Factors Affecting the Oxygen Isotope Ratio of Plant Organic Material

This chapter outlines the current understanding of the factors affecting δ 18 O p , formalizes this understanding in mathematical models and then, describes potential applications of the technique. The oxygen isotope ratio of soil water taken up by a plant depends, to a first approximation, on the temperature of droplet formation for rain and snow falling at the site. Precipitation becomes more depleted in 18 O as temperature decreases at higher latitudes and altitudes. There is also an amount effect, with high precipitation sites having more depleted water than would be expected from site temperatures. The temperature dependence of δ 18 O of precipitation (618Or), and its record of variation in plant tissue have been recognized for some time, and prompted Libby et al. to suggest that it may be possible to use cellulose from annual tree rings as “isotopic thermometers.”