Where is the ooid factory?

Abstract:

Ooids, concentrically laminated carbonate grains, are found in high-energy shallow water environments (the ooid factory). Consequently, ooid laminae are thought to precipitate around suspended grains. The role of microbes in this process is debated: abiotic models neither explain how ooids acquire a high organic content, nor do they account for the fast abrasion and loss of ooid carbonate in highly agitated areas. Here we probe the role of microbes and physical processes in ooid accretion on an oolitic beach in the leeward coast of Cat Island, the Bahamas. Grain size and petrography, microbial community composition and physical factors are compared along a cross-shore transect. A hydro-morphodynamic model is used to analyze sediment transport and sorting across the beach and shallow shelf. We find that the surf zone has a barren seafloor, and it is dominated by shiny and rounded ooids whose size decreases seaward, as predicted by physical grain sorting. Dull ooids and grapestones, irregular coated grains that are thought to form by microbially-mediated precipitation of carbonate around ooids and other grains, are present outside of the surf zone. The bulk size of these grains increases seaward, and they contain more abundant and diverse microbial communities than agitated ooids. The inverted sorting trend indicates that the time scale for grain accretion in this region is shorter than the decadal time scale for grain sorting. Modeling and field observations suggest that carbonate precipitation of both ooids and grapestones occurs in sediments that are colonized by diatom- and cyanobacteria-rich mats and reworked during storms. Periodic storms transport ooids to the surf zone, where they are rounded by abrasion and gain a shiny exterior in less than a day of reworking. Small ooids are periodically transported back to the microbially colonized areas, and the accretion cycle restarts. Grains too large to be frequently transported exit the accretion-erosion conveyor belt, remain trapped outside the surf zone and become grapestones. These observations suggest that ooid growth requires a specific balance between the timescales of microbial growth, colonization and carbonate precipitation. A similar mechanism might occur in tidal shoals, where large bedforms provide protected areas for temporary mat growth.