Date of Award
Doctor of Philosophy (PhD)
Earth & Environmental Science
During Neoproterozoic time an irreversible increase in the oxygen levels present in Earth's atmosphere resulted in the emergence of complex life forms at the end of Proterozoic time. The processes whereby this increase in oxygen occurred has not been explained. To begin to understand the reasons for this rise in oxygen concentration, we must study the global cycles of carbon, oxygen, and sulfur, which are inextricably linked. I studied the ~830 Ma Browne Formation of the Officer Basin, Western Australia to reconstruct the marine chemistry, the sulfur cycle, and the depositional environment during this enigmatic period in geologic history.
I performed a petrographic study of the Browne Formation using optical and electron microscopy (SEM-EDS) and X-ray diffraction (XRD). To document the marine origin of the Browne Formation evaporites, I measured the bromine content of several rock-salt samples using X-Ray Fluorescence. The Browne Formation was deposited within an extensive, subcontinental-scale shallow saline system, or "saline giant", a geologic province not represented on Earth today.
I present the first direct characterization of seawater chemistry from the Mid-Neoproterozoic, and can thus impose a tighter constraint on the much-debated level of oceanic sulfate than has been possible heretofore. My results extend documentation of seawater chemistry by ~300 Ma, and hence represent chemical characterization of the oldest seawater ever measured by fluid-inclusion analysis. I analyzed primary fluid inclusions in primary halite using the CRYO-SEM-EDS technique. The major-ion composition of mid-Neoproterozoic seawater was very different from present-day seawater chemistry; the concentration of Ca2+ exceeded that of SO42-, such as it did in Cambrian, Silurian, Devonian, Jurassic, and Cretaceous seawater.
From a review of the literature describing the isotopic composition of marine sulfate for this Mid-Neoproterozoic time, and using the same petrographic and sedimentologic criteria I applied to my samples, I was able to narrow the known range of isotopic data on seawater sulfate for this period of time. I measured the isotopic composition of marine sulfur from primary anhydrite crystals associated with primary halite using a Mass Spectrometer (Finnigan MAT CHN 1108 Analyzer).
My research offers far-reaching implications concerning Proterozoic marine-sulfate and oxygen levels, and the chemical co-evolution of the early oceans, atmosphere, and biosphere. These data make it possible to explore the relationships between major-ion chemistry of seawater and the evolution of marine life during this important interval of Earth history.
Spear, Natalie, "A Multidisciplinary Analysis of Evaporite Rocks from the Mid-Neoproterozoic Browne Formation, Officer Basin, Western Australia" (2013). Publicly Accessible Penn Dissertations. 703.