Experimental and theoretical effects of microorganisms, organic molecules, and atom exchange rates on the Ca isotopic composition of gypsum: Implications for the use of Ca isotopes as a geochemical proxy

Open Access
Harouaka, Khadouja
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
October 30, 2015
Committee Members:
  • Matthew Scott Fantle, Dissertation Advisor
  • Jennifer Macalady, Committee Member
  • James David Kubicki, Committee Member
  • Christopher Aaron Gorski, Special Member
  • Calcium isotopes
  • gypsum
  • surface selective inhibition
  • exchange rates
  • morphology
The Ca isotopic composition of minerals in the geologic rock record is a valuable proxy used to understand the processes that shaped the physical, chemical and biological evolution of our planet. However, correctly applying the Ca isotope proxy requires an in depth knowledge of how Ca isotopes fractionate during mineral precipitation. The mechanisms that control Ca isotopic fractionation during abiotic mineral precipitation are not yet clearly understood, and there is very little known about the mechanisms that govern biologically induced Ca isotopic fractionation during biomineralization. The main goal of this dissertation is to determine how biology can control Ca isotopic fractionation by quantifying the effect microbial life has on the Ca isotopic composition of gypsum and determining the underlying mechanisms that produce the measured Ca isotopic composition. The results of the work will be used to evaluate the viability of Ca isotopes as a biosignature of microbial life, as well as the extent to which the presence of organics can impact the Ca isotope record. The presence of Acidithiobacillus thiooxidans microbes in the gypsum precipitation solution resulted in a Ca isotopic fractionation factor that was 0.3 permil lighter than gypsum precipitated from abiotic controls. The change in tge fractionation factor due to the presence of microbes was strongly correlated with crystal morphology, such that biotic precipitates have smaller aspect ratios than abiotic precipitates produced under the same conditions. The change in morphology led to the hypothesis that microbially produced soluble and insoluble organic molecules inhibit gypsum precipitation rate, altering the fractionation factor. Simple gypsum precipitation experiments in the presence of basic amino acids confirmed this hypothesis, as amino acid inhibition resulted in lowering the fractionation factor by 0.2 permil relative to controls. Further, the difference in the energy of a surface upon which an amino acid is detached and attached, as determined by DFT theory, can vary up to 3 eV between multiple gypsum faces, which implies that organic molecule inhibition of gypsum precipitation is a surface selective process. It was also found, based on Gaussian frequency calculations of the same faces, that the fractionation factor associated with growth on the individual faces can vary by as much as 1.4 permil. Hence, surface selective inhibition of gypsum can allow for a greater expression of the fractionation factor of uninhibited faces relative to the inhibited faces, resulting in an isotopically distinct crystal. The empirical and theoretical evidence presented in this dissertation lead to the hypothesis that the presence of organic molecules can influence the fractionation factor of minerals by surface selective inhibition of a growing particle. Each crystal face is theorized to have a distinct fractionation factor, and surface selective inhibition results in compensatory growth on alternate faces, allowing for their distinct fractionation factor to be expressed in the bulk isotopic composition of the mineral. The results in this work imply that Ca isotopes are not suitable as a standalone biosignature of microbial life, as the presence of organic molecules, which can also be produced abiotically, are the likely trigger of the biological isotopic fractionation effect. However, the presence of organic molecules during mineral formation are now an additional interpretation of variation in Ca isotope record on the order of 0.2-0.3 permil. Gypsum morphology was also found to have important implications in the preservation of the original Ca isotopic composition of gypsum over long time scales. Larger minerals exchange with solution at much slower rates than microcrystalline materials, and therefore can potentially make better proxy archives that are less susceptible to diagenesis.