Stable isotope fractionation and trace element partitioning in marine, terrestrial, and laboratory-synthesized carbonates
- Author(s): Mitnick, Elizabeth Horner
- Advisor(s): DePaolo, Donald J
- et al.
This dissertation is about trace element and isotope fractionation in natural and laboratory-synthesized calcite and aragonite. Quantifying various isotope (δ13C and δ44Ca) and trace element (Sr/Ca and Mg/Ca) ratios in carbonates allows us to probe the kinetics of various reactions at the mineral-fluid interface. Understanding the way in which these signals depend on various parameters (e.g., precipitation rate, temperature, fluid composition) elucidates molecular-scale fundamentals of crystal growth. Similarly, this framework allows us to use these isotopic and trace element probes as proxies for interpreting natural carbonates, and in many cases, understand ancient environments. Furthermore, by identifying the nature of carbonate growth in solutions of varying chemistry and temperature, we can best harness the ability of these minerals to sequester CO2 and fluid contaminants in response to anthropogenic pollution and climate change.
In Chapter 2, I use the Ca and Sr concentrations of marine sedimentary pore fluids to estimate rates of authigenic carbonate precipitation in modern sediments. Using the extensive Ocean Drilling Program database, these rates of authigenic carbonate production can be compared to coeval rates of organic carbon and biomineralized carbonate deposition, thereby quantifying relative fractions of each contributing carbon flux. While there are many sedimentary environments in which the relative authigenic carbonate fraction is significant in comparison to the organic and biomineralized fractions, the δ13C of these carbonates is almost identical to the biomineralized fraction in nearly all cases. Within the context of a carbon isotope mass balance framework, I demonstrate that the biomineralized and authigenic carbonate fractions can be suitably accounted for via a single inorganic carbonate pool. This finding has implications for interpreting the geologic record, indicating that some of the fluctuations in δ13C over the course of Earth history cannot be explained by the ongoing and extensive production of authigenic carbonate. I explore the few marine sedimentary environments that do produce authigenic carbonate with δ13C distinct from that of the biomineralized fraction, and thereby could be possible exceptions to the main finding of this work. The processes responsible for creating this isotopically anomalous authigenic carbonate are not thought to be steady-state phenomena throughout most of Earth history. The production of authigenic carbonate in these environments therefore suggests that δ13C variations in the sedimentary record could not have been the result of authigenic carbonate formation. Rather, the formation of authigenic carbonate with a δ13C composition distinct from that of biomineralized carbonate would be a consequence of a non-steady state perturbation to the carbon cycle. Consequently, these exceptional environments do not change the finding that a single inorganic carbon reservoir can be used to encapsulate the ongoing biomineralized and authigenic fluxes in marine sediments from an isotopic mass balance standpoint.
In Chapter 3, I study the formation of calcite and aragonite from the travertine-precipitating hot springs of Bridgeport, California. These travertines form as an elongate fissure ridge of intergrown calcite and aragonite, precipitating along the edges of a thin fluid conduit with systematically changing fluid pH, temperature, and CO2 degassing. The ability to readily measure modern precipitates from predictably evolving ambient fluids makes this system a unique natural laboratory in which to probe kinetics of carbonate growth in a high-temperature (>60°C) system. While travertines around the world have been the subject of a substantial body of literature, only one other study has reported the Ca isotope fractionation that occurs between travertine carbonates and fluid. In this work, δ13C, δ44Ca, Sr/Ca and Mg/Ca ratios in bulk carbonates and fluids are measured. Although the Sr/Ca ratios in the bulk solid are much higher than expected based on experimental studies, I argue that they may not be anomalous for thermogene travertines. The spatially evolving δ13C and Sr/Ca composition of the fluid and solid indicates decreasing aragonite content down-conduit, suggesting that the Ca isotope fractionation will reflect the relative change in calcite and aragonite fractions. Although aragonite is expected to produce a larger magnitude fractionation than calcite, we do not observe any trend with distance from the inlet corresponding to the systematically changing mineral fractions. This observation can be explained by ongoing recrystallization of carbonate in the porous conduit walls, demonstrating the complex nature of multi-mineral travertine deposits and the impact of diagenetic alteration on Ca isotope systems in nature.
Finally, in Chapter 4, I probe calcite growth at the smallest spatial and temporal scales studied in this dissertation. I present results from an experimental study in which calcite was precipitated from solutions of varying Mg/Ca ratios with a constant saturation index (SI) under controlled conditions. The aim of this project was to determine the effect of Mg on calcite growth as reflected in the Ca isotope fractionation between solid and solution. Mg is thought to primarily inhibit calcite precipitation via a kink blocking mechanism. However, the change in Ca isotope fractionation with increasing Mg/Ca and decreasing precipitation rate at constant SI demonstrates that in addition to kink blocking, Mg also impedes growth via incorporation inhibition. This additional mechanism alters the relative Ca attachment and detachment fluxes at the crystal surface, resulting in the observed fractionation. This finding has important implications for the interpretation of Ca isotope fractionation in both marine inorganic and biomineralized calcite precipitating from Mg-bearing seawater and sedimentary pore fluids.