Stable isotope (chlorine, hydrogen and oxygen), geochemical and field evidence for continental fluid flow vectors in the Al-Khiran sabkha (Kuwait)

Abstract

The elevated coastal barriers of southern Kuwait minimize the effects of seawater intrusion into the Al-Khiran sabkha by focusing continental fluids into a now sedimented lagoon. This topographic–hydrologic regime is reflected in the chemistry and isotopic character of the waters, where a steady evaporative concentration gradient is recorded from inland to the coastal basins. Presently the sabkha is in a state of depositional equilibrium. Two east–west traverses were conducted orthogonal to the main facies boundaries, providing new data on δD‰, δ 18O‰ and δ 37Cl‰ isotopic compositions of the marine and coastal sabkha waters. Observations on the E–W traverse into the desert, show that samples near the tidal creek but going west increase in heavy isotopes. δD and δ 18O values for the brines range from −11 to +38‰ and −0.1 to +10.3‰, respectively. This trend is explained without invoking tidal inundation of the sabkha or excessive seawater mixing. Rather, the data fit an “ascending-brine model” whereby continental waters dominate the reflux media. The main batch of waters sampled form a group isotopically distinct from the local meteoritic water and the Neogene formation waters. They represent an apparent evaporation trend from old formation waters from the two main Kuwait aquifers (Damman Limestone and Kuwait Group gravels), with a characteristic increase in heavier isotopes. Only a narrow band of the data indicate sabkha waters with an evaporitic seawater signature (adjacent to the tidal creeks). Zones of mixing are represented by low pH and progressive gradients in chemical species (SO 4, Cl, Mg, K, Br and Ca), and the sectors within the sabkha are considered to be anomalously narrow (<100s m) by contrast with the classic Trucial Coast sabkhas (10s km). The range of δ 37Cl values, −0.5 to 0.3‰, extends well above the range predicted for simple evaporation of seawater but largely decoupled from the δD and δ 18O trends. δ 37Cl isotopic signatures suggest that dissolution of evaporites occurs in the bare sabkha facies probably in response to laterally descending ephemeral meteoritic waters. A combination of wind-transported Cl and diffusion explains the observed δ 37Cl values. This study emphasizes the important aspects of continental–marine brine interface which can be used for modelling economic and environmental aspects of sabkhas.