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Brucite microbialites in living coral skeletons: Indicators of extreme microenvironments in shallow-marine settings

¹ School of Natural Resource Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia
² U.S. Geological Survey, 600 Fourth Street South, St. Petersburg, Florida 33701, USA
³ Department of Geological Sciences, State University of New York at Binghamton, Binghamton, New York 13902-6000, USA
⁴ Inorganic Materials Research Program, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia

Brucite [Mg(OH)₂] microbialites occur in vacated interseptal spaces of living scleractinian coral colonies (Acropora, Pocillopora, Porites) from subtidal and intertidal settings in the Great Barrier Reef, Australia, and subtidal Montastraea from the Florida Keys, United States. Brucite encrusts microbial filaments of endobionts (i.e., fungi, green algae, cyanobacteria) growing under organic biofilms; the brucite distribution is patchy both within interseptal spaces and within coralla. Although brucite is undersaturated in seawater, its precipitation was apparently induced in the corals by lowered pCO₂ and increased pH within microenvironments protected by microbial biofilms. The occurrence of brucite in shallow-marine settings highlights the importance of microenvironments in the formation and early diagenesis of marine carbonates. Significantly, the brucite precipitates discovered in microenvironments in these corals show that early diagenetic products do not necessarily reflect ambient seawater chemistry. Errors in environmental interpretation may arise where unidentified precipitates occur in microenvironments in skeletal carbonates that are subsequently utilized as geochemical seawater proxies.

Key Words: brucite • microbialite • scleractinian coral • paleoclimate • Mg/ Ca ratio • marine chemistry

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