Articles | Volume 11, issue 12
Biogeosciences, 11, 3149–3161, 2014
Biogeosciences, 11, 3149–3161, 2014

Research article 17 Jun 2014

Research article | 17 Jun 2014

17O excess traces atmospheric nitrate in paleo-groundwater of the Saharan desert

M. Dietzel1, A. Leis2, R. Abdalla1, J. Savarino4,3, S. Morin5, M. E. Böttcher*,6, and S. Köhler1 M. Dietzel et al.
  • 1Graz University of Technology, Institute of Applied Geosciences, Rechbauerstrasse 12, 8010 Graz, Austria
  • 2Joanneum Research, Institute of Water Resources Management, Graz, Austria
  • 3CNRS, LGGE, 38000 Grenoble, France
  • 4Univ. Grenoble Alpes, LGGE, 38000 Grenoble, France
  • 5Météo-France – CNRS, CNRM-GAME UMR 3589, 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, which exceeds the World Health Organization limit for drinking water, but the origin 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 20 mol % of total dissolved NO3- originated from the Earth's atmosphere (x[NO3]atm), where the remaining NO3 refers to microbially induced nitrification in soils. 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 accumulated disposition of atmospheric NO3 on plants, soil surfaces and in vadose zones 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 mineralised groundwater as end members without considering evaporation. Using the δ34SSO4 and δ18OSO4 isotope signature of dissolved SO42−, no indication is found for a superimposition by denitrification, e.g. involving pyrite minerals within the aquifers. It is suggested that dissolved SO42− originates from the dissolution of CaSO4 minerals during groundwater evolution.

Final-revised paper