Brachiopod fabric, classes and biogeochemistry: implications for the reconstruction and interpretation of seawater carbon-isotope curves and records

Shell calcite of modern and ancient brachiopods is an important proxy in reconstructing oceanographic conditions of seawater. Our investigation reveals significant magnesium and carbon isotope relationships between two classes of ancient brachiopods. Brachiopods of the extinct Class Strophomenata constructed two- or three-layer shells with a primary layer of randomized granular calcite, a secondary layer of cross-bladed laminar calcite and, at times, a tertiary layer of large prismatic calcite. Contemporaneous, but extant, Rhynchonellata also have two- or three-layer shells, but their secondary layer consists of calcite fibers instead of cross-bladed laminar calcite. Furthermore, inter-crystalline space is limited in between the fibers of the secondary layer of the Rhynchonellata but higher in the laminar secondary layer of Strophomenata. δ18O values of the two classes of brachiopods from the same stratigraphic interval of the Lopingian Nesen Formation of North Iran show a considerable overlap with no significant difference (p=0.263). In contrast, carbon isotope values are significantly different (p=0.001) between contemporaneous Strophomenata (mean: +2.18‰) and Rhynchonellata (mean: +4.13‰). An evaluation of literature data of Permian (Asselian) and Carboniferous (Visean and Serpukhovian) Strophomenata (mean: +2.32‰, N=59) and Rhynchonellata (mean: +4.94‰, N=105) confirms a similar differentiation in their carbon isotope values.Its higher magnesium contents suggest that the Strophomenata probably secreted shell calcite at a faster rate than their coeval Rhynchonellata. This process may account for the δ13C differentiation observed between specimens of these two brachiopod classes. However, the different amount and composition of the organic matter of the Strophomenata may also play a role in shaping their more negative carbon isotope signature. Thus, the class of brachiopod and their associated different fabric must be adjusted for in their δ13C results and taken into consideration during the reconstruction of Paleozoic global δ13C carbonate-based seawater curves. Invariably, in some earlier studies undetected impacts imposed by the class-fabric on δ13C may account for some of the variation or trends documented in Deep-Time seawater-13C curves reconstructed with brachiopod populations from North America, Europe and Russia.