Oxygen Isotopes of Pedogenic Nitrate: New Paleoproxy for Precipitation in Arid regions.

Pedogenic nitrate is found in most arid and semiarid environments, usually in association ýwith atmospherically derived chloride. While the chloride mass balance techniques have ýbeen used to assess hydrologic recharge rates over time and as a soil chronometer, the use ýof nitrate as a paleoproxy has largely been ignored. This is because nitrate is not a ýconservative in the root zone, it being consumed by plants and microbes that use it as a ýsource of fixed N, and nitrate production via by oxidation of ammonium by nitrifying ýbacteria. The accumulation of pedogenic nitrate is thus a two source problem: ýAtmospheric deposition and biologic formation. I will show that the atmospheric ýcomponent of the nitrate budget has a unique 17O composition relative to ýmicrobial nitrification. This Δ17O signal is can be used to assess the ratio of ýbiogenic/inorganic nitrate accumulation. This ratio is primarily impacted by the change in ýthe biogenic component which is related to soil moisture/precipitation. Thus soil nitrate ýýΔ17O can be used a new paleoproxy for precipitation rates in arid/semi arid ýregions. ý