Constraints on Global Carbon Cycling, Basin Formation, and Early Animal Evolution During the Neoproterozoic and Early Cambrian

Abstract

Within this dissertation, I document different time intervals during the Neoproterozoic through the early Cambrian (1000 – 525 Ma), one of the most dynamic and non-uniformitarian intervals in the history of the Earth. This thesis focuses on two time periods of change within this time interval: 1) The pre-Cryogenian-Cryogenian transition (~800-635 Ma) and 2) the Ediacaran-Cambrian Transition (~551-525 Ma).

To begin, I use a combination of detailed mapping, regional stratigraphy, and geochronology to reinterpret the depositional and tectonic setting of the ~750-730 Ma Beck Spring Dolomite and its bounding units in Death Valley, California. Instead of being deposited in an interior seaway in a long-lived aulacogen as Rodinia rifted apart, I suggest that this unit was accommodated by strike-slip motion that could have represented a margin-wide event along the west coast of Laurentia. The true rift to drift did not occur until the latest Ediacaran to earliest Cambrian.

Late Ediacaran to early Cambrian datasets from the Great Basin in the western USA are integrated to refine the biological and environmental changes that occur across the Ediacaran-Cambrian Boundary. In my third chapter, two new horizons of exceptionally preserved Ediacaran fauna that include Conotubus and Gaojiashania are documented. These fossils were previously only known from South China. The Conotubus horizon occurs just below the most negative δ13C values in the basal Cambrian δ13C excursion, establishing it as the youngest Ediacaran fossil to date. These fossils are placed into a high-resolution δ13C chemostratigraphic framework, allowing for regional and global correlation. Three δ13C chemostratigraphic curves from the Great Basin combined with five sections from Western Mongolia show that instead of a single, rapid δ13C excursion at the Ediacaran-Cambrian Boundary, there is one broad excursion with more secondary structure than has been previously documented. Together, these data suggest the nadir of the δ13C excursion is younger than 541 Ma and provide a new framework to interpret Ediacaran-Cambrian biostratigraphy, chemostratigraphy, and U/Pb ash ages globally.

The final chapter focuses on the earliest Cambrian in Southwest Mongolia. By using extensive field data, I construct a basin-wide facies model and age model for the Zavkhan Basin. I place the early Cambrian fossil horizons into this new framework and correlate the composite dataset from Mongolia with others globally. In doing so, I reinterpret the tempo and patterns of evolution during this critical interval in Earth history.

The following chapters cover diverse events in Earth history that better constrain topics ranging from the breakup of the supercontinent Rodinia to environmental change across the Ediacaran-Cambrian Boundary to rates of evolutionary change during the early Cambrian. All of these topics, however, are united by a multidisciplinary approach that combines original field observations with geochemical, geochronological, stratigraphic, and paleontological datasets to constrain the origin and timing of the coevolution of life and the environment in the deep past.