NESSF Fellowship for Levitt: Clumped Isotopes in Carbonates as a Potential Biosignature

Recent developments in the understanding of H2O-CO2-carbonate system equilibrium and kinetic fractionations associated with biological and inorganic reactions requires a critical reassessment of traditional and non-traditional stable isotope science and implications for biosignatures in carbonates as well as extraterrestrial climate reconstructions and links between early Earth climate and microbial evolution. The new geochemical method of “clumped isotopes” analyses has recently been employed to further characterize “vital effects” recorded in carbonates through biomineralization processes. The application of this new method has profound potential as a means of biomarker identification. In addition to delineation of biogenic and inorganic fractionation patterns, clumped isotopes analyses have also been used to determine temperature and other fluid conditions surrounding formation of carbonates. Better understanding of Earth system paleoclimate has been an early beneficiary of this application. However, analyses of extraterrestrial material using clumped isotopes methods have also provided insights into hydrologic conditions, gas phase CO2 dynamics, and aqueous dissolved inorganic carbon in the early Martian surface environment. Similar application of clumped isotopes science lends itself to investigation of early Earth carbonates that may contain clues to development of life and how this phenomenon influenced conditions conducive to the evolution. The study of clumped isotopes involves quantification of the preferential bonding between rare stable isotopes (e.g., 13C to 18O) in multiply substituted isotopologues as compared to stochastic (random) bonding predicted for that chemical compound or mineral lattice. Chemical bonds involving isotopes of higher relative atomic mass are stronger due to reduced bond vibration frequencies and therefore, zero-point energy. This thermodynamic bonding preference imparts temperature dependence to isotope clumping. Application of clumped isotope geochemistry for carbonate analysis requires empirical and/or theoretical characterization of 13C-18O bonding frequency (magnitude denoted by #47) and parameter dependence over the range of possible carbonate formation conditions. Published experimental and theoretical studies aiming to determine the temperature dependence of isotope clumping in carbonates have produced discrepant results. However, related field, laboratory, and modeling work investigating kinetic fractionation in carbon, oxygen, and #47 isotopes due to rapid carbonate precipitation and degassing of CO2 from solution may explain discrepant T-#47 empirical and theoretical data. Understanding isotopic trends and patterns recorded by inorganic precipitation is paramount to the understanding vital effects recorded by biomineralization. Recent examination of corals, foraminifera, and other biogenic carbonates finds non-equilibrium fractionation of carbon and oxygen but equilibrium fractionation (as compared to experimental studies) of 13C-18O bonds. The proposed scope work is motivated by the hypothesis that experimental T-#47 relationships determined previously were influenced by kinetic fractionation due to CO2 degassing and that kinetic fractionation must be avoided to accurately characterize inorganic parameters affecting 13C-18O clumping in carbonates. This project proposes to use the chemostat precipitation method to characterize inorganic conditions influencing 13C-18O bonding in carbonates precipitated from solutions at a constant partial pressure of CO2 and constant solution isotopic composition. These experimental methods will facilitate the reconciliation of experimental and theoretical studies, allow for more accurate reconstruction of environmental conditions using clumped isotope geochemistry, and enhance understanding of vital effects stored in biogenic carbonates carbon and oxygen isotope anomalies.