Carbon-isotope stratigraphy of the uppermost Cambrian in eastern Laurentia: implications for global correlation

The delta C-13 profile from an interval of the Martin Point section in western Newfoundland (Canada) spans the upper Furongian (uppermost Cambrian). The interval (similar to 90 m) is a part of the Green Point Formation of the Cow Head Group and consists of the Martin Point (lower) and the Broom Point (upper) members. It is formed of slope marine carbonates alternating with shales (rhythmites) and conglomeratic interbeds. The preservation of the investigated micritic carbonates was meticulously evaluated by multiple petrographic and geochemical screening tools. The delta C-13 and delta O-18 values (-0.5 +/- 0.8 parts per thousand VPDB and -7.1 +/- 0.3 parts per thousand VPDB, respectively) exhibit insignificant correlation (R-2 = 0.002) and similarly the correlation of delta C-13 values with their Sr and Mn counterparts, which supports the preservation of at least near-primary delta C-13 signatures that can be utilized to construct a reliable high-resolution carbon-isotope profile for global correlations. The delta C-13 profile exhibits two main negative excursions, a lower broad excursion (similar to 3 parts per thousand) that reaches its maximum at similar to 70 m below the Martin Point / Broom Point members boundary and an upper narrow excursion (similar to 2.5 parts per thousand) immediately below the same boundary. The lower excursion can be correlated with the global latest Furongian HERB event (TOCE), which is also recognized in the C-isotope profile of the GSSP boundary section at Green Point whereas the upper excursion matches with that of the Cambrian-Ordovician boundary in the same section. The peak of the HERB delta C-13 excursion is correlated with positive shifts on the Th/U and Ni profiles (redox and productivity proxies).