Exploring relationships amongst δ13C, δ18O, Δ47, and δ11B estimates of calcifying fluid pH in cultured marine biogenic carbonates

Biominerals record environmental parameters in the form of isotope and trace element signatures. Oftentimes, biomineralization is a highly-controlled process, causing the resulting bones, shells, or teeth to be out of equilibrium with the organism's environment. These biologically-induced deviations are not well understood and vary from organism to organism. Multiply-substituted carbonate "clumped" isotope (Δ ₄₇ ) analysis potentially circumvents this complication, permitting paleothermometry without knowledge of the isotopic composition of organisms' growth environments by measuring the abundance of ¹³ C- ¹⁸ O in the mineral lattice. However, this still assumes carbonate formation occurs in equilibrium with the organism's surroundings or solely varies with growth temperature, which has been shown not to be true in a number of controlled biogenic and abiogenic mineral-precipitating organisms. To examine the processes driving disequilibrium, we use boron isotope analysis (δ ¹¹ B), which correlates with calcifying fluid pH and not with temperature, making it ideal for observing pH effects in paleoenvironmental proxies. We quantified δ ¹³ C, δ ¹⁸ O, and Δ ₄₇ and explored their relationships with δ ¹¹ Β in several species of calcifying marine invertebrates, from phyla including Mollusca, Echinodermata, Cnidaria, Annelida, Rhodophyta, and Chlorophyta. These samples were cultured in laboratory conditions at 25 °C and across a range of p CO ₂ . Initial work revealed relationships amongst these proxies to δ ¹¹ B, including positive, negative, and no-relationship, but that these relationships are highly species-specific. These relationships show potential coupling with, and thus pH effects on, these isotope signals. Exploring these relationships should elucidate processes the processes dominating disequilibration and what ultimately gets recorded in the final biomineral, thereby improving our ability to use biomineral proxies to reconstruct paleoenvironments.