Mechanisms of Micritization Revealedby Petrography, Mg/Ca and Sr/Ca Ratios of the Carbonate Sediments of theArabian (Persian) Gulf, Abu Dhabi

The solidity and porosity of calcium carbonate rocks are of major interest for oil-reservoir
evaluation, groundwater flow studies, and civil engineering applications. Micritization—an
early diagenetic process that converts carbonate shells and skeletal grains into
microcrystalline carbonate—significantly affects these rock properties, yet its underlying
mechanisms remain poorly constrained. The coastal environments of Abu Dhabi provide
natural laboratories for studying micritization, as they are modern analogues of the low-angle
carbonate ramps that were widespread in epeiric seas throughout much of the geological past.
In this study, we investigate calcium carbonate muds and associated pore waters from a range
of depositional environments, including mangroves, tidal channels, sabkhas, and offshore
settings, to better understand the processes controlling micritization. We apply an integrated
approach combining sedimentological, mineralogical, and geochemical methods. Preliminary
results indicate that the carbonate mud is predominantly composed of aragonite, with minor
amounts of low magnesium calcite. Boring intensity increases with depth, particularly in
tidal-channel environments, and is closely associated with physical erosion by endolithic
fauna. In contrast, crystal morphologies observed in sabkha sediments suggest that chemical
precipitation processes are more dominant in these settings.
Trace element systematics reveal that micritization is accompanied by systematic changes in
Sr/Ca and Mg/Ca ratios. In all tested environments, grain size reduction (i.e., micritization) is
associated with a significant increase in Mg/Ca, while Sr/Ca is much less sensitive to the
same process. While both Sr/Ca and Mg/Ca are incorporated within the diagenetic aragonite
lattice according to their respective partition coefficients, Mg/Ca ratios are strongly increased
by adsorption during micritization-related grain size changes. The decoupling of Mg and Sr
during the micritization process may provide new constraints on the question of the
mechanism of micrite formation.