POLYMORPH MINERALOGY OF CYANOBACTERIAL CACO₃ IS CONTROLLED BY THE MG/CA RATIO OF AMBIENT SEAWATER: IMPLICATIONS FOR STROMATOLITES OVER DEEP GEOLOGIC TIME

Cyanobacterial mats have formed laminated calcareous structures (stromatolites, thrombolites, microbialites) in shallow marine environments for at least the past 3.5 billion years. These geologically-significant structures represent some of the first modes of rock-forming life on the planet and were among the predominant bio-assemblages preserved until the end of the Proterozoic.

Independent lines of evidence from the geological record (fluid inclusions, marine evaporites, abiotic and biotic CaCO3 precipitates, Mg/Ca of fossil echinoderms) suggest that the molar Mg/Ca of seawater has oscillated between 1.0 and 5.2 over the last 540 million years, thereby favoring the precipitation of calcite when seawater mMg/Ca < 2.0 and aragonite + high-Mg calcite when mMg/Ca > 2.0. A hydrothermal brine – river water mixing model, driven by historical rates of ocean crust production (determined from sequence stratigraphy during the Phanerozoic and granitic plutonism during the Precambrian), predicts Mg/Ca ratios which are consistent with the empirical evidence for “calcite- and “aragonite seas” since the Late Archean. Such secular variation in seawater Mg/Ca would have subjected calcifying cyanobacteria to 8 periods of aragonite seas and 7 periods of calcite seas over the past 2.8 billion years.

To evaluate the effect of seawater Mg/Ca on cyanobacterial calcification, modern cyanobacterial mats were cultured in artificial seawaters formulated over the range of Mg/Ca ratios believed to have occurred since the Late Archean.

Cyanobacterial mats cultured in modern aragonite seawater (mMg/Ca = 5.2) produced the majority (two-thirds) of their CaCO3 as the aragonite polymorph, and one-third as the high-Mg calcite polymorph (19% MgCO3). Significantly, cyanobacterial mats cultured in the artificial calcite seawater (mMg/Ca = 1.5) produced exclusively the calcite polymorph (7% MgCO3).

The similarity between these results and the results of comparable experiments on abiotic calcification suggests that cyanobacteria are inducing essentially abiotic precipitation of CaCO3 through the removal of CO2 via photosynthesis. It is likely that the polymorph mineralogy of calcifying cyanobacterial mats has varied in synchroneity with that of ooids, crystal fans, and marine cements throughout the “calcite- and “aragonite seas” of the geologic past.