CYANOBACTERIAL FOSSILIZATION THROUGH THE FORMATION OF CALCIUM CARBONATE AND CLAY MINERALS

Variations in sea water chemistry may have influenced changes in cyanobacterial fossilization through time. However, specific factors that drove the fossilization trends remain poorly understood. We investigate the precipitation of carbonate and clay minerals in enrichment cultures of aggregate and mat forming filamentous cyanobacteria. The cultures are grown in artificial sea water with varying concentrations of Mg, Ca, and Si, and on different substrates (silica sand, carbonate sand, agar and suspended without substrate). Ca- or Mg-rich alumino-silicate precipitates form in actively growing cultures after about two months. Precipitates primarily occur around cyanobacterial filaments wider than 1.5 µm and grown on silica sand and/or in the presence of high (1.0 mM) silica concentrations. Uniformly thin, smooth mineral coatings precipitate around cyanobacterial filaments in the presence of 1.0 mM silica and silica sand substrate, whereas coarser, patchily distributed precipitates are found around some filaments grown on agar and in the absence of added silica. Mineral precipitates were not observed around thinner (less than 1.5 µm-wide) filaments or beadlike (possibly heterocystous) cyanobacteria, suggesting that these organisms are less likely to be preserved under all examined experimental conditions. Calcium carbonate also precipitated in cyanobacterial cultures, and the mineralogy and distribution of calcite or aragonite did not exhibit an obvious relationship with Mg/Ca ratios, the presence of silica or the type of substrate. These experiments suggest that silica concentrations from 0.4 to 1.0 mM and the presence of siliciclastic substrate promote the precipitation of clay minerals around filamentous cyanobacteria. The same factors may have promoted cyanobacterial fossilization in shales and other siliciclastic sediments.