Thirty-hour continuous observation revealed that cyanobacterial metabolism caused daily lamination of a stromatolitic travertine in the Shionoha hot spring, SW Japan. The travertine is mainly composed of dendritic calcite crystals intercalated with bands of micritic particles at sub-millimeter intervals. Cyanobacteria remained in the spaces between the calcite crystals during the daytime, rose to the travertine surface to receive illumination as light decreased and formed a biofilm around sunset. A micritic band developed on the biofilm where glutinous substances trapped detrital ferrihydrite-rich particles transported from upcurrent. The trapped particles covered the biofilm, and in-situ precipitation of calcite crystals became dominant before mid-night. The depositional model of daily lamination of the Shionoha travertine may provide insight into the poorly understood origins and depositional rate of ancient stromatolites.
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