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

[摘要] Saharan paleo-groundwaterfrom the Hasouna area of Libya contains up to 1.8 mM of nitrate, whichexceeds the World Health Organization limit for drinking water, but theorigin is still disputed. Herein we show that a positive ¹⁷O excess inNO₃⁻ (Δ¹⁷O_NO3 =Δ¹⁷O_NO3 − 0.52 δ¹⁸O_NO3) ispreserved in the paleo-groundwater. The ¹⁷O excess provides an excellenttracer of atmospheric NO₃⁻, which is caused by the interaction ofozone with NO_x via photochemical reactions, coupled with anon-mass-dependent isotope fractionation. OurΔ¹⁷O_NO3 data from 0.4 to 5.0 ‰ (n = 28)indicate that up to 20 mol % of total dissolved NO₃^- originatedfrom the Earth's atmosphere (x[NO₃⁻]_atm), where theremaining NO₃⁻ refers to microbially induced nitrification in soils.High Δ¹⁷O_NO3 values correspond to soils that arebarren in dry periods, while low Δ¹⁷O_NO3 valuescorrespond to more fertile soils. Coupled highΔ¹⁷O_NO3 and high x[NO₃⁻]_atmvalues are caused by a sudden wash-out of accumulated disposition ofatmospheric NO₃⁻ on plants, soil surfaces and in vadose zones withinhumid–wet cycles. The individual isotope and chemical composition of theHasouna groundwater can be followed by a binary mixing approach using thelowest and highest mineralised groundwater as end members without consideringevaporation. Using the δ³⁴S_SO4 and δ¹⁸O_SO4 isotope signature of dissolved SO₄²⁻, noindication is found for a superimposition by denitrification, e.g. involvingpyrite minerals within the aquifers. It is suggested that dissolvedSO₄²⁻ originates from the dissolution of CaSO₄ minerals duringgroundwater evolution.

[学科分类] 地球化学与岩石