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

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 17 O-excess in NO − 3 (∆ 17 O NO 3 = δ 17 O NO 3 − 0.52 δ 18 O NO 3) is preserved in the paleo groundwater. The 17 O-excess provides an excellent tracer of atmospheric NO − 3 , which is caused by the 5 interaction of ozone with NO x via photochemical reactions, coupled with a non-mass dependent isotope fractionation. Our ∆ 17 O NO 3 data from 0.4 to 5.0 ‰ (n = 28) indicate that up to x[NO − 3 ] atm = 20 mol % of total dissolved NO − 3 originated from the Earth's at-mosphere. High ∆ 17 O NO 3 values correspond to soils that are barren in dry periods, while low ∆ 17 O NO 3 values correspond to more fertile soils. Coupled high ∆ 17 O NO 3 10 and high x[NO − 3 ] atm values are caused by a sudden wash out of dry deposition of atmospheric NO − 3 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 δ 34 S SO 4 and δ 18 O SO 4 isotope 15 signature of dissolved sulfate, no indication is found for a superimposition by denitrifi-cation, 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.