In-Situ Production of Peloidal Nuclei on Ooid Shoals

Ooids form in high-energy environments where mud is scarce or absent. Yet, the majority of nuclei in Bahamian ooid shoals are peloids. The periphery of the shoals contains peloids but also abundant skeletal fragments, ooids, grapestones and lithoclasts. Here, we propose that a) the majority of ooid nuclei are peloids produced in situ from mud deposition on the shoal by storms, and b) fecal-pellet producing organisms act as microbial vectors by transferring indigenous microbes to the pelleted mud, thus assisting in cortex accretion. To test this hypothesis, we characterized and quantified the ooid nucleus composition in six active shoals and one beach by point counting in thin sections. In all study sites, the nuclei are predominantly peloids of ovoidal, spherical or rod-shaped morphologies, ranging from 82.5% to 90.2%. This abundance is in contrast with the diversity of grains from surrounding areas. It is well-documented that storms deposit mud layers on the shoal, which are subsequently reworked by bioturbation and turned into fecal pellets by the mud shrimp, Callianassa. The fecal pellets need to harden to avoid disintegration before it can serve as a nucleus for ooid formation. This process of cementation is documented with a series of SEM images, which reveals that cementation within peloids is aided by microbial organomineralization processes, similar to those described in the formation of ooids. This similarity suggests that microbial communities responsible for the organomineralization in ooids are likely introduced into the mud by the organisms that form the fecal pellets.