Over the past two decades, carbon isotope chemostratigraphy has emerged as the most important correlation tool for the Precambrian. The dominant paradigm amongst chemostratigraphers is that delta13C in carbonate rocks (delta13Ccarb) reflects the delta13C of dissolved inorganic carbon (DIC) in contemporaneous sea water. If this assumption is true, then why do we observe negative delta13Ccarb excursions lasting millions of years that dip well below the canonical value for mantle carbon input to the ocean-atmosphere system (-6?)? Do these excursions in fact represent diagenetic alteration away from DIC-derived values? Even if diagenetic, are the excursions global in nature, making them still useful for correlation? To explore these questions, we propose work on the deepest delta13Ccarb excursion in Earth history -- the `Shuram' anomaly, recorded in Ediacaran carbonate sediments on at least seven paleo-continents during the early evolution of animals. We will combine detailed field mapping and sequence stratigraphy with isotope chemostratigraphy and U-Pb geochronology of ash beds to constrain the timing and duration of the Shuram anomaly. In particular, isotope conglomerate tests will constrain the relative timing of delta13Ccarb acquisition, canyon cutting, subaerial exposure, meteoric diagenesis, and burial diagenesis. Our investigation of the unprecedented isotopic variability during the Ediacaran will test whether delta13Ccarb is a record of the evolving global carbon cycle and may be used as a correlation tool -- vitally important for every Precambrian story (climatic, evolutionary, geochemical, tectonic) of global change.
The most negative carbon isotope anomaly in Earth history is found in limestones of the Ediacaran Period (635-542 Ma). Geologists long have recognized the broad coincidence of this isotope anomaly and the rise of abundant macro-scale fossils in the rock record. Partly for this reason, geochemists have interpreted this carbon isotope anomaly in the context of a changing redox state of Ediacaran oceans and the requisite rise in atmospheric oxygen for animal life. However, in light of the fairly drastic implications of such a large carbon isotope anomaly, other geochemists have begun to interpre the signal in terms of secondary diagnetic processes related to the modification of the limestone rocks by ground water. To address this debate, we studied this carbon isotope anomaly at an unprecedent scale, making over 4000 d13C--d18O measurements and 250 trace element, d44Ca, d26Mg and 87Sr/86Sr measurements from 15 measured stratigraphics sections and numerous maps spanning the full scale of the basin hosting the Wonoka Fm. in South Australia. By pairing detailed field observations to chemostratigraphic measurements, we were able to show that the carbon isotope anomaly must have been recorded by the limestones at Earth's surface and not during subsequent diagenetic reactions. Furthermore we were able to demonstrate stratigraphic changes in original carbonate mineralogy, and have contributed new constraints on the range of processes that could have caused this carbon isotope anomaly. However, deciphering a singular origin for this Ediacaran negative carbon isotope anomaly will have to wait until we acquire U-Pb zircon age constraints on the timing (are these carbon isotope anomalies found globally truly synchronous in time?) and duration (how much oxidative power is required to reduce enough carbon to create this isotope anomaly?) of the carbon isotope excursion.
