First discovery of dolomite and magnesite in living coralline algae and its geobiological implications

摘要

Dolomite is a magnesium-rich carbonate min-eral abundant in fossil carbonate reef platforms but surpris-ingly rare in modern sedimentary environments, a conun-drum known as the "Dolomite Problem". Marine sedimen-tary dolomite has been interpreted to form by an uncon-firmed, post-depositional diagenetic process, despite mini-mal experimental success at replicating this. Here we show that dolomite, accompanied by magnesite, forms within liv-ing crustose coralline alga, Hydrolithon onkodes, a prolific global tropical reef species. Chemical micro-analysis of the coralline skeleton reveals that not only are the cell walls cal-citised, but that cell spaces are typically filled with magne-site, rimmed by dolomite, or both. Mineralogy was con-firmed by X-ray Diffraction. Thus there are at least three mineral, phases present (magnesium calcite, dolomite and magnesite) rather than one or two (magnesium calcite and brucite) as previously thought. Our results are consistent with dolomite occurrences in coralline algae rich environ-ments in fossil reefs of the last 60 million years. We re-veal that the standard method of removing organic material prior to Xray Diffraction analysis can result in a decrease in the most obvious dolomite and magnesite diffraction patterns and this may explain why the abundant protodolomite and magnesite discovered in this study has not previously been recognized. This discovery of dolomite in living coralline algae extends the range of palaeo-environments for which biologically initiated dolomite can be considered a possible source of primary dolomite.