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

摘要

Dolomite is a magnesium-rich carbonate mineral abundant in fossil carbonatereef platforms but surprisingly rare in modern sedimentary environments, aconundrum known as the "Dolomite Problem". Marine sedimentary dolomite hasbeen interpreted to form by an unconfirmed, post-depositional diageneticprocess, despite minimal experimental success at replicating this. Here weshow that dolomite, accompanied by magnesite, forms within living crustosecoralline alga, Hydrolithon onkodes, a prolific global tropical reef species. Chemicalmicro-analysis of the coralline skeleton reveals that not only are the cellwalls calcitised, but that cell spaces are typically filled with magnesite,rimmed by dolomite, or both. Mineralogy was confirmed 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 andbrucite) as previously thought. Our results are consistent with dolomiteoccurrences in coralline algae rich environments in fossil reefs of the last60 million years. We reveal that the standard method of removing organicmaterial prior to Xray Diffraction analysis can result in a decrease in themost obvious dolomite and magnesite diffraction patterns and this mayexplain why the abundant protodolomite and magnesite discovered in thisstudy has not previously been recognized. This discovery of dolomite inliving coralline algae extends the range of palaeo-environments for whichbiologically initiated dolomite can be considered a possible source ofprimary dolomite.