Biogeochemical characterization of micritized carbonate grains in the shallow-marine Al-Kharrar Lagoon (Red Sea, Saudi Arabia)

Micritization of marine carbonate grains is an early diagenetic process involving the alteration of the original carbonate fabric and the formation of cryptocrystalline textures through cycles of dissolution and reprecipitation. Microorganisms play a significant role in this process, actively contributing to the formation of constructive and destructive aragonite and high Mg–calcite envelopes. However, the complex interdependencies among sedimentological, geochemical and microbiological processes governing micritization in modern shallow-marine settings remain poorly understood. To address this knowledge gap, four sediment cores were collected across an inlet to the Al-Kharrar Lagoon on the Red Sea coast of Saudi Arabia. A comprehensive multidisciplinary analysis of the Holocene sedimentary sequences from these cores (<1 m depth) was conducted, combining petrographic, mineralogical, geochemical and microbial ecology analyses. Scanning electron microscopy displayed carbonate sediments covered by mud-sized aragonite needles resulting from the breakdown of coarser carbonate grains and/or microbial activity. Additionally, the presence of Mg-rich calcite, mini-micrite crystals (<1 μm) and biofilms on the grains' surface indicated microbial-mediated precipitation. Microborings and carbonate fillings were also found beneath the surfaces. Besides the clear signature of biological residues on grains, DNA-based analysis confirmed the presence of bacteria typically associated with carbonate precipitation and microboring, including cyanobacteria (potential endoliths), aerobic heterotrophs and sulphate-reducing bacteria (both potential precipitation inducers). These bacteria were widely distributed throughout the cores, with the highest abundance observed in the upper sediment layers. Overall, the results confirmed the active processes of dissolution and micrite formation within the carbonate sediments of the Al Kharrar lagoon, positioning the site as a valuable modern analogue for Mesozoic micritic limestones. These results advance the understanding of the microbial communities involved in the micritization process and provide useful insights into the evolution of microporosity, a defining characteristic of many Middle Eastern reservoirs.