Biogeochemical Model and Simulation of the Effect of Precambrian Algae on the Formation Process of Certain Laminated Cherts

Abstract The source of silica in the formation of the Precambrian laminated cherts has long remained a problem to be solved. Through experiments on cherts and living blue‐green algae, the authors found that the collected chert samples probably come from primary deposits, and there is a great biomass of fossil algae in chert, among which the filamentous algae can be compared with the living blue‐green algae Oscillatoria; that a higher Pco2 of the gas would be favourable for the increase of the biomass of living blue‐green algae and consequently raises the pH value of the water body; and that lack of free oxygen and a higher concentration of SiO2 in the water have no apparent influence on the biomass of blue‐green algae. Based on the evidence above, a biogeochemical model concerning the origin of Precambrian laminated chert has been set up, in which the photosynthesis of algae under the presumed atmospheric conditions of the Precambrian might raise the pH value of the water body and promote the dissolution of silicate minerals, thus providing a source of colloid SiO2 for the formation of Precambrian laminated chert.Furthermore, a simulation experiment device has been designed successfully, which can control the temperature (30 ± 0.5°), Pco2 (50662.5 Pa) and Po2 (about zero Pa) of the gas, the rate of photosynthesis of algae and the movement of the water carrying opal. In the simulation experiments, separate measurements have been made on the rate of photosynthesis of algae, pH value and concentration of SiO2 of the water body, with the results indicating that under the conditions similar to the presumed Precambrian atmosphere, the photosynthesis of algae can make the pH value of the water body go up steadily to over 9.7, leading to the dissolution of the silicate minerals, with the concentration of SiO2 measured reaching as high as 84 mg/l. Finally, through the vaporation of water, a phenomenon of colloid floccilation has been observed.The simulation experiment has verified the proposed biogeochemical model illustrating the origin of Precambrian laminated chert. Moreover, the device and method of its kind may also be applied to the research on the relationship of the Precambrian algae with the formation of some other mineral deposits such as of Fe, Mn, U and carbonates.