Skeletal carbonate mineralogy of New Zealand bryozoans

Abstract

On many cool-water carbonate shelves, both modern and ancient, bryozoans are the dominant sediment contributor. Because skeletal carbonate mineralogy can be related to both taxonomic and environmental controls, and has important implications for understanding the geochemistry and diagenesis in carbonate deposits, it is highly relevant to develop a comprehensive database for bryozoan skeletal mineralogy. This X-ray diffraction study of modern New Zealand bryozoans more than doubles the published data available on bryozoan mineralogy and allows for statistical analysis of variability in mineralogy, particularly the hitherto unknown background variation within individuals and within populations. Such a database allows evaluation of differences among populations, taxa, and growth forms. The 412 analyses of 49 bryozoan species fall into four mineralogical categories: (a) 61% are single calcite, which ranges widely from low to high wt% MgCO 3 varieties; (b) 6% (in the genera Cellaria and Macropora) comprise dual calcites, a dominant one low in Mg ( X=1.8 wt% MgCO 3) and the other much higher ( X=9.0 wt% MgCO 3); (c) 27% are mainly calcite with some aragonite; and (d) 6% are mainly aragonite with some calcite. The bimineralic calcite–aragonite skeletons can arise from different processes, including biologically mediated changes in precipitation and diagenetic alteration following death. There is small but significant variation in Mg content in calcite both within colonies (Mg increasing with age) and within populations, for a total of within-species variance of 0.526. We found, however, more variation among species in both calcite/aragonite ratio and Mg content than that within species. Whereas some genera and families are reasonably consistent mineralogically (e.g., low-Mg calcite cyclostomes such as Telopora buski), others are highly variable (e.g., Chaperia acanthina). Among the higher taxa, cyclostomes tend to be entirely or mainly calcite with a low to moderate range of MgCO 3 ( X=2.1 wt%), whereas cheilostomes are far more variable, with anascans ( X=4.1 wt%) having a lower MgCO 3 content than ascophorans ( X=5.6 wt%). All dual-calcite species are anascans in the Calloporoidea. The proportions of aragonite and of Mg in calcite are greater in evolutionarily younger taxa, at least at the higher levels. In relation to growth forms, erect rigid varieties are calcitic except for several robust-branching genera such as Adeonellopsis. Encrusting forms, particularly unilaminar sheets, are highly variable and contain most of the bimineralic calcite–aragonite species. Erect flexible forms are either single or dual calcite, and extremely variable in Mg content. Free-living or vagrant forms are usually aragonitic. In relation to latitude and temperature variation, few species show any consistent or significant mineralogical trends, although Schizoporella unicornis deposits increasing amounts of aragonite at lower (warmer) latitudes. Mecynoecia purpurascens includes more (but still low) Mg in its calcite skeleton with increasing latitude, a reversal of the Mg–temperature relationship noted in several other skeletal carbonate producers.