Chemical fingerprints of five modern brachiopod species and their application to support fossil geochemical studies

The calcite of fossil brachiopods is a prime target for the reconstruction of palaeoenvironmental conditions through much of the Phanerozoic, utilizing a growing number of isotopic and chemical proxies. However, parameters such as intra-specific and intra-specimen variability of key chemical indices are still largely unknown for modern analogues, limiting the confidence with which such tools can be used for fossil specimens. Here we study the geochemistry of primary and secondary shell layers for multiple specimens of five extant brachiopod species (Liothyrella uva, Magasella sanguinea, Magellania venosa, Terebratella dorsata, and Notosaria nigricans) belonging to the orders Terebratulida (including suborders Terebratellidina and Terebratulidina) and Rhynchonellida. We report geochemical fingerprints and their variability for commonly studied proxies (Mg/Ca, Mn/Ca, Fe/Ca, Sr/Ca), adding also data for the rarely quantified Na/Ca and S/Ca ratios.
Well known element enrichments in primary shell calcite are quantified for a broad range of taxa for the first time. Mg is elevated by a factor of two and Na and Sr by c. 60 % over typical levels in the secondary calcite. Primary shell layer samples are compositionally offset from extrapolated Na-Sr and Mg-S trends in secondary shell layer calcite in Liothyrella uva and Magellania venosa, whereas there is strong overlap in Terebratella dorsata. While some species-specific spatial control on element/Ca ratios is seen, intra-specific heterogeneity in secondary shell calcite is moderate. While this finding suggests that representative element/Ca ratios can be defined for extant species that can guide interpretations of element/Ca ratios in fossil taxa, such heterogeneity needs to be taken into consideration where element/Ca ratios are targeted as palaeoenvironmental proxies. Median element/Ca ratios cover a broad range in extant taxa, but their intra-specific variability is similar and akin to that of screened fossil examples. Most studied species exhibit strong co-variation of Na/Ca with Sr/Ca ratios as well as Mg/Ca with S/Ca, but expression of these correlations may be modulated by differences in shell secretion behaviour between orders/suborders. Intra-specific chemical variability and co-variation of Mg/Ca with S/Ca are identified as additional tools for assessing fossil preservation