Skeletal Carbonate Mineralogy of Brachiopods: Smith, Freeman, Dixon-Anderson and Lee

Here we combine published and new mineralogical data on brachiopods from all over the world, including most major taxa, to investigate patterns in and controls on brachiopod carbonate mineralogy. Measurements of 1711 specimens in 147 species included 253 fossil specimens: living species range from 79°N to 74°S and from intertidal to almost 4000 m deep. Calcareous brachiopods create strong and resilient valves, formed of very low-Mg calcite (χ ̅ =1.3 wt% MgCO3 in the phylum). The Craniida (χ ̅ = 8.9 wt% MgCO3) and Thecideida (χ ̅ = 6.5 wt% MgCO3) are unusual in precipitating calcite with higher Mg content; these taxa are unusual in that they cement themselves to the substrate. This is the first study to find bimineralic brachiopods; a few species show a combination of low-Mg (χ ̅ = 0.8 wt% MgCO3) and intermediate-Mg calcite (χ ̅ = 7.2 wt% MgCO3). While Mg in calcite varies systematically among valve layers and sometimes along the growth axis, we found no consistent difference between valves of the same individual. A weak latitudinal signal indicates some overall temperature control of Mg, but in general brachiopods are strongly controlling, precipitating low-Mg calcite even when Mg:Ca ratios is seawater are high. The drivers and influences on brachiopod mineralogy range from individual to environmental to phylogenetic – the resulting variability is complex. Assuming simple mineralogy in brachiopods for paleoceanographic studies may need to be reexamined.