The preservation of francolite-associated sulfate in lingulid brachiopods

Lingulid brachiopods - the namesake "living fossils" of Darwin - have a continuous record from the Cambrian to the present and possess phosphatic valves, rendering them largely unique among extant invertebrates. Due to their poorly understood vital effects and susceptibility to taphonomic breakdown, they have not been widely adopted as geochemical archives. However, inorganic sulfate is abundant in lingulid bioapatite, which may be more resistant to diagenetic alteration than carbonate-based minerals. As such, it is possible that lingulid shells may provide an alternative, independent archive of Phanerozoic sulfate seawater chemistry, provided that the distribution and preservation of sulfur within lingulid shells can be identified. Here, we leverage sulfur K-edge micro X-ray Fluorescence (μ-XRF) imaging, X-ray Absorption Near Edge Structure (XANES) spectroscopy, and petrography to characterize sulfur speciation and distribution within extant and fossil lingulid brachiopods. In modern Lingula anatina, inorganic sulfate is dominant along francolite-rich laminae, whereas organic sulfur species are mostly observed in non-mineralized laminae and the periostracum. In the Cambrian fossil Ungula ingrica, inorganic sulfate is generally dominant in regions with the lowest sulfur abundances, with higher sulfate abundances along compact laminae than on baculate laminae. In contrast, the distribution of diagenetic pyrite is heterogenous, although largely concentrated along shell edges and particular laminae. Notably, the XANES spectrum of francolite-associated sulfate is characterized by a unique post-edge feature at 2494.0 eV. Collectively, these results suggest that carefully selected lingulid specimens - and from intra-valve laminae with minimal diagenetic phases - have the potential to be insightful for reconstructing Phanerozoic sulfate seawater chemistry.