Sawda Nathil is one of a series of inland depressions that extend nearly continuously from Umm Bab along the Saudi Arabia-Qatar border to Khor Al Adid. Six to eight thousand years ago, these depressions were marine embayments that separated the peninsula of Qatar from the Kingdom of Saudi Arabia. Narrow land bridges connected Qatar, restricting the migration of people and animals. These embayments infilled rapidly with dune sands blown southward from Qatar. Since then, they have become progressively more evaporitic. Inland depressions like Sawda Nathil represent unique environments of Qatar. Most depressions are close to or below sea level. This brings groundwater to the surface in the driest parts of Qatar. Evaporation to salt saturation creates thick gypsum and salt crusts (sabkhas), as well as shallow hypersaline ponds (salinas) with spectacular microbial gypsum stromatolites. The present ground surface is a mosaic of relict marine facies, deflated dune sands, inland sabkhas, and salinas. The study area is located along the eastern margin of the Sawda Nathil depression. Four shallow cores were taken at the sabkha area surrounding the salina. A total of 30 sediment samples and an additional 5 samples from the stromatolitic salina deposits were collected for thin section and X-ray diffraction (XRD) analyses. Radiocarbon (AMS) and optically stimulated luminescence (OSL) age-dating were carried out on three samples. Scanning electron microscope (SEM) analyses was carried out on samples from a gypsum stromatolite. Radiocarbon and optically stimulated luminescence dates for marine shells and the enclosing sediment provide ages of approximately 6,000 years before present (yr BP) and coincide with a well-documented highstand, when sea level was 2 to 4 meters higher than present. These beach deposits are interpreted to represent remnants of the post-glacial Holocene transgression which began about 18,000 years ago and reached its highest level during the Late Holocene about 6,000 years ago. The sabkhas (gypsum and salt flats) and salinas (saline ponds) are younger, associated with infilling of the embayments, related to a drop of sea level to present day. SEM examination of gypsum stromatolite samples show gypsum crystals developing in close spatial association with microbial biofilms (filamentous structures). Whether this is a purely passive microbially-influenced gypsum mineralization process or whether the microorganisms actively control the mineralization process in order to obtain ecological advantages, remains to be evaluated. Studying and documenting different types of microbial sedimentary structures preserved in gypsum is of particular interest, not only in the field of petroleum geology, but also in the field of exobiology. Whereas carbonate minerals are quantitatively the most important sediment for preserving morphological biosignatures on Earth, the most relevant chemical sediments on Mars are likely sulfate minerals. Gypsum has been widely detected on Mars and is interpreted to have formed under evaporitic conditions broadly similar to those characterizing terrestrial sabkha and salina environments of Qatar.


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