Density-Driven Free-Convection Model for Isotopically Fractionated Geogenic Nitrate in Sabkha Brine.

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