Improving chronology of the Pre-Sturtian isotope excursions using Bayesian modelling and improved maximum depositional ages.

The Neoproterozoic era was a time period of massive climatic change, including global glaciations and large global carbon and oxygen isotopic anomalies, which are likely also linked to climatic shifts. Also, during this time, the supercontinent Rodinia broke apart while the younger supercontinent Gondwana formed. On top of the climatic and tectonic disturbances, the Neoproterozoic experienced important advances in the evolution of life with the expansion of eukaryotic diversity culminating in the appearance of metazoans. Much of this geological diversity has been discovered in sedimentary sequences containing interbedded siliciclasitcs and carbonates where direct age control is difficult to obtain. Sparse U-Pb ages from ash layers have been used to provide precise and accurate age constraints on isotopic excursions prior to the snowball earth events in a minority of the stratigraphic sections worldwide. The paucity of age control limits our ability to correlate disparate sections which is crucial for our understanding of the evolution of life and its relation to environmental variability.

In this study we intend to develop new Bayesian age models for Neoproterozoic sections from the Ugab Subgroup which is part of the Otavi/Swakop Group in Northwestern Namibia. These sections contain Cryogenian snowball earth related diamictites, along with two older carbon and oxygen isotope excursions recorded in marine carbonates. Volcanic flows and tuffs at the base of the section and in the overlying glaciogenic Chuos Formation, implying that volcanism was likely ongoing throughout deposition of the Ugab Subgroup. We will use Bayesian statistical modelling to generate an age depth model for the stratigraphic sections which will allow us to estimate the timing and duration of the isotope excursions, which can then be compared to estimates in other basins and sections around the world to develop an integrated stratigraphic record of the leadup to the Cryogenian Snowball earth. The data for our age depth model will include existing age constrains (such as ages on surrounding strata) and chemically abraded detrital zircon laser ablation U-Pb data from siliciclastic sediments within the Ugab Subgroup, under the assumption that the youngest grains represent zircon from a volcanic eruption, close to the timing of sediment deposition. The age of the eruptions will be generated using a Bayesian model, and then fed into a Bayesian age depth model to estimate the age of each sedimentary unit in the stratigraphic column. The age depth model enforces the laws of superposition, and will be also constrained by the timing of the known events (the snowball earth diamictite, and U-Pb ages, generated in this study, from the tuffs at the base of the section). The estimated timings of the isotope excursions calculated by our model will be compared to absolute U-Pb age constraints on these excursions that have been published for a section in Ethiopia. Our new approach to generate precise age control in stratigraphic sections without obviously datable horizons, is likely to be extremely useful for other sections worldwide.

To ensure that our U-Pb data are accurate and precise, we will also develop inhouse zircon reference materials, and acquire gravimetrically calibrated U-Pb spikes that will be available for future users of the Geotop U-Pb labs.