Comparison of the sedimentary characteristics and diagenesis of modern carbonate deposits between tidal channels and mangrove marshes, Abu Dhabi, United Arab Emirates

As a typical modern low-angle carbonate ramp sedimentary system, the shallow-marine mangrove marsh and tidal channel environments adjacent to Abu Dhabi Island contain a wide variety of allochems and diverse early-diagenetic processes. Integrating data from cores, thin sections, cathodoluminescence (CL), and scanning electron microscopy (SEM) allows for quantifying the abundance of components and providing new insights into various diagenetic events. The sediments show various skeletal and non-skeletal carbonate components, as well as a minor amount of terrigenous siliciclastic minerals (quartz and feldspar) and organic matter. The low energy within protected intertidal systems accounts for the greater abundance of carbonate mud (average 76%) in marshes, compared to tidal channels (average 3%). The high energy of tidal currents resulted in more abundant peloids and bioclasts (including bivalves, gastropods, foraminifera, algae, corals, and bryozoans) in the tidal channels. In addition, siliciclastic grains are more abundant in marshes near dredged channels. The anthropogenic activities, such as channel excavation, may increase the input of Pleistocene aeolian deposits into the modern carbonate sediments around dredged channels. Aragonite and high-Mg calcite cements with diverse crystal morphologies are slightly more common in tidal channels than in marshes. This is attributed to relatively strong hydrodynamic conditions and permeable sandy sediments, which cause higher rates of marine porewater flow. The intensity and extent of micritization and microboring of the allochems are greater in tidal channels than in marshes. The micrite envelopes of aragonite-dominated fragments are non-luminescent, whereas the micrite envelopes of bioclasts dominated by high-Mg calcite exhibit dull orange luminescence. There are no discernible differences in the elemental composition between host bioclasts and their micrite envelopes. Dolomite cement in modern sediments, which is rarely found in previous studies, mainly occurs as scattered single rhombic crystals or aggregates in the deposits from tidal channels. Energy dispersive X-ray spectrometer (EDS) data shows that patchy dolomite cements are surrounded by high-Mg microcrystalline cements. Extracellular polymeric substances (EPS) and metabolic processes of organisms may facilitate dolomite precipitation. Besides, the relatively high evaporation rate leads to an increase in the Mg/Ca ratio in pore water, thereby promoting the formation of dolomite.