Biogeosciences Discuss., 10, 20079-20111, 2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper is under review for the journal Biogeosciences (BG).
17O-excess traces atmospheric nitrate in paleo groundwater of the Saharan desert
M. Dietzel1, A. Leis2, R. Abdalla1, J. Savarino3,4, S. Morin5, M. E. Böttcher6,*, and S. Köhler1,**
1Graz University of Technology, Institute of Applied Geosciences, Rechbauerstrasse 12, 8010 Graz, Austria
2Joanneum Research, Institute of Water Resources Management, Graz, Austria
3CNRS, Institut National des Sciences de l'Univers, France
4Laboratoire de Glaciologie et de Géophysique de l'Environnement, Université Joseph Fourier, Grenoble, France
5Météo-France – CNRS, CNRM-GAME URA 1357, CEN, Grenoble, France
6Biogeochemistry Department, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany
*now at: Leibniz Institute for Baltic Sea Research, Geochemistry & Isotope Geochemistry Group, 18119 Warnemünde, Germany
**now at: Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden

Abstract. Saharan paleo groundwater from the Hasouna area of Libya contains up to 1.8 mM of nitrate, the origin of which is still disputed. Herein we show that a positive 17O-excess in NO317ONO3 = δ17ONO3 – 0.52 δ18ONO3) is preserved in the paleo groundwater. The 17O-excess provides an excellent tracer of atmospheric NO3, which is caused by the interaction of ozone with NOx via photochemical reactions, coupled with a non-mass dependent isotope fractionation. Our Δ17ONO3 data from 0.4 to 5.0‰ (n = 28) indicate that up to x [NO3]atm = 20 mol % of total dissolved NO3 originated from the Earth's atmosphere. High Δ17ONO3 values correspond to soils that are barren in dry periods, while low Δ17ONO3 values correspond to more fertile soils. Coupled high Δ17ONO3 and high x [NO3]atm values are caused by a sudden wash out of dry deposition of atmospheric NO3 on plant or soil surfaces within humid-wet cycles. The individual isotope and chemical composition of the Hasouna groundwater can be followed by a binary mixing approach using the lowest and highest mineralized groundwater as end-members without considering evaporation. Using the δ34SSO4 and δ18OSO4 isotope signature of dissolved sulfate, no indication is found for a superimposition by denitrification, e.g. involving pyrite minerals within the aquifers. It is suggested that dissolved sulfate originates from the dissolution of calcium sulfate minerals during groundwater evolution.

Citation: Dietzel, M., Leis, A., Abdalla, R., Savarino, J., Morin, S., Böttcher, M. E., and Köhler, S.: 17O-excess traces atmospheric nitrate in paleo groundwater of the Saharan desert, Biogeosciences Discuss., 10, 20079-20111, doi:10.5194/bgd-10-20079-2013, 2013.
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