Potassium isotope fractionation in the cultured brachiopod Magellania venosa

Seawater potassium (K) isotopes have the potential to trace the silicate cycle in Earth’s history, yet a reliable archive that can faithfully record the temporal seawater K isotope composition (δ⁴¹K) is still lacking. Brachiopods present a potentially unparalleled marine fossil archive for reconstructing seawater chemistry, owing to their long evolutionary history and shell resistance to diagenetic alteration. However, the mechanisms of K incorporation and isotopic fractionation during brachiopod calcification remain unknown. In this study, we combined elemental, isotopic and synchrotron-based spectroscopic analyses to study the variations in K-bearing phases and δ⁴¹K compositions of the brachiopod species Magellania venosa, cultured under well-constrained experimental conditions. When corrected for culture medium δ⁴¹K, the Δ⁴¹K of oxidatively cleaned cultured M. venosa specimens ranges from –0.41 to 0.45 ‰, without an apparent relationship to temperature, carbonate and seawater chemistry in the culturing tanks. These values are within the range of previously published δ⁴¹K values in natural brachiopods (Li et al., 2021) and demonstrate that δ⁴¹K is not influenced by environmental parameters. Potassium K-edge X-ray absorption near edge structures (XANES) spectroscopy analyses of uncleaned shell powders showed that K is hosted in both with calcite and organic phases, with most K associated with the organics. Following oxidative cleaning, elemental and isotopic data suggest that most K originates from the calcite phase, although some insoluble organic material remains and is depleted in heavy K isotopes. We suggest that the observed wide spread of K isotope compositions in brachiopods, along with other biogenic carbonate δ⁴¹K values, is best explained by mixing between calcite (∼0.3 ‰) and organic (∼–1 to –2‰) phases. Collectively, our findings imply that clean brachiopod calcite may exhibit a consistent offset from the modern seawater of ∼+0.3 ‰, supporting its potential as a seawater δ⁴¹K archive, provided that organic phases are effectively removed before isotopic analysis. Future sequential cleaning and carbonate synthesis experiments will be essential to further establish the fidelity of fossil brachiopod δ⁴¹K records as proxies for past seawater δ⁴¹K.