Calcium carbonate deposition and fossil record

Many species of cyanobacteria have calcium carbonate in the enveloping mucilage of the cells. In freshwater these algae usually grow in water where carbonate crystallizes out by non-biological physicochemical mechanisms, and the crystals of calcite become trapped in the mucilage of the algae. Normally, only 1% to 2% of the calcium carbonate is actively precipitated by the cells (Pentecost, 1978). There are a large number of different forms of carbonate deposits attributed to the Cyanophyceae (for a review, see Golubic´, 1973).

In the marine environment, cyanophycean depositions are the result of trapping and binding of the sediments as well as carbonate precipitation. Stromatolithic heads are probably the best known of these forms (Figs. 2.53, 2.55). These heads are firmly gelatinous to almost cartilaginous in texture, predominantly hemispherical, and show fine concentric lamination. In the shallow subtidal waters of south Florida and the Bahamas, they are produced by a single species of Schizothrix (Monty, 1967). During the day there is growth of algal filaments, resulting in the algae covering the surface of the stomatolite head. During the night, growth ceases, and sediments accumulate on the surface

of the head, forming sediment-rich laminae up to 100 m thick. In the early part of the day, the algae penetrate through this deposited sediment, and grow a hyaline layer, 200 m thick, with a low concentration of entrapped sediment. These alternating periods of growth and deposition give a laminated structure to the stromatolite (Reid et al., 2000) (Fig. 2.55). Stromatolites first formed 3500 million years ago although the first stromatolites were formed by non-biological deposition of CaCO3 (Arp et al., 1999) with the first stromatolites formed by cyanobacteria (Fig. 2.54) appearing about 2700 million years ago (Buick, 1992; Dalton, 2002; Brasier et al., 2002).

The production of laminae in stromatolites depends on fluctuations ultimately derived from the physical movements of the earth, sun, and moon, and requires some kind of rhythmicity that causes discontinuity in the accretionary process. The periodicity of laminae is due primarily to the daily photosynthetic cycle of the organisms in the stromatolites. In addition, stromatolites are helio- tropic and grow toward sunlight (Awramik and Vanyo, 1986). This heliotropism allows the calculation of the extent of a year’s deposition in a stromatolite. The yearly cycle of movement of the sun causes the sun to be higher in the sky in the summer and lower in the winter. The heliotropism of the stromatolites causes the stromatolites to grow in a sine waveform over the course of a

Fig. 2.54 Fossil cyanobacteria preserved in silicified stromatolites of the 1400 million-year-old Gaoyuzhang formation of northern China. (a) Eoentophysalis belcherensis. (b) Palaeolyngbya barghooriana. (c) Oscillatoriopsis. (From Golubic and Seong-Joo, 1999.)

year (Fig. 2.55). Counting the number of laminae in one sine wave has allowed paleontologists to calculate the number of days in a year for a geological period. Such studies have shown that the solar year has varied considerably. For example, approximately 1000 million years ago, the solar year consisted of approximately 435 days (Vanyo and Awramik, 1985).

Up to 2000 million years ago, there were no grazing and boring organisms; thus stromatolites grew uncontested. Without competition, Precambrian stromatolites freely populated enormous areas and most likely grew in water down to a depth of 10 m. The occurrence and size of stromatolites declined dramatically after the evolution of grazing and boring organisms. Today stromatolites grow only in warm waters that are inhospitable to grazers and borers – waters such as the hypersaline waters in Shark Bay, Australia (Fig. 2.53), or in waters with a high tidal current that inhibit borers and grazers, such as in the Bahamas where stromatolites grow to a height of 2 m (Dill et al., 1986).