Title: Stable isotopes in Neoproterozoic and early Paleozoic earth history
Description: The stable isotope record of carbon and oxygen preserved in ancient carbonate rocks provides a means to investigate the evolution of marine geochemistry. In this dissertation I employ a combination of field observations and stable isotope chemostratigraphy to study the history of Earth's surface during three important intervals: (1) the early Neoproterozoic Era, (2) the Early Cambrian System, and (3) the Ordovician-Silurian transition. In each case carbon isotope variations allow for the temporal correlation of globally-distributed rock successions. The final study also includes an analysis of variability of oxygen isotopes preceding and during the end-Ordovician glaciation. I report the first high-resolution carbon isotope curve for the early Neoproterozoic Shaler Supergroup on Victoria Island, arctic Canada. The results provide the basis for a revised correlation between the Shaler and Mackenzie Mountains supergroups. I suggest that the lower portions of the successions are diachronous, and that a previously unrecognized hiatus exists in the middle of the Wynniatt Formation. Stratigraphic and stable isotope studies of the Khubsugul Basin in northern Mongolia demonstrate the existence of two Cryogenian glacial deposits, a thin record of the Ediacaran Period, and a thick succession of Early Cambrian carbonates above a sub-Tommotian unconformity. The results constrain the age of the Khubsugul phosphorite, one of the largest sedimentary phosphorite deposits in the world, to the Ediacaran Period. This is an important revision of earlier work that suggested an Early Cambrian age.

The sedimentary record of the Ediacaran and Early Cambrian is also exposed in the Zavkhan Basin of southwestern Mongolia. Preliminary field mapping and chemostratigraphic work indicates that the Tsagaan Oloom and Bayan Gol formations were deposited diachronously, likely in a migrating foredeep. Strata developed along the Early Paleozoic margin of Laurentia host sedimentological and isotopic evidence for sea level change and climate deterioration at the end of the Ordovician Period. I report new carbon and oxygen chemostratigraphic data from Nevada, British Columbia, Nunavut, and Quebec that suggest a long-term cooling in the tropics preceded the Hirnantian glaciation. Orbital variability may have played an important role in the development of Gondwanan ice sheets during a interval that was otherwise characterized by ice-free conditions.
Subjects: Carbon, Isotopes