oa Growth rate of Holocene reefal microbialites ? implications for use as environmental proxies, Heron Reef southern Great Barrier Reef

Microbialites are important constituents of reefrock in modern coral reefs, and they have proven to be reliable proxies for some aspects of seawater geochemistry. We used radiocarbon dating to evaluate their potential use as environmental archives by establishing their growth rates and temporal relationship with associated coral-algal communities. At Heron Reef, southern Great Barrier Reef, microbialites occupy ~7 % of framework volume, having formed in low-light settings in framework cavities within two metres below the reef flat. Microbialite growth generally was initiated within a few 100’s of years of the death of the surface-dwelling coral or clam substrates upon which they grew after those substrates had been incorporated into the framework by successive framework growth. Microbialites accreted at a mean rate of 2.9 mm/100 yrs and grew for only a few 100’s of years, presumably while conditions were optimal. Hence, microbialites represent ecological factors in reef growth, operating over similar time frames to associated coral-algal communities, but offset in time by as much as hundreds of years, compared to immediately adjacent skeletal framework. Therefore, microbialites can serve as proxies for marine geochemistry over averaged century-scale time frames, but they lack the temporal resolution afforded by, for example, scleractinian corals.