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.本研究整合了全球范围内已发表及全新的腕足动物（brachiopod）矿物学数据，涵盖绝大多数主要类群，旨在探究腕足动物碳酸盐矿物学的分布模式及其调控机制。本次研究共对147个物种的1711件标本开展了检测，其中包含253件化石标本；现生腕足动物的分布范围覆盖北纬79°至南纬74°，栖息环境从潮间带延伸至近4000米水深区域。钙质腕足动物拥有坚固且韧性极佳的壳瓣，其壳体由极低镁方解石（low-Mg calcite，该门腕足动物平均MgCO₃含量为1.3 wt%）构成。穿孔贝目（Craniida，平均MgCO₃含量为8.9 wt%）与被壳贝目（Thecideida，平均MgCO₃含量为6.5 wt%）较为特殊，它们沉淀的方解石镁含量更高；该两类群的独特之处在于会将自身固着于基底之上。本研究首次发现双矿物型腕足动物：部分物种同时兼具低镁方解石（平均MgCO₃含量为0.8 wt%）与中镁方解石（intermediate-Mg calcite，平均MgCO₃含量为7.2 wt%）两种矿物组分。尽管方解石中的镁含量在壳瓣的不同层位间存在系统性变化，部分类群还会沿生长轴呈现规律性差异，但我们未发现同一壳体的不同壳瓣间存在稳定的组分差异。虽存在微弱的纬度分布信号，表明整体温度会对镁含量产生一定调控作用，但总体而言腕足动物自身对矿物组分具有较强的调控能力：即便海水Mg:Ca比值较高，它们仍能沉淀低镁方解石。影响腕足动物矿物学特征的因素涵盖个体、环境及系统发育多个维度，由此产生的矿物组分变异极为复杂。古海洋学研究中假设腕足动物仅具备单一矿物组分的做法，或需重新审视。