Early Neoproterozoic Global Change Through the Lens of the Tambien Group, Northern Ethiopia

Abstract:

The early Neoproterozoic is a crucial period in the evolution of life and climate on Earth. Basins that developed during the time contain a record of the diversification of eukaryotic life as well as large-scale changes to the carbon cycle and paleogeography during the period leading up to Cryogenian glaciation. Understanding global change leading up to Cryogenian glaciation is key for interpreting the boundary conditions that resulted in the beginning of dramatic climate and geochemical oscillations during this critical interval. Existing age models for Neoproterozoic nonglacial intervals, such as the time leading up to Cryogenian glaciation, largely have been based on correlation of carbonate $\delta^{13}$C values, but there are few tests of the assumed synchroneity of these records between basins. In contrast to the ash-poor successions typically targeted for Neoproterozoic chemostratigraphy, the Tonian to Cryogenian Tambien Group (Tigray region, Ethiopia) was deposited in an arc-proximal basin where volcanic tuffs suitable for U-Pb geochronology are preserved within the mixed carbonate-siliciclastic sedimentary succession. We use physical and isotopic stratigraphic data sets from Tambien Group sedimentary rocks in concert with high-precision U-Pb dates from intercalated tuffs to establish global synchroneity of large scale carbon isotopic change. These new temporal constraints strengthen the case for interpreting Neoproterozoic carbon isotope variation as a record of large-scale changes to the carbon cycle. Furthermore, these dates strengthen the temporal framework for interpreting paleogeographic change, geochemical cycling, and environmental evolution during the radiation of early eukaryotes.