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Gypsum Stromatolites From Sawda Nathil: A Geological Relict From Salinas Along the Fourth Coastline of Qatar

Abstract

Sawda Nathil is one of a series of inland depressions (inland sabkhas and salinas) that extend nearly continuously along the Southern Qatar border with Saudi Arabia. Six to eight thousand years ago, these depressions were marine embayments that separated the peninsula of Qatar from the mainland. These embayments in-filled rapidly with dune sands, blown southeastward across Qatar. Since then, they have become progressively more evaporitic. Inland depressions like Sawda Nathil host a set of unique environments within Qatar. Most depressions are close to or below sea level, resulting in near-surface water tables in the driest parts of Qatar. Evaporation to salt saturation creates thick gypsum and halite 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, and inland sabkhas and salinas. Core material of four short push cores was photographed, sedimentologically described, and sampled for petrographic thin-section and X-ray diffraction (XRD) analyses. Radiocarbons (AMS) as well as optically stimulated luminescence (OSL) age-dating were carried out on three samples. Scanning electron microscopy (SEM) was used to characterize samples from a gypsum stromatolite. Radiocarbon dating of marine shells provides ages of approximately 6600 un-calibrated 14C years before present (yr BP), coinciding with a well-documented sea-level highstand, approximately 2 to 4 meters higher than present. During that time, Qatar was mostly an island, only connected in the South with the Arabian Peninsula by a narrow isthmus. SEM examination of gypsum stromatolites show gypsum crystals developing in close spatial association with microbial biofilms. Whether this is a purely passive microbial-influenced gypsum mineralization process or an example where 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.