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

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 NO₃⁻ (Δ¹⁷O_NO3 = Δ¹⁷O_NO3 − 0.52 δ¹⁸O_NO3) 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 NO_x via photochemical reactions, coupled with a non-mass-dependent isotope fractionation. Our Δ¹⁷O_NO3 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 Δ¹⁷O_NO3 values correspond to soils that are barren in dry periods, while low Δ¹⁷O_NO3 values correspond to more fertile soils. Coupled high Δ¹⁷O_NO3 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 δ³⁴S_SO4 and δ¹⁸O_SO4 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.