Density-driven free-convection model for isotopically fractionated geogenic nitrate in sabkha brine

Subsurface brines with high nitrate (NO₃⁻) concentration are common in desert environments as atmospheric nitrogen is concentrated by the evaporation of precipitation and little nitrogen uptake. However, in addition to having an elevated mean concentration of ∼525 mg/L (as N), NO₃⁻ in the coastal sabkhas of Abu Dhabi is enriched in ¹⁵N (mean δ¹⁵N ∼17‰), which is an enigma. A NO₃⁻ solute mass balance analysis of the sabkha aquifer system suggests that more than 90% of the nitrogen is from local atmospheric deposition and the remainder from ascending brine. In contrast, isotopic mass balances based on Δ¹⁷O, δ¹⁵N, and δ¹⁸O data suggest approximately 80 to 90% of the NO₃⁻ could be from ascending brine. As the sabkha has essentially no soil, no vegetation, and no anthropogenic land or water use, we propose to resolve this apparent contradiction with a density-driven free-convection transport model. In this conceptual model, the density of rain is increased by solution of surface salts, transporting near-surface oxygenated NO₃⁻ bearing water downward where it encounters reducing conditions and mixes with oxygen-free ascending geologic brines. In this environment, NO₃⁻ is partially reduced to nitrogen gas (N₂), thus enriching the remaining NO₃⁻ in heavy isotopes. The isotopically fractionated NO₃⁻ and nitrogen gas return to the near-surface oxidizing environment on the upward displacement leg of the free-convection cycle, where the nitrogen gas is released to the atmosphere and new NO₃⁻ is added to the system from atmospheric deposition. This recharge/recycling process has operated over many cycles in the 8000-year history of the shallow aquifer, progressively concentrating and isotopically fractionating the NO₃⁻.