Sea-level variation and the zonation of microbialites in Hamelin Pool, Shark Bay, Western Australia

The application of modern methods of time-series analysis to a record of sea-level variation at Flint Cliff, Hamelin Pool, between October 1983 and April 1985, shows that astronomical tides account for only one of the following five key components of the record: a seasonal oceanic cycle; a short-term irregular cycle; the complex astronomical tidal system in the Pool; isolated major events; and less marked variations probably reflecting wind stress, still able to defeat the astronomical tide in the short-term. We have compared the inundation record with precisely surveyed elevation ranges of various microbial communities. The dominance of a seasonal cycle is the fundamental determinant of variation in the duration of immersion and exposure determining the littoral zonation of microbial mats in Hamelin Pool. The astronomical tide is not the major cause of this variation. The microbial communities fall into three zones. In Zone 3, the microbialite-forming colloform mat is virtually never exposed. In Zone 2, smooth, reticulate and mamillate mats colonise the lower littoral environment. Here, many of the exposed microbialites have been stranded by the falling sea level, and are colonised by intermittently submerged microbial communities that modify the stranded lithified microbialites. Zone 1 is inundated only under exceptional circumstances and microbial communities are ephemeral.