EVOLUTION OF SOLUTES IN COASTAL SABKHA BRINES OF THE UAE

Shallow water-table brines beneath the coastal sabkhat of the United Arab Emirates are forming today in a hydrologically simple system. These circumstances allow contemporary measurement of solute and water fluxes as well as identification of boundary conditions for evaluation of solute evolution models. When formed, the Holocene coastal aquifer was filled with marine solutes and seawater but over 8000 years has evolved into system dominated by continental solutes and local rainfall. Currently over 90% of the water in the aquifer is from local precipitation while 90% of the solutes are derived from upward leakage from underlying formations, that is, solutes and water are from different sources. In this aquifer, water and solutes are transported to the surface by capillary forces where water is lost from the system as a vapor and the solutes are precipitated as salts. Rain episodically dissolves salts on the surface forming dense brines that transport both water and solute into the aquifer by density driven convection fingers. This solute-recycling process is repeated at each recharge event and when coupled with new solutes entering from below, increases the solute concentration in the aquifer with time altering the initial seawater signature. δ⁸¹Br, ²²⁶Ra isotopes and solute ratios are consistent with a continental source of solutes and ³H activities are consistent with local precipitation as a source of water; seawater is not a significant source of either water or solutes at this stage of the evolution. Rapid transgression of the aquifer into the Gulf relative to horizontal discharge fluxes creates a chemical gradient of decreasing continental solutes distally. Solutes in the aquifer are largely sodium and chloride where as the permanent solid phases are currently low solubility retrograde gypsum, anhydrite, calcite, and dolomite that form in the capillary zone beneath the surface. Because of this difference between solute composition (chloride dominated) and associated solid phase (sulfate dominated), care must be taken in estimating the paleo solutes based on analysis of solid phases of geologic deposits. That is, some of the variability of sabkha composition in the geologic record may result from capturing temporal variability associated with evolution and not necessarily reflecting different solute sources.