Lithification of pelletal grains by microbially mediated organomineralization

Fecal pellets are key components of the biological carbon pump and a source of nutrients. When cohesively bound and cemented, they become the most abundant non-skeletal grains in shallow-water carbonates and an important nucleus for ooid formation. Herein, we describe the mineralization processes contributing to pellet cementation using scanning electron microscopy in conjunction with stable isotope and mineralogical analyses of the bulk sediments on Great Bahama Bank, known for its abundance of peloids and fecal pellets. Results show biogenic signatures consistent with the initial stages of organomineralization. These include, curved-tubular filaments and bacilli bacteria associated with extracellular polymeric substances (EPS); fibrils of EPS binding aragonite needles; and the coexistence of decayed EPS with amorphous calcium carbonate that once it develops, coalesces and crystalizes into aragonite. While δ13Cinorg do not show large variations, slightly higher values are observed in areas where pellets are indurated, likely attributed to photosynthetic activity on the pellets. The δ13Corg values tend to be more elevated in indurated pellets, suggesting that heterotrophs are preferentially using enriched carbon sources. In contrast, friable pellets from nearshore areas show more negative δ13Corg values, suggesting microbial exposure to depleted C3 and CAM plant sources. Fingerprints of nitrogen fixation and denitrification are also documented and supported by δ15Norg values. Altogether, these findings suggest that EPS and a microbial consortium, including gut-microbial flora and free-living surface colonizers, are responsible for inducing early cements, which are later complemented by secondary cements. Microorganisms involved in pellet lithification may subsequently also aid in ooid cortex development.