Micritization and Microbial-related Diagenetic Features in Modern Shallow Marine Carbonates (Red Sea, Arabian Sea and Arabian Gulf)

Pores smaller than 10 microns in diameter (microporosity) can make up more than 90% of the total porosity in giant Arabian carbonate reservoirs. While a lot of research has been done to understand the distribution of microporosity, the diagenetic processes initiating its development are still debated. Since microporosity occurs in highly diagenetically overprinted rocks the involvement of early syn-sedimentary processes are generally overlooked. Micritization is a process happening during early diagenesis in the first centimeters of depth in which parts of carbonate grains are reworked to cryptocrystalline textures. The fundamental drivers of micritization are still somewhat debated, however, more and more evidence points to the involvement of microbes such as cyanobacteria, algae, or fungi. So, how can we decipher the diagenetic sequence that ancient limestones have experienced and predict microporosity distribution if the initial steps are poorly understood?

The hypothesis driving this research places microbial micritization as the first step toward the creation of microporosity in limestones. Here, we present the first results undertaken as part of a multidisciplinary research project, at the interface of geology and microbiology and coupling field sampling and laboratory experiments. We compare the rates of micritization and the variety of microbial-related diagenetic features encountered between different carefully selected intertidal locations from the Red Sea and Arabian Sea (Saudi Arabia), and the Arabian Gulf (United Arab Emirates). A series of 1 m long sediment cores has been collected at low tide, and subsamples were extracted from every 10 cm for systematic petrographic and geochemical analyses. Thin section petrography revealed extensive microborings and associated micritization in the sediments. XRD analysis has been carried out to establish the mineral variations through the locations and depth, and SEM imagery further confirmed the presence of organic biofilms and mucous. The results from the metagenomic analysis revealed the microbial diversities and provide further understanding of the specific microbial drivers that play a key role in micritization processes. The work presented here hence aims to enhance the fundamental understanding of micritization in shallow marine carbonate sediment, the role of microbes in early diagenetic processes and their potential impact on microporosity development.