Boron, carbon, and oxygen isotopic composition of brachiopod shells: Intra-shell variability, controls, and potential as a paleo-pH recorder

The boron isotopic composition of biogenic carbonates has been used to reconstruct seawater pH and atmospheric pCO₂ on Pleistocene and Cenozoic timescales. Because of their excellent preservation and extensive fossil record throughout the Phanerozoic, brachiopods are a promising candidate for extending the boron isotope record as far back as the Cambrian. Here we present stable carbon, oxygen, and boron isotopic measurements of modern Terebratulid brachiopod calcite in comparison with environmental pH estimates calculated from oceanographic data. Geochemical transects along the length and depth of single shells confirm previously published trends in carbon and oxygen isotopic composition. In the outer surface (primary and outermost secondary layers), δ¹¹B covaries with δ¹³C and δ¹⁸O, with more negative values in the outer and more positive values in the middle of the shell. However, δ¹¹B deviates from δ¹³C and δ¹⁸O in the inner part of the secondary layer, where the δ¹³C and δ¹⁸O values are more positive and near equilibrium, whereas δ¹¹B returns to more negative values. A comparison of different specimens of the species Terebratalia transversa (Sowerby, 1846) and Laqueus californianus (Küster, 1844) microsampled from the middle part of the fibrous secondary layer demonstrates a clear correlation to ambient pH with a sensitivity similar to other empirical calibration curves for cultured planktic foraminifers, corals, and inorganic calcite. The relationship in other species is less clear and significantly offset, necessitating the use of a single species or a cross-calibration method with other species in paleo-pH reconstructions.