Mineralogy of Arctic bryozoan skeletons in a global context

Bryozoans are major carbonate producers in some ancient and Recent benthic environments, including parts of the Arctic Ocean. Seventy-six species of bryozoans from within the Arctic Circle have been studied using XRD to determine their carbonate mineralogies and the Mg content of the calcite. The majority of species were found to be calcitic, only four having bimineralic skeletons that combined calcite and aragonite, and none being entirely aragonitic. In almost all species, the calcite was of the low- (< 4 mol% MgCO3) or intermediate-Mg (4-11.99 mol% MgCO3) varieties. Previous regional studies of bryozoan biomineralogy have found higher proportions of bimineralic and/or aragonitic species in New Zealand and the Mediterranean, with a greater number of calcitic species employing intermediate- and high-Mg calcite. The Antarctic bryozoan fauna, however, has a similar mineralogical composition to the Arctic. The lesser solubility of low-Mg calcite compared to both Mg calcite and aragonite in cold polar waters is most likely responsible for this latitudinal pattern. However, it is unknown to what extent environmental factors drive the pattern directly through eliciting an ecophenotypic response from the bryozoans concerned or the pattern reflects genetic adaptations by particular bryozoan clades.