17O excess traces atmospheric nitrate in paleo-groundwater of the Saharan desert
Résumé
Saharan paleo-groundwater from the Hasouna area of Libya contains up to 1.8mM 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 NO−3 (117ONO3 = 17ONO3−0.52 18ONO3 ) is preserved in the paleo-groundwater. The 17O excess provides an excellent tracer of atmospheric NO−3 , which is caused by the interaction of ozone with NOx via photochemical reactions, coupled with a non-mass-dependent isotope fractionation. Our 117ONO3 data from 0.4 to 5.0‰ (n = 28) indicate that up to 20 mol% of total dissolved NO−3 originated from the Earth's atmosphere (x[NO−3 ]atm), where the remaining NO−3 refers to microbially induced nitrification in soils. High 117ONO3 values correspond to soils that are barren in dry periods, while low 117ONO3 values correspond to more fertile soils. Coupled high 117ONO3 and high x[NO−3 ]atm values are caused by a sudden wash-out of accumulated disposition of atmospheric NO− 3 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 34SSO4 and 18OSO4 isotope signature of dissolved SO2−4 , no indication is found for a superimposition by denitrification, e.g. involving pyrite minerals within the aquifers. It is suggested that dissolved SO2−4 originates from the dissolution of CaSO4 minerals during groundwater evolution.
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