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

Abstract

Through early lithification, cyanobacterial mats produced vast amounts of CaCO 3 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 CaCO 3 saturation state ( Ω CaCO 3 ) in the mat. However, it is uncertain which microbial processes produce the Ω CaCO 3 increase and to which extent similar Ω CaCO 3 shifts were possible in Precambrian oceans whose chemistry differed from that of the modern ocean. I developed a numerical model that calculates Ω CaCO 3 in cyanobacterial mats and used it to tackle these questions. The model is first applied to simulate Ω CaCO 3 in modern calcifying cyanobacterial mats forming at Highborne Cay (Bahamas); it shows that while cyanobacterial photosynthesis increases Ω CaCO 3 considerably, sulphate reduction has a small and opposite effect on mat Ω CaCO 3 because it is coupled to H 2S oxidation with O 2 which produces acidity. Numerical experiments show that the magnitude of the Ω CaCO 3 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 Ca 2+ in ambient waters. With oceanic Ca 2+ concentrations greater than a few millimolar, an appreciable increase in Ω CaCO 3 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 CaCO 3 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 Ω CaCO 3 shift in cyanobacterial mats throughout Earth’s history produced by a decrease in salinity and temperature of the ocean, a decrease in atmospheric pCO 2 and an increase in solar irradiance. Such a trend would explain how the formation of the microsparitic stromatolite microtexture was possible as the Ω CaCO 3 of the ocean decreased from the Paleoproterozoic to the Phanerozoic.