Biogeochemistry and inorganic geochemistry as indicators of the paleoenvironment and paleohydrology of the Al-Azraq Basin, Jordan

Abstract

This study investigates the biogeochemical and inorganic geochemical indicators for past environments and paleohydrology of a high resolution sediment record from the Al-Azraq Basin, Jordan. The second largest basin in Jordan, it is an important aquifer for a majority of the population in this arid region. Thirteen hundred samples were collected from fifty one meters of cored playa and lacustrine sediments. Stable isotopic composition of bulk organic matter and carbonate are the main proxies in addition to results from supporting methodologies including: grain size analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Based on the lithology of the core and geochemical data, the core is divided into three main zones: Zone 1 with six subzones; Zone 2 with four subzones; and Zone 3 with two subzones. Carbonate diagenesis by water enriched with sulfate occurs in Zone 1 and Zone 3, and affects the values of carbon isotopic compositions in some samples. Also, dolomitizition occurs in Zone 1, but does not affect the values of oxygen isotopic composition. Bulk organic matter revealed no effect of diagenesis except in Zone 3b, which impacts the carbon isotopic composition values. Mineralogy and oxygen isotopic composition revealed the presence of low lake levels punctuated by dry periods during Zone 1. C/N ratios of bulk organic matter indicate the source of organic matter primarily from aquatic algae in Zones 1a, 1b, and 1c; whereas Zone 1d indicates mixing of aquatic and terrestrial organic matter sources. This is considered a transition zone between marsh deposits and lake sediments. During this time the influx of materials to the basin increased, also indicating high precipitation and a wet climate. Zone 2 reveals changes in the climate by the presence of a lake during this period. Zone 3a exhibited the presence of marsh deposits as indicated by increasing abundant organic carbon concentrations. C/N ratios identify the presence of aquatic and land plants as the source of organic matter. Zone 3b represents a dry period from the presence of dolomite with increased sand particles and angular chert grains. This research identifies detailed environmental patterns of marsh to lake to playa environments with multiple cycles of seasonal deposition, indicative of significant climate shifts throughout the Middle to Late Pleistocene. Holocene age sediments are missing. The oxygen isotope record for the upper sediments correlate well with regional records from the eastern Mediterranean through Marine Isotope Stage (MIS 5). The base of the core dates between MIS 15 and 9 (570 to 300 ka) reflecting climates generally similar to a deglaciation.