OXYGEN ISOTOPES IN SOIL NITRATE: A NEW PROXY FOR PALEO-PRECIPITATION IN ARID ENVIRONMENTS?

Developing new proxies for evaluating changes in precipitation in the past is important for developing an understanding how rainfall patterns might be altered by future climate change. In arid regions, nitrate is a common anion is soil, and its oxygen isotope composition is influenced by the availability of water, therefore its isotope composition may be used as a novel precipitation proxy. Soil nitrate in arid regions has two sources: Atmospheric deposition and nitrification and these sources have very distinct oxygen isotope compositions. Atmospheric nitrate, produced by photochemical oxidation of NO_x has large mass-independent isotope compositions, with Δ¹⁷O values between 20 and 30 permil. Nitrification on the other hand, is the kinetic oxidation of ammonium by microbes, which follows normal mass dependent isotope fractionation laws and has Δ¹⁷O of zero. Nitrification is often limited by the availability soil moisture in desert environments so it decreases with increasing aridity. Soil nitrate can be viewed as a two component mixture of these two sources with the nitrification source becoming more important with increasing water availability. We will show how decreases in soil nitrate Δ¹⁷O values are correlated with mean annual precipitation in arid regions. This proxy is then applied using a 700 ky old paleo-surface in the Atacama Desert in northern Chile that was sampled to a depth of 2 meters. We observe changes in precipitation on 100 ky timescales suggesting South American precipitation regimes are modulated by Milankovitch Cycles. This new precipitation proxy may be useful in other arid to hyper-arid regions for inferring how regional scale precipitation changes under different climate forcing scenarios.