The northern limit of Qatar represents a unique depositional environment, reflecting the interplay of coastal processes and sea-level history along the windward margin of the peninsula. The coastline faces directly into prevailing winds, creating distinctive subenvironments. The ecological and geological history of this area is documented for the first time by integrating autonomous underwater vehicle (AUV) surveys and dive-based sampling and documentation of existing maritime data. An AUV collected side-scan sonar imagery, bathymetry, water temperature profiles, sub-bottom profiles and underwater photo mosaics data. Divers followed AUV transects making observations and collecting forty unconsolidated sediment and non-living coral samples. The area of study extends from 500 meters offshore from the island of Jazeerat Reken (Ras Rakan Island), clockwise for 10 kilometers southeast, offshore of the village of Al-Mafyar (Umm Tays Island). North of Jazeerat Reken the seafloor shows a steep margin from shallow subtidal to depths of approximately -7 meters. In contrast the eastern margin of the area, near Al-Mufyar shows a gentle slope, dipping to -6 meters over a 2 kilometer distance. The steep margin along the north coincides with the limits of a fringing reef system that built seaward during the Holocene. Radiocarbon age dates indicate reef growth initiated as early as 5000 years before present, when sea level was 2 to 4 meters higher than today. Two isolated sub-Recent patch reef (reef bommies) trends were discovered offshore of the main fringing reef slope, west of Jazeerat Reken. These offshore isolated patch reefs start at a water depth of about -7 meters and rise up close to present day sea level. The steepest slope of the fringing reef was observed at the northern tip of Qatar, north of Jazeerat Reken Island, where the base of the reef is at about -6 meters. Side-scan sonar imagery and sub-bottom profiling was carried out, revealing a fringing reef slope, the isolated patch reefs and the thickness of the unconsolidated sediment cover of the seabed. Samples from non-living corals were taken for radiocarbon age dating analyses. A sea-level drop approximately 2000 years before present may account for the end of fringing and patch reef growth. Since the sea-level fall, the reef top has been characterized by mobile, coarse coral-algal sands and beach-rock stabilized islands, like Jazeerat Reken. The unique grain size and shape of coral-algal beach sands provide a unique nesting ground for hawksbill turtles (Eretmochelys imbricata). The seabed east of Al-Mufyar (Umm Tays Island) is dominated by carbonate sand bars, separated by brown algae beds attached to the underlying bedrock or hardgrounds. Sediment samples were taken along a 2000 meter long transect perpendicular to the shoreline and side-scan sonar and sub-bottom profiling were carried out in this area. The results of our study provide new insights into changing carbonate platform geometries and related sediment distribution through time; controlled by sea-level fluctuations, as well as wind, tidal, wave, and current conditions. It also provides a template for conducting cost-effective, easily executed ecological and geological studies.


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