Stable isotope studies and the hydrological regime of sabkhas in southern Kuwait, Arabian Gulf

Abstract

The marine sabkhas in Al-Khiran, Kuwait were formed by sedimentary offlap and relative fall in sea level since the last Holocene transgressive maximum ∼ 4500–4700 years BP. In the absence of significant marine flooding of the coastal sabkhas today (due to the presence of chenier ridges), their hydrological regime is almost entirely controlled by sub-surface continental groundwater influx. This topographic-hydrologic framework is also reflected in the isotopic data, and to some extent in the chemistry of the groundwaters, where the major ions show a steady evaporative concentration from inland to coast and from coast to sabkha, with a “high” chloride plateau in the central parts of the marine sabkha. Monitoring of the groundwater table (GWT) over a three-year period indicates that: (a) when the GWT depths are < 1.2 m, the sabkha is in a state of depositional equilibrium; (b) GWT fluctuations (due to drawdown or recharge) exceed 0.5 m yr −1; and (c) calculations based on measured fall in GWT levels, porosities of the sediments and the volume of anhydrite formed in the sabkha suggest that in the last 4700 years since progradation commenced, only about 2–4 cm yr −1 (net) of groundwater is lost from the sabkha surfaces—a surprisingly low rate for this arid region. This is interpreted as due to recharge of the marine sabkha (hosted by marine sediments) by significant lateral flow of continental groundwater from the regional system—a process that maintains long-term stability of the sabkhas and without which deflationary processes would dominate. The oxygen and deuterium isotopic compositions of the marine and continental sabkha waters form a group ( δ 18 O=0 ± 2‰ ; δD = − 10 ± 7‰) isotopically distinct from local meteoric water and Pleistocene formation waters. They represent evaporation of the old formation waters, with a characteristic increase in the heavier isotopes. Only sabkha waters in areas of tidal flooding (the narrow creek areas) can be shown to be evaporated seawater. The δ 18 O and δD values of the residual brine increase to about +6‰ and +23‰, respectively, when they reverse and are arrested at the halite precipitation point. Only a minor amount of mixing between these two evaporated waters could take place. Isotopic analyses of sulphur and oxygen in dissolved sulphate of sabkha waters and precipitated mineral sulphate confirm a minor contribution of seawater sulphate ( δ 34 S = +19 ± 2‰ ; δ 18 O = + 11‰ ) in the tidal areas, but show that the bulk of the marine sabkha water sulphate ( δ 34 S = + 14 ± 2‰ ; δ 18O + 14 ± 1%.) and coexisting sulphate minerals is dominantly of continental origin. At Al-Khiran, Gulf of Sirte (Libya) and in Abu Dhabi (United Arab Emirates), δ 34S values of sabkha sulphate decrease to +14‰ on the landward side: close to the values between + 13 and + 16‰ for continental sabkhas and continental getch (gypsum). This pervasive homogeneous sulphate source may be recycled particulate sulphate distributed by major dust storms. These results have important implications in evaporite sedimentology.