--> The Truth Behind Microbes and Ooids

AAPG Annual Convention and Exhibition

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The Truth Behind Microbes and Ooids

Abstract

Despite the importance of ooids as hydrocarbon reservoirs and major contributors to global carbonate budget, their genesis remain controversial. While the tradional view of ooid formation relies on abiotic factors, others contend the importance of microbes on ood genesis. Herein, we use a suite of analyses that include DNA molecular analyses, solid state 13C NMR, geochemical and scanning electron microscopy analyses to identify signatures indicative of processes associated with ooid accretion and carbonate precipitation in the active shoals of the Bahamas. This study reveals that modern marine ooids not only harbor a rich and diverse community of microbes with a wide range of metabolic processes associated with CaCO3 precipitation (e.g. denitrification, photosynthesis, sulfate reduction, etc.) but ooids undergo a complex diagenetic history throughout their lifespan driven by the syntrophic interaction of microbes with capabilities for carbonate precipitation or dissolution. Furthermore, the ubiquitous presence of exopolysaccharide (EPS) matrices along with EPS producers in the outer and inner cortices of the ooids, supports the imminent role of biofilm communities in CaCO3 precipitation. This study also provides for the first time direct evidence on the presence of amorphous calcium carbonate (ACC), a metastable carbonate polymorph that appears to act as a transient precursor phase in the early phases of ooid cortex accretion. The latter is supported by multiple lines of evidence that include SEM analyses documenting nanograin structures similar in size (20 nm-150 nm) and morphology (e.g. spherical, subspherical) to typical ACC grains together with the Mg/Ca of leached ooids (0.89 to 1.66 mol mol-1), which follows those of biogenic produced ACC. Further line of evidence is supported by solid-state NMR with1H-13C cross-polarization analyses, which documents a broad resonance peak (168.5 ppm) similar to those of pure-synthetic ACC. Together, these findings indicate that a redox dependent microbial consortium may influence CaCO3 precipitation in the form of ooid accretion, cementation and micritization. It is also inferred that ooid are not suitable indicators of paleoclimate as ooids are affected throughout their life by a complex chain of abiotic and biological processes, which can lead to large-geochemical offsets.