Organic and Biogeochemical Patterns in Cryptic Microbialites

Abstract

Cryptic non-cyanobacterial microbialites from reef caves at Lizard Island (Great Barrier Reef, Australia) and the Salt River Canyon, St.Croix (US Virgin Islands) exhibit very distinctive organic patterns. The Lizard Island microbialites consist of various types of automicrite. The biofacies is mainly characterized by highly diverse biofilms, foraminifers, sponges, brachiopods, and bryozoans. Biomarker characteristics of various microbes and sponges (e.g. demosponge acids and mid-chain branched, monomethylalkanoic acids) support histological observations. The microbialite sequence exhibits a light-decreasing facies succession --- in this particular case a transgressive sequence with an age of {\textasciitilde}3500 years B.P. Macromolecular analyses of the automicrites reveal an excess of acidic macromolecules, which are a feature of calcifying systems. Microbialites from the Salt River Canyon consist of peloids and thin automicritic layers (calcified biofilms) and also exhibit a light-decreasing sequence. The peloids are automicrites that are formed in acidic mucilage in the presence of bacterial remains and mucus. Biomarkers indicate a significant input of terrestrial plant remains and autochthonous heterotrophic bacteria, particularly anoxygenic ones. Sponge and other metazoan biomarkers were not observed in microbialites of the Salt River Canyon. In neither case cyanobacteria were noted, neither in histological sections nor by using biochemical methods.