Development of a Lingulid Brachiopod Proxy for Evaluating Phanerozoic Seawater Calcium Isotope Evolution

Calcium isotopic ratios offer the potential to reconstruct calcium cycle variability over geologic timescales, in turn elucidating long-term controls on ocean chemistry, climate, and biologic productivity. The most robust reconstructions of Phanerozoic seawater calcium isotope trends are based on skeletal carbonates and vertebrate phosphates. However, there are still considerable discrepancies between different archives, particularly in pre-Cenozoic rocks, potentially due to (i) the difficulty of calibrating species-dependent vital effects in extinct lineages and (ii) the susceptibility of many biogenic archives to geochemical resetting, even in some exceptionally preserved fossils.

Linguliform brachiopods may offer a unique approach to reconstructing the Phanerozoic calcium cycle. Lingulid shells are composed of francolite, which is markedly more resistant to diagenesis than carbonate and hydroxyapatite minerals. This clade originated in the basal Cambrian, allowing for a near-continuous Phanerozoic record. Its shell morphology has changed little since the mid-Paleozoic, suggesting isotopic fractionation calibrations based on modern organisms may be robust in the geologic record. Finally, linguliform brachiopods are archetypal disaster taxa during mass extinction intervals, rendering them ideal archives for exploring calcium cycle instability in these critical periods. Together, these factors suggest that lingulid brachiopods may offer an alternative, independently derived approach for reconstructing the calcium isotopic evolution of Phanerozoic seawater.

Here, we present a calibration of the fractionation between ambient seawater and modern lingulid brachiopod phosphate, including an overview of analytical protocols, intra- and inter-valve variability in individual Lingula specimens, and geographic variability. Preliminary results suggest that lingulid shells possess remarkably high calcium isotope ratios (avg. δ44/40Ca of -0.7‰ relative to seawater), substantially heavier than all known vertebrate apatite and inorganic sedimentary phosphorites. We will explore the implications of these heavy calcium isotope values for understanding lingulid biomineralization processes and applying lingulid geochemistry in the geologic record.