons. In all these cases the fractionation follows the paleotemperature equation of Longinelli and Nuti. In molluscs and in fish the δ 18O of the skeleton (δ p) depends on the δ 18O of the environmental water (δ w) and on ambient temperature. Thus, as in other isotopic paleothermometers, estimation of δ w is necessary for temperature calculation. Mammal δ p, which is not affected by changes in ambient temperature, is useful for estimating δ w. Simple linear relations exist between δ p of experimental rats, their body water isotopic compositions (δ BW) and δ w of their drinking water. Body water composition depends on δ w of intake water as well as on the rate of metabolism. When the metabolic rate is high compared to the rate of drinking, bone phosphate will not be very sensitive to changes in δ w of the environmental water. In free ranging deer a significant amount of the intake water is water contained in leaves that the deer eat. In arid environments leaf water becomes highly enriched in δ 18O due to isotopic effects of evaporation. This enrichment is reflected in the bone phosphate. Modern people consume significant amounts of water from sources other than those in their immediate living environment. Consequently, their bone phosphate is not useful for estimating the isotopic composition of environmental water. However, this was not true for ancestral and more recent man, and therefore the δ p of skeletons might be useful for tracing past human migrations.