Calcium isotope values of modern and fossil cephalopod shells – Trophic level or proxy for seawater geochemistry?

Abstract

Carbonate hardparts of marine organisms are frequently explored archives of their ambient seawater composition. Among the various materials used, the hardparts of molluscs are particularly relevant because their accretionary growth allows for the compilation of time-resolved proxy data sets. Here, we explore to which extent the calcium isotope values of aragonitic cephalopod hardparts represent proxies for seawater composition or record a trophic level signal. In a first step, we collect Ca isotope data from a diverse set of modern cephalopods, including Nautilus, Sepia and Spirula. These taxa differ in their external/internal hardparts, life spans, trophic levels, habitat depth, hinterland geology, and resulting runoff. In a second step, we have put these concepts to the test and explore the calcium isotope signatures of Lower Cretaceous (lower Albian) well-preserved cephalopods (nautiloids, ammonoids and belemnites) and bivalve shells from Madagascar. In the sense of a methodological comparison, two measurement approaches (double spike (δ44/40Ca, TIMS) and bracketing standard (δ44/42Ca, MC-ICP-MS) techniques are applied. Calcium from organic matter and intra-shell variations on bulk δ44/40Ca isotope values are explored. Four protocols (H2O2, NaOH, HClO4, incineration up to 250 °C including subsequent rinsing with deionised water and Ca removal) are applied to destroy organic matter in the shell material and to dissolve any non‑carbonate bound Ca, i.e., its metastable secondary Ca reservoir. These are equally successful, and the Ca isotope values of the purified carbonate phase plot on the equilibrium mass fractionation line. Samples not treated to destroy or remove their organic matter do not fall on the equilibrium mass fractionation line. Limitations of sample pre-treatment are reflected by a slight shift towards lower values in the case of incineration experiments at 650 °C. This feature is best explained in the context of a mineralogical phase transformation of aragonite to calcite. Calcium isotope values show no significant variability when sampling hardparts with different microstructures (callus, nacre, prismatic layer) and mineralised during different ontogenetic phases (outer shell, septa). Perhaps most relevant, mean δ44/40Ca (‰ SRM-915a) isotope values for Nautilus (0.72‰ ±0.19 2SD), Sepia (0.79‰ ±0.18 2SD), and Spirula (0.50‰ ±0.18 2SD) lack evidence for a significant control by trophic levels. Nitrogen isotope data and known diets for all three cephalopods serve as a litmus test for these data, which do not correlate with this dietary proxy information. These results are considered encouraging and suggest that Ca isotope values of cephalopod shells are proxies for their ambient seawater Ca isotope signatures. According to our data, however, it remains unclear to which extend cephalopod hardparts record the Ca isotopic composition of past seawater. Specifically, the data from three different modern species point to a mean Δ44/40Ca(ceph-seawater) fractionation of −1.21‰ (± 0.15 2SD). Based on our work, we present a best practice guide for cephalopod Ca isotope analysis and interpretation.