The calcium carbonate saturation state in cyanobacterial mats throughout Earth’s history

Through early lithification, cyanobacterial mats produced vast amounts of CaCO3 on Precambrian carbonate platforms (before 540 Myr ago). The superposition of lithified cyanobacterial mats forms internally laminated, macroscopic structures known as stromatolites. Similar structures can be important constituents of Phanerozoic carbonate platforms (540 Myr to present). Early lithification in modern marine cyanobacterial mats is thought to be driven by a metabolically-induced increase of the CaCO3 saturation state (ΩCaCO3ΩCaCO3) in the mat. However, it is uncertain which microbial processes produce the ΩCaCO3ΩCaCO3 increase and to which extent similar ΩCaCO3ΩCaCO3 shifts were possible in Precambrian oceans whose chemistry differed from that of the modern ocean. I developed a numerical model that calculates ΩCaCO3ΩCaCO3 in cyanobacterial mats and used it to tackle these questions. The model is first applied to simulate ΩCaCO3ΩCaCO3 in modern calcifying cyanobacterial mats forming at Highborne Cay (Bahamas); it shows that while cyanobacterial photosynthesis increases ΩCaCO3ΩCaCO3 considerably, sulphate reduction has a small and opposite effect on mat ΩCaCO3ΩCaCO3 because it is coupled to H2S oxidation with O2 which produces acidity. Numerical experiments show that the magnitude of the ΩCaCO3ΩCaCO3 increase is proportional to DIC in DIC-limited waters (DIC < 3–10 mM), is proportional to pH when ambient water DIC is not limiting and always proportional to the concentration of Ca2+ in ambient waters. With oceanic Ca2+ concentrations greater than a few millimolar, an appreciable increase in ΩCaCO3ΩCaCO3 occurs in mats under a wide range of environmental conditions, including those supposed to exist in the oceans of the past 2.8 Gyr. The likely lithological expression is the formation of the microsparitic stromatolite microtexture—indicative of CaCO3 precipitation within the mats under the control of microbial activity—which is found in carbonate rocks spanning from the Precambrian to recent. The model highlights the potential for an increase in the magnitude of the ΩCaCO3ΩCaCO3 shift in cyanobacterial mats throughout Earth’s history produced by a decrease in salinity and temperature of the ocean, a decrease in atmospheric pCO2 and an increase in solar irradiance. Such a trend would explain how the formation of the microsparitic stromatolite microtexture was possible as the ΩCaCO3ΩCaCO3 of the ocean decreased from the Paleoproterozoic to the Phanerozoic.