Morphological patterns of (sub)recent marine aragonite crystals in transects from the shoreline to open coastal waters of Abu Dhabi, United Arab Emirates

Aragonite crystals, usually described as fibrous or needle-like, are common early diagenetic fabrics in modern (sub)tropical seas worldwide. This study presents findings from landward to seaward transects in the Abu Dhabi lagoon (United Arab Emirates), revealing a far greater diversity in the morphology of aragonite crystals than previously reported from natural settings. The observed crystals include hexagonal, tabular, chisel-shaped, and needle to bladed types. Across the transition from the shoreline through the lagoon and into the coastal waters, distinct first-order spatial and morphological patterns emerge. These range from flat-terminated crystals, such as hexagonal and tabular forms, to pointed-terminated crystals, including needle-like, bladed, and chisel shapes. Based on experimental studies and in situ seawater data, we propose that the seawater Mg2+ concentration, Mg/Ca ratios, and temperature are the primary controlling factors on crystal morphology. In nearshore, restricted environments with high Mg2+ concentrations, low Mg/Ca ratios, and warm seawater temperatures, flat-terminated aragonite cement crystals are prevalent. In lagoonal to more open marine areas, high Mg/Ca seawater ratios, low Mg2+ concentrations, and cooler temperatures result in pointed-terminated aragonite crystals. Lower-order variations in the aragonite morphologies, whether flat-terminated or pointed, exhibit subtle variations due to small-scale fluctuations in seawater Mg2+ concentrations and CO2 levels. A nonclassical aragonite crystallization pattern is superimposed on these environmental factors. This research highlights the crucial role of spatiotemporal studies, when combined with data from experimental studies, in understanding diagenetic environments. Our findings reveal insights that enhance our understanding of the environmental information hidden within the intricate morphological patterns of marine aragonite minerals.