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

Abstract

Subsurface brines with high nitrate (NO 3 − ) 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), NO 3 − in the coastal sabkhas of Abu Dhabi is enriched in 15 N (mean δ 15 N ∼17‰), which is an enigma. A NO 3 − 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 Δ 17 O, δ 15 N, and δ 18 O data suggest approximately 80 to 90% of the NO 3 − 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 NO 3 − bearing water downward where it encounters reducing conditions and mixes with oxygen-free ascending geologic brines. In this environment, NO 3 − is partially reduced to nitrogen gas (N 2 ), thus enriching the remaining NO 3 − in heavy isotopes. The isotopically fractionated NO 3 − 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 NO 3 − 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 NO 3 − .