Stable isotopic and geochemical analysis of lacustrine carbonates from the Al-Azraq basin, Jordan: implications for paleoenvironment and paleohydrology

Abstract

This study tests the geochemical basis for the paleoenvironmental history of 31 m of cored lacustrine sediments in the Al-Azraq basin, in the eastern desert of Jordan. The analysis of carbon and oxygen isotopes of carbonates distinguishes patterns of aridity and increased moisture over greater than 250 ka. Initial analysis examined the isotope data identifying oxygen isotopes of carbonate that reflect precipitation and evaporation trends which are further supported by the paleoenvironmental proxies of ICP-AES, SEM, XRD, and magnetic susceptibility. The study identifies seven environmental zones that fluctuate between phases of aridity punctuated with periods of higher moisture and occasional aeolian deposition. The covariance of δ13Ccarb and δ18Ocarb indicates the paleolake was a closed-basin system throughout its history. Sediments from 31.69 m to the surface transition from lake to increasingly arid environments with multiple wet periods and the upper most sediments reflecting aeolian deposition determined by significant fluctuations in isotopes, grain size, and element ratios. Fe/Mn demonstrates a saw-toothed pattern for the first 10 m averaging below 100. The ratio then fluctuates substantially into the 200 and 300s in Zones F and G. The ratio of Sr/Ba averages 1.46 yet fluctuates strongly in arid zones of F and G, and to as much as 43.48 in Zone C. X-ray diffraction and SEM reveal the presence of carbonate minerals including authigenic calcite, dolomite, gypsum, and celestite in arid zones. Zones B and D signal higher moisture environments with negative δ18Ocarb values and increases in clay percent. IRSL determinations provide a sequence of three ages reaching saturation limits of greater than 250 ka at 10 m. The AZ1 lacustrine basin record has significantly older sediment ages than the fringing marsh and oasis sites. Correlation with speleothem records from local and northern Israel caves generally corresponds with MIS 9, MIS 8, and MIS 6. Correlation with the youngest age could be either MIS 5, a significant wet period recognized regionally and along the basin margins, or MIS 2, but neither are strong signals in the central basin.