Aloisi, Giovanni (2008) The calcium carbonate saturation state in cyanobacterial mats throughout Earths history. Geochimica et Cosmochimica Acta, 72 . pp. 6037-6060. DOI 10.1016/j.gca.2008.10.007.

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Abstract

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 (XCaCO3 ) in the mat. However, it is uncertain which microbial processes produce the XCaCO3
increase and to which extent similar XCaCO3 shifts were possible in Precambrian oceans whose chemistry differed from that
of the modern ocean. I developed a numerical model that calculates XCaCO3 in cyanobacterial mats and used it to tackle these
questions. The model is first applied to simulate XCaCO3 in modern calcifying cyanobacterial mats forming at Highborne Cay
(Bahamas); it shows that while cyanobacterial photosynthesis increases XCaCO3 considerably, sulphate reduction has a small
and opposite effect on mat XCaCO3 because it is coupled to H2S oxidation with O2 which produces acidity. Numerical experiments
show that the magnitude of the XCaCO3 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 XCaCO3 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 XCaCO3 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 XCaCO3 of the ocean decreased from the Paleoproterozoic to the Phanerozoic.

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