Old Fort Point Formation: Neoproterozoic Negative d13C Excursion Recorded in a Deep-Water Carbonate Succession
The Precambrian recorded dramatic atmospheric and geochemical fluctuations, such as the “Snowball Earth,” the most severe glaciation in geologic history, and the Earth’s most significant negative carbon isotope excursion, the Shuram Excursion, which has been identified in multiple, globally-disparate locations. The Excursion recorded a rapid negative δ13C shift to approximately -12‰ and recovery over hundreds of meters in some units, which are predominantly shallow-marine carbonates. The excursion’s poor age constraints currently imply a maximum of some forty million years, between 635 Ma and approximately 591 Ma. This requires a large input of carbon-12 over a significant period of geologic time, and while debate exists over whether it is primary or diagenetic, evidence now leans in favour of a primary origin. This study presents δ13C data from a deep-water turbiditic carbonate in the Canadian Rockies that has produced a precise Re-Os date of 607.8 ± 4.7 Ma, known as the Old Fort Point Formation. Significantly, this unit records a δ13C excursion of -11.5‰ before the Re-Os date. The distinctive δ13C signal record implies it is correlative to the Shuram Excursion, and moves the Shuram Excursion farther back than previously estimated. The presence of multiple excursions between the Shuram Excursion and the Precambrian-Cambrian boundary indicate that the carbon cycle was unstable following the meltdown of the Marinoan glaciation. The OFPF helps to constrain the Shuram Excursion but more work to is needed to determine if current stratigraphic correlations have mis-correlated multiple excursions or not.