A MODEL OF OXYGEN-ISOTOPE FRACTIONATION IN BODY-WATER OF LARGE MAMMALS

A model is proposed for oxygen isotope fractionation in body water of terrestrial, herbivorous mammals larger than 1 kg. The goal of this mo del is to estimate the oxygen isotopic composition (delta(18)O) of int ake water in order to reconstruct paleoclimate from the delta(18)O of fossil biogenic phosphate. The principal oxygen inputs are liquid wate r, atmospheric O,. and oxygen in food. The principal outputs are water (liquid and vapor) and CO2. Body mass-dependent scaling equations are used to assign O-2, H2O, and CO2 fluxes. The model predicts that the delta(18)O of body water is higher than the delta(18)O of intake water and approaches the delta(18)O of intake water with increasing body si ze, as observed in empirical data. This reflects the increasing import ance of liquid water flux relative to atmospheric O-2, CO2. and water vapor flux at larger size (i.e., water flux increases relatively faste r than metabolic rate and surface area with increasing body size). The se results suggest that the largest fossil taxa should be used for pal eoclimate reconstruction because ( 1) potential errors are smallest at large body sizes and (2) drinking water forms a larger proportion of the oxygen intake. Paleoclimate reconstruction based on the delta(18)O of biogenic phosphates can thus be corrected for body-mass fractionat ion effects, a significant cause of previously uncharacterized intersp ecific variation.