ENVIRONMENTS OF PREFERENTIAL, ABIOTIC CARBONATE PRECIPITATION IN HOLOCENE STROMATOLITES, A MODEL TO GUIDE THE SEARCH FOR PRECAMBRIAN MICROFOSSILS

In Holocene marine stromatolites from the Bahamas, syndepositional lithification mediated by microbes produces only paper-thin crusts (Browne, 2001; Reid and others, 2002) It is the abiotic interstitial precipitation of carbonate below the zone of living cyanobacteria that develops well-cemented columnar stromatolities with the potential for selective preservation of microbes. In Bahamian occurrences, the precipitation is promoted by tidal flushing. In the Western Australian (Shark Bay) occurrence, elevated salinity enhances precipitation. If these relationships apply to some Precambrian stromatolies, they can provide lithologic guides to prospecting for preserved microbiotas.

The abiotic, fiberous aragonite of well-cemented Holocene stromatolites from the Bahamas (Dill and others, 1999). is widespread in other environnments like that of beachrock (Ginsburg, 1953), the sea-facing margins of coral reefs (Ginsburg and James, 1979) and platform margins and as submarine hardgounds, especially in carbonate sands frequently flushed by waves and tides on shallow shelves or platforms.. With this model, the most likely candidates for preserved microbes in stromatolites would be those associated with oolitic sands, intraclasts of stromatolites, beach deposits, platform margins and evaporites.