[B-GM02] Rock-Bio Interactions and its Applications

[BGM02-P01] Decompositional processes of microbial carbonates in Lagoa Vermelha, Brazil

*Fumito Shiraishi¹, Yusaku Hanzawa¹, Jiro Asada², Leonardo Fadel Cury³, Anelize Manuela Bahniuk³ (1.Earth and Planetary Systems Science Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 2.INPEX Solutions, 3.Federal University of Parana)

In Lagoa Vermelha, Brazil, a lagoonal stromatolite and saltpan microbial mat were investigated to understand the effects of anthropogenic changes and progressive evaporitic condensation on microbial carbonates. The lagoonal stromatolite, mainly composed of magnesian calcite and aragonite, was developed on a dolomite-containing carbonate crust. While many stromatolites had been eroded to the water surface level, some smaller, green stromatolites retained a domal shape below the water surface. However, the domal stromatolite surface was dominated by endolithic cyanobacteria with conspicuous microborings. In addition, microbial aerobic respiration caused carbonate dissolution in the dark condition, and metazoans were observed to have grazed the inner surface of the stromatolite and excreted fecal pellets. This indicated that the formation of lagoonal stromatolites had ceased and they were decomposing, probably due to changes in lagoonal water chemistry in recent years. On the other hand, the microbial mat about 3 cm thick developed in a saltpan pond precipitating carbonate and gypsum, and it contained quartz, magnesian calcite, aragonite, and gypsum. At the time of the investigation, CaCO₃ precipitation was absent and dissolution was occurring due to the dominance of heterotrophs relative to phototrophs. This might have reflected progressed saltpan evaporation, which had suppressed the cyanobacteria population and consequently their photosynthetic precipitation of CaCO₃. This examination of two carbonate deposits in Lagoa Vermelha suggests that environmental changes have potentially caused their decomposition not only by metazoan grazing but also by microbial metabolism.