Photosynthesis-induced biofilm calcification and calcium concentrations inphanerozoic oceans

Photosynthetic carbon assimilation is commonly invoked as the cause of calc ium carbonate precipitation in cyanobacterial biofilms that results in the formation of calcareous stromatolites. However, biofilm calcification patte rns in recent lakes and simulation of photosynthetically induced rise in ca lcium carbonate supersaturation demonstrate that this mechanism applies onl y in settings Low in dissolved inorganic carbon and high in calcium. Taking into account paleo-partial pressure curves for carbon dioxide, we show tha t Phanerozoic oceans sustaining calcified cyanobacteria must have had consi derably higher calcium concentrations than oceans of today. In turn, the en igmatic lack of calcified cyanobacteria in stromatolite-bearing Precambrian sequences can now be explained as a result of high dissolved inorganic car bon concentrations.