STRENGTHENING NEOPROTEROZOIC CHEMOSTRATIGRAPHIC CORRELATIONS WITH CARBON AND STRONTIUM ISOTOPES OF THE TAMBIEN GROUP, NORTHERN ETHIOPIA

We present marine carbon and strontium isotope data from the Tambien Group in northern Ethiopia constrained by several radiometric ages of intercalated volcanics and within a revised lithostratigraphic framework. This paired isotopic record significantly strengthens Neoproterozoic chemostratigraphic correlations leading up to the Sturtian glaciation. The Tambien Group is ~5 km thick and consists of mixed siliciclastic and carbonate rocks deposited in an arc-related basin that later became part of West Gondwana during the late Neoproterozoic East African Orogeny. Due to proximal volcanic activity, numerous volcanic ashes and tuffs are intercalated with marine carbonates in the Tambien Group, from which we developed a detailed chemostratigraphic framework. ID-TIMS U-Pb zircon ages from four of the ten volcanic units sampled constrain the timing of Tambien Group deposition to between 820 and < 775 Ma. These data agree with previous interpretations that correlate a diamictite with exotic clasts at the top of the stratigraphy with the ca. 720 Ma Sturtian glaciation and demonstrate that the Tambien Group is an extended record of pre-Sturtian Earth history. Our new global chemostratigraphic correlation constrained by these and previously published ages supports the interpretation that the Bitter Springs carbon isotopic stage is a globally synchronous feature and that the Islay carbon isotopic anomaly occurred > 10 Ma before the onset of the Sturtian glaciation. This correlation is supported by 40 strontium isotope measurements from the Tambien Group that are consistent with and build upon existing strontium isotope records from Svalbard, northwest Canada, and the Tambien Group itself. Our Sr isotope correlation confirms that seawater ⁸⁷Sr/⁸⁶Sr decreased leading up to the Sturtian glaciation after broadly increasing since before the Bitter Springs stage. This decreasing trend coincides with emplacement of several large igneous provinces (LIPs) at low latitudes during the breakup of Rodinia and is qualitatively consistent with the “fire & ice” hypothesis for triggering the Sturtian glaciation. However, preliminary modeling suggests that weathering of mafic LIPs with known aerial extents cannot account for the downturn in pre-Sturtian seawater ⁸⁷Sr/⁸⁶Sr.