Searching for Disequilibrium: Clumped Isotope and Stable Carbon and Oxygen Isotope Signatures in Cultured Biogenic Marine Carbonates

Carbonate clumped isotope analysis is a technique constraining the formation temperature of carbonate minerals by measuring the bond ordering of 13C-18O in the mineral lattice. However, this proxy assumes mineral formation occurs in equilibrium with the external environment, which has been shown to not be the case for oxygen isotope fractionation in many types of carbonate biominerals. Here we explore the hypothesis that the high-level of control organisms exhibit during biomineralization can affect the resulting carbonate clumped isotope (Δ47) signature recorded in the final lattice in ways dependent on the calcification strategy used (i.e., transient phases, pH regulation, etc.). We quantify δ13C, δ18O, and Δ47 using mass spectrometry in more than a dozen species of marine calcifiers, some of which are used for paleoclimate reconstruction, grown at approximately 25 ^oC and at a range of pCO2 levels in order to explore the extent to which these isotopic ratios record the biomineralization strategy used. Initial results do not support the hypothesis that biominerals from a diverse array of carbonate-precipitating organisms grown at the same temperature yield identical Δ47 values and, instead, yield evidence of possible disequilibrium calcification in certain taxa. Measurements to date indicate that the strongest departure from predicted (i.e., equilibrium) Δ47 values is exhibited by a coralline red alga species that is known to elevate pH at the site of biomineralization. Observation of nonuniform Δ47 signatures with marine carbonates precipitated from similar environmental conditions could indicate that these recorded isotopic signatures correlate to conditions within the aqueous medium at the crystal-solution interface. Planned work includes boosting sample replication and diversity of species studied, as well as comparing results to experimental and theoretical predictions for the isotopic disequilibrium associated with specific geochemical processes.