FOSSILIZED BUBBLES IN MESOPROTEROZOIC STROMATOLITES: MORPHOLOGICAL EVIDENCE FOR OXYGENIC PHOTOSYNTHESIS OF CYANOBACTERIA IN ANCIENT MICROBIAL MATS

Proterozoic stromatolites are commonly interpreted as microbial mats constructed by cyanobacteria, although direct morphological evidence for oxygenic photosynthesis is rare. Oxygen bubbles trapped in microbial mats could potentially be fossilized through early diagenetic silicification processes and can provide direct evidence for oxygenic photosynthesis and hence cyanobacterial participation in mat construction. Here we report bubble-like structures in silicified stromatolites of the Mesoproterozoic Gaoyuzhuang Formation (1400-1600 Ma) in the Pangjiapu area of North China. These bubbles are surrounded by tangentially oriented filamentous microfossils and they tend to occur in the pinnacle of small conical stromatolites. They are interpreted as oxygen bubbles produced by cyanobacteria and trapped within microbial mats. The preservation of oxygen bubbles in Mesoproterozoic stromatolites not only confirms the key role of cyanobacteria in mat construction, but also provides useful information about the biophysics and paleoenvironmental conditions of ancient microbial mats.