Chemostratigraphy of the Ediacaran Old Fort Point Formation in the southern Canadian Cordillera

The Ediacaran Old Fort Point Formation in the southern Canadian Cordillera records a large, highly negative carbon isotope excursion, with a minimum in the ${\delta }^{13}$C values of marine carbonates of –12‰. Carbon isotope excursions are often interpreted to be broadly synchronous and to reflect shifts in the ${\delta }^{13}$C value of marine dissolved inorganic carbon, possibly reflecting either fluctuations in the global proportion of organic matter burial, or post-depositional diagenesis. As diagenesis can create large discrepancies between the original ${\delta }^{13}$C_carb value of the sediment and what is preserved today, it is essential to determine to what extent diagenesis has impacted these records. We measured carbon and oxygen isotopes on eight stratigraphic sections (n = 360), carbonate clasts from debris-flow deposits from the Old Fort Point Formation (n = 968), and major and trace elemental abundances (n = 249). We used these geochemical datasets to investigate whether local processes leverage a larger control on carbon and oxygen isotopic values rather than global processes. We argue that the carbonate strata with the lowest ${\delta }^{13}$C_carb values ($\sim$ –12‰) are now calcitic but were neomorphosed from an aragonite precursor in a fluid-buffered, early diagenetic environment conducive to dolomite growth. Additionally, the recovery of the expressed excursion is preserved in carbonates that underwent early-marine diagenesis under more sediment-buffered conditions, with its geochemistry closer to the aragonite precursors. Tremendous variability, up to 17‰, is observed in ${\delta }^{13}$C_carb values of clast populations from individual breccia horizons found in the in-fill of submarine paleocanyons, filled with material from underlying carbonates of the Temple Lake and Geikie Siding Members. The presence of such a large range of clast ${\delta }^{13}$C_carb values requires an early acquisition of the observed Old Fort Point Formation ${\delta }^{13}$C_carb values (pre-brecciation and submarine canyon incision) and precludes late-stage burial diagenesis. Moreover, despite disagreement with previously published geochronological constraints, we propose that the carbon isotope excursion recorded in the Old Fort Point Formation is equivalent to the widely studied Shuram excursion, the most negative carbon isotope excursion recorded in Earth history, and thus further geochronological work in the southern Canadian Cordillera is needed to test this proposition.