Environmental and physiological influences on isotopic and elemental compositions of brachiopod shell calcite: Implications for the isotopic evolution of Paleozoic oceans

Abstract

Brachiopods from the Demissa Bed (Middle Devonian, Hamilton Group) biologically regulated the incorporation of Mg, Sr and Na into their shell calcite. Significant taxonomic differences in the elemental contents of three species (Athyris spiriferoides, Mediospirifer audacula and Mucrospirifer mucronatus) may be related to differences in calcification processes. In contrast, no significant difference or “vital effects” were observed between the isotopic values of Mediospirifer audacula and Athyris spiriferoides from Erie (Δ¹⁸O=0.03‰, p=0.949; Δ¹³C=0.76‰, p=0.083) and Genessee Counties (Δ¹⁸O=0.39‰, p=0.471; Δ¹³C=0.06‰, p=0.854). This suggests that these brachiopods did not exert a biological control over their isotopic compositions, and that their shell calcites reflect ambient physicochemical conditions.

Isotopic compositions in unaltered shell calcites of brachiopods from Genessee County, which was close to the basin depocentre, are heavy for carbon (δ¹³C, $\text{χ}$=+5.01‰, PDB) and oxygen (δ¹⁸C, $\text{χ}$=−2.85‰, PDB) compared to the species sampled at the basin's edge (Erie County; (δ¹³C, $\text{χ}$=+2.79‰, PDB; δ¹⁸O, $\text{χ}$=−3.83‰, PDB). There is a significant separation in isotopic values between the deeper- and shallower-water brachiopods of the basin ((δ¹⁸O=0.98‰, p=0.0005; Δ¹³C=2.22‰, p=0.0005). The δ¹⁸O variation suggests a temperature/salinity change with water depth, whereas the change in δ¹³C composition probably records an enrichment /depletion of organic matter with water depth. This observation has significant implications for Paleozoic ocean isotopic-evolution studies, because many global changes in marine δ¹⁸O and δ¹³C are based on isotopic shifts of similar magnitude.