Spatial Variability in Sediment Composition and Seawater Properties along a transect in Eastern Offshore Abu Dhabi

Sedimentology of coastal carbonate sediments and evaporites of Abu Dhabi has been intensively studied. However, information about deeper subtidal deposits and seasonal variations of water properties is more limited. This investigation explores the sediment composition of six cores and water column properties (temperature, salinity, dissolved oxygen, pH) along a 150 km long SSE-NNW transect in offshore eastern Abu Dhabi, with water depths ranging from 11 to 45 meters. Situated in the southeastern Arabian Gulf, the transect includes diverse oceanographic and sedimentological zones, serving as a crucial benchmark for understanding seasonal variability and regional environmental dynamics.

CTD surveys conducted in August 2024 captured temperature, salinity, pH, and dissolved oxygen profiles during the warmest part of the seasonal cycle. Temperature profiles revealed stratification of the distal water body, with surface temperatures reaching 36°C and a thermocline between 10 and 28 meters. The proximal water body showed no temperature stratification and uniform temperatures from 35-36°C from the coast to 80 km offshore and down to 28m water depth. Salinity ranged from 39 PSU offshore to 45 PSU nearshore, with a uniform surface layer and increasing stratification with depth. Dissolved oxygen peaked at 5 ml/l in surface waters, declining to 2 ml/l at the bottom, indicating significant stratification but no hypoxia in the deep-water body. Surface pH values were high (above 8.45), decreasing gradually to 8.2 in bottom waters.

The grain size distribution of the cores reveals a correlation between core penetration depth and sediment characteristics. Coarse-grained sediments yielded shorter cores (10–20 cm), while muddy sediments at the deepest site produced a 50 cm core. Grain size analysis identified a progression of mean grain size from fine and medium sand in shallower cores to coarse sand further offshore and muddy sediments at greater depths (> 40m), illustrating dynamic sedimentary processes

The sediment composition predominantly includes small bivalves, gastropods, benthic foraminifera, echinoid spines, ostracods, and occasional planktonic components such as pteropods. The high abundance of micromollusks (bivalves and gastropods < 2mm) compared to similar carbonate depositional settings highlights their potential role in carbonate production.

Further analyses include the quantification of sediment compositions across grain-size fractions and stable isotope analyses of bioclasts to determine seasonal carbonate production patterns. Analyses of seawater and sediments provide insights into nutrient dynamics, seawater acidification and oxygenation, enhancing the understanding of the response of already extreme marine environment of the Arabian Gulf to climate change.