Discovery of dolomite and magnesite in living coralline algae

Abstract

This Masters started as a consideration of the impacts of ocean acidification on high-magnesium calcite coralline algae. I shifted focus part way through to consider the composition of magnesium calcite sediments from the Great Barrier Reef. While investigating some inconsistencies in X-ray diffraction (XRD) results, I discovered that the coralline algae Hydrolithon onkodes samples I had analysed contained significant amounts of dolomite and magnesite and that this explained inconsistent results not only from my work but also possibly most published work on coralline mineralogy for the past 50 years. My focus again shifted to investigate the occurrence of this dolomite and magnesite, and the final paper presented herein details the discovery of dolomite and magnesite in coralline algae. Early research into coralline algae mineralogy noted the discrepancy between bulk magnesium measurements for mol % MgCO{u2083} and results of those calculated from XRD. This offset was attributed to the presence of amorphous magnesium hydroxide (brucite) or magnesium calcites ranging up to 30 mol % MgCO{u2083}. Some earlier researchers considered that dolomite might be present, but this was unable to be proven. Sedimentary dolomite has been thought to form by a post-depositional diagenetic process that converts limestone to dolomite over long periods of time, perhaps up to millions of years. Marine sedimentary dolomite in fossil coral reefs is typically found in association with coralline algae. Our discovery revealed that dolomite and magnesite can form within living coralline algae. Chemical micro-analysis of the coralline skeleton reveals that not only are the cell walls calcified, but that cell spaces are typically filled with magnesite, rimmed by dolomite or both. Our results are consistent with dolomite occurrences in coralline algae rich environments in fossil reefs of the last 60 million years. Furthermore, this discovery demonstrates that dolomitisation, rather than being a lengthy process over thousands of years or more, can take place contemporaneously with living processes in coralline algae. The question of why this prolific dolomite mineralisation hadn't been discovered previously is considered and comparative treatment tests reveal that the standard process of bleaching prior to XRD analysis can result in a decrease of the intensity of dolomite and magnesite diffraction patterns. Consideration is given to the role of dominant paradigms in obscuring scientific progress in this area of study, and I reflect on how that may have impacted previous research. Finally, I conclude that the XRD method is suitable for estimating mol % MgCO{u2083} for tropical coralline algae cell wall Mg-calcite, that dolomite formed by coralline algae can account for primary dolomite found in Cenozoic reefs, and that the role of biologically formed dolomite needs to be considered when reconstructing past environments.