Formation of lithified micritic laminae in modern marine stromatolites (Bahamas): The role of sulfur cycling

Microbial mats on the surfaces of modern, marine stromatolites at Highborne Gay, Bahamas, were investigated to assess the role of microbial processes in stromatolite formation. The Highborne Cay stromatolitic mats contain Sch izothix as the dominant cyanobacterium and show millimeter-scale lamination : Some layers in the mat are soft (unlithified), whereas other layers are c rusty (lithified). Lithified layers within the mats correspond to micritic horizons composed of thin (20-50 mu m) micritic crusts, which commonly over lie truncated, micritized carbonate sand grains. These features are identic al to lithified laminae in the underlying stromatolite; the micritic crusts are similar in thickness to micritic laminae in many ancient stromatolites . Biogeochemical parameters in a representative stromatolitic mat from High borne Cay were measured to identify the role of bacteria in precipitation a nd dissolution of CaCO3. Depth distributions of O-2, HS-, and pH were deter mined with microelectrode measurements in the field. Oxygen profiles were u sed to calculate photosynthesis and aerobic respiration. Sulfate reduction was determined using (SO42-)-S-35 and sulfide oxidation potential was measu red in homogenized samples. Our results indicate that cyanobacterial photos ynthesis, sulfate reduction, and anaerobic sulfide oxidation in stromatolit ic mats at Highborne Cay are responsible for CaCO3 precipitation, whereas a erobic respiration and aerobic sulfide oxidation cause CaCO3 dissolution. A close coupling of photosynthesis and aerobic respiration in the uppermost few millimeters of the mats results in no, or very little, net lithificatio n. Sulfur cycling, on the other hand, shows a close correlation with the fo rmation of lithified micritic layers. Photosynthesis, combined with sulfate reduction and sulfide oxidation results in net lithification. Sulfate redu ction rates are high in the uppermost lithified layer and, on a diel basis, consume 33-38% of the CO2 fixed by the cyanobacteria. In addition, this li thified layer contains a significant population of sulfide-oxidizing bacter ia and shows a high sulfide oxidation potential. These findings argue that photosynthesis coupled to sulfate reduction and sulfide oxidation is more i mportant than photosynthesis coupled to aerobic respiration in the formatio n of lithified micritic laminae in Highborne Cay stromatolites.