Biomineralizing organisms record information about the environments they grew in through their geochemistry (e.g., elemental and isotopic ratios). Their skeletal remains have become powerful tools for reconstructing biological and environmental processes, past and present. Organisms exert varying levels of control over biomineralization, and this control can alter the resulting geochemistry recorded in the biomineral. Research into these vital effects has shown that they are often species-specific, suggesting the potential to advance our understanding of how organisms biomineralize using geochemistry and the plausibility of geochemical identities. My dissertation explores geochemical patterns in skeletal remains, and probes biomineralization mechanisms. I report the results of the study of a diverse array of species that were cultured, and the experimental transformation of abiotic amorphous calcium magnesium carbonates (ACMC), which is a widespread biomineralization strategy, using different elemental and isotopic systems. In Chapters 1 and 2, I present some of the largest and most diverse datasets of elemental and isotopic data for benthic invertebrates. I report 9 element-to-calcium ratios (Li, B, Na, Mg, Zn, Sr, Cd, Ba, and U) (Chapter 1), and singly- and multiply-substituted isotopes (δ13CVPDB, δ18OVPDB, Δ47, I-CDES, and Δ48, CDES) (Chapter 2). Chapter 1 reports data, from 18 species of benthic marine invertebrates spanning a range of biogenic carbonate polymorph mineralogies (low- and high-Mg calcite, aragonite, and mixed mineralogy) belonging to a range of phyla (including Mollusca, Echinodermata, Arthropoda, Annelida, Cnidaria, Chlorophyta, and Rhodophyta), all cultured at a single temperature (25 °C) and at several carbon dioxide treatments (ca. 409, 606, 903, and 2856 ppm). Chapter 2 reports data for 12 species. I explore controls over elemental and isotopic partitioning in biogenic marine carbonates, including species-level and biomineralization-pathway-level controls, internal pH regulation compared to external pH changes, and short-term responses to changing seawater chemistry. Notably, these datasets enabled some of the first explorations of broad-scale geochemical patterns in an array of biogenic carbonates cultured in and across controlled environmental conditions.
These chapters statistically analyze the elemental and isotopic data using univariate, multivariate, and phylogenetic analyses and demonstrate that marine calcifiers predominantly do not mimic how geochemistry is recorded in abiotic minerals. Instead, I uncover high phylogenetic signals in the data. With the elemental data, I show that phylogenetic relationships explain more of the variation in the data than differences in mineralogy or carbon dioxide levels in the cultures. I observe similar phylogenetic patterns in the isotopic data. I show that by using novel dual clumped isotope analysis (i.e., pairing two clumped isotopologues), it becomes possible to resolve differences in subcellular mechanisms associated with the movement and transformation of dissolved inorganic carbon species during biomineralization, between diverse taxa.
In Chapter 3, I use our mechanistic knowledge of geochemical tracers (Mg/Ca, Li/Ca, δ7Li, δ26Mg, and δ44Ca) to provide a perspective on a widespread biomineralization strategy, ACMC and synthesis and transformation into crystalline carbonates. To understand how this pathway affects biomineral geochemistry, synthesized ACMC that was transformed at different temperatures was measured to determine elemental and isotopic compositions, from which I make fundamental interpretations of the kinetics and thermodynamics. I find that ACMC transformation involves concurrent dissolution of amorphous particles and carbonate mineral precipitation, with a transition from kinetic to thermodynamic dominance with rising temperatures. While the experimental setup does not directly replicate natural biomineralization processes, the persistence of non-equilibrium fractionations suggests that geochemical proxies based on these systems should account for the influence of amorphous precursor phases and transformation kinetics.
In all, my results underscore the importance of accounting for biological processing when using a diverse set of geochemical tools as geobiological or paleoclimatic proxies. They also suggest a link between biomineral elemental and isotopic geochemistry, specific biomineralization strategies, and evolutionary history. These findings center geochemistry as a powerful tool for understanding evolution as a keystone process, from climate to life.
