Murphy, Ashley Elizabeth. Characterization of stromatolitic biosignatures after secondary dolomitization. Retrieved from https://doi.org/doi:10.7282/t3-9t76-f948
DescriptionThe earliest signatures of life preserved in Earth’s rock record are found within fossilized organosedimentary structures called stromatolites. Many of these geologically ancient structures are dolomitized, but the effects of this diagenetic process on microbial biosignature preservation have yet to be properly evaluated. This dissertation research investigates the effects of dolomitization on the preservation of microbial biosignatures by evaluating the physiochemical controls on the alteration of ancient stromatolitic biosignatures in ~500 Ma stromatolites from the Allentown Formation in New Jersey, USA. Further, dolomitization experiments performed on modern stromatolites from The Bahamas are used to determine if dolomite may be used as a potential mineralogical biosignature. Additionally, a new Raman spectroscopy method was developed that provides the relative crystal orientation of dolomite and calcite minerals. Petrology, geochemistry and geothermometry results show the occurrence of organic carbon in the Allentown stromatolites is exclusive to the first generation of dolomite, and the burial and temperature alteration of the host rock matches that of the organic carbon within, which suggests that the carbon is likely biological in origin and indigenous and syngenetic with the stromatolite depositional setting. Experimentally dolomitized modern stromatolites show that the presence of organics in natural samples (sand, sediment, and stromatolites) affects dolomitization by speeding up the recrystallization process and producing distinct dolomite crystal habits and surface textures (globular, skeletal, hollow). Additionally, we found a strong correlation between dolomites’ Raman v1 mode and degree of cation order, which has not been reported in other studies. The relative Raman peak intensity ratio of the carbonate lattice modes T and L reveals the crystallographic orientation of calcite and dolomite with respect to the incident light polarization. This new Raman method yields information related to growth and deformation during diagenetic and metamorphic alteration, and may be used to identify preferred crystal orientation in microbialite fabrics. This dissertation research sheds light on the role secondary dolomitization plays in obscuring and preserving biosignatures in ancient rock, and what instrumentation would best detect these. Our findings are essential for biosignature detection in ancient carbonate rocks on Earth and Mars.