Stromatolites

By the 1960s, word was percolating through the international palaeontological community that living laminated mound structures had been found in very shallow and tropical hypersaline coastal waters in Shark Bay, Western Australia. Called stromatolites (meaning 'mattress of rock'), they had been known about for some time by the biological community, but the news took some time to cross the subject boundary to the palaeontological world. Now they are known from other shallow tropical waters with high evaporation rates such as the Bahama banks and Gulf of Mexico, and they have been subject to intense investigation.

Essentially, the laminated mounds are built by communities of microorganisms (mostly cyanobacteria and occasionally algae), which grow across sediment surfaces where there is little current to disturb them. Their tiny filaments interweave, binding the sediment surface and spreading out as thin mats. The mucilagenous surface of the mats traps more sediment particles, forming a sheet-like veneer of sediment. The veneer prevents sunlight from reaching the micro-organisms, which need its 'photo' energy for photosynthesis, and so the organisms respond by growing through the surface sediment to reach the light and establish new microbial mats, which in turn trap more sediment and so on ad infinitum. The result is a micro-laminated sheet-like structure.

Since few shallow-water environments are entirely devoid of currents, such thin microbial mats readily wrinkle and tear. Renewal of the surface depends on the initial pattern of disrupted mats, with the result that variously shaped domes and columns develop over time. The extent of upward growth depends on subsidence or rising water levels, otherwise the mats become exposed at the surface and die off. The sediment is typically carbonate mud, which can harden

(lithify) quickly; or at least in geological terms it is rapid. Once lithified into limestone, stromatolites can be remarkably resistant to erosion and therefore have a good record of preservation in the rock record.

The problem has been that, although their construction is essentially mediated by microbes, what is preserved is generally just the laminated sediment. The microbes decay without trace, which is why it took so long for the critical organic traces to be recovered and that involves the intervention of an unusual mode of preservation. At Gunflint, on Lake Superior, it was the rapid lithification of chert that did the trick.

Stromatolites are now known to be widespread throughout Precambrian times, with some perhaps as old as 3.5 billion years. The remarkable thing is that they are so conservative. Those that are billions of years old look very much like 750-million-year-old stromatolites or the living ones of Shark Bay. However, stromatolites have not been nearly as common since late Proterozoic times (around 700 million years ago) as they were throughout much of Precambrian times.

The reason for this is competition for space on the seafloor. Seaweeds (marine algae) became increasingly common from late Proterozoic times, as did mobile animals, which ploughed the sediment surface and burrowed into it. Today, stromatolites are largely restricted to lakes and shallow marine environments with extreme conditions that deter competition and predators. Stromatolites do however occasionally make brief comebacks after major extinction events. Back in the Grand Canyon, its Precambrian strata still had something new to contribute to our understanding of Precambrian life.

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