Mass dependent processes can generate an isotope anomaly: the effect of ordinary diffusion on Δ17O in solids

Abstract

Environmental contextThe nitrate oxygen isotope anomaly has been suggested as a tool for quantifying the atmospheric input of this essential nutrient to terrestrial environments. However, it is calculated that the most important transport process in minerals, diffusion, may also be responsible for measurable anomalies. The signal in Chilean nitrate minerals could be associated with diffusion during crystal growth so the belief that the nitrate is entirely of atmospheric origin is therefore questioned. AbstractCalculated diffusion profiles assuming the classical mass dependence of diffusion coefficients indicate that important Δ17O gradients could be measured in geological samples. The belief that the MIF (mass independent fractionation) signature can be used as a tracer of atmospheric processes because mass dependent processes cannot significantly alter the signal is therefore questioned. Oxygen delta values in a three-isotope plot could lie on a straight line of slope 0.5 or 1 when diffusion is approximately a linear process. Diffusion during nitratine (NaNO3) growth could be partially responsible for measured Δ17O in nitrate deposits from Atacama Desert (Chile) and Turpan-Hami Basin (China).