Redox conditions across the Cambrian-Ordovician boundary: Elemental and isotopic signatures retained in the GSSP carbonates

Lime mudstone samples (rhythmites) were collected at high resolution from outcrops of the Cambrian-Ordovician GSSP boundary section at Green Point (western Newfoundland, Canada). The sequence (~45m-thick) consists of slope carbonates with alternating shale and siltstone interbeds, and it spans the boundary located between the Martin Point and Broom Point members of the Green Point Formation (Cow Head Group). Samples were extracted from micritic rhythmites by microdrilling and subsequently screened using petrographic and geochemical criteria to evaluate their degree of preservation. Although the δ¹³C_org profile (-29.7 to -25.6‰ VPDB) shows insignificant variations, the TOC values (0.1 to 4.1%) exhibit a generally upward decreasing trend. A negative δ¹³C_carb excursion, reflecting a sealevel rise, marks a geochemical anomaly that correlates with an increase in the N contents (0 to 2.9%) of organic matter and the δ¹⁵N_org values (-0.6 to +6.0‰), which suggests a change to more reducing oceanic conditions. The U contents vary from 0.1 to 3.0ppm and the δ²³⁸U values (-0.97 to -0.18‰) generally decrease with the negative δ¹³C_carb excursion. The U isotopic variations suggest a widespread increase in reducing conditions associated with sealevel rise during this interval. The investigated sedimentary rocks were slope carbonates where dysoxic conditions likely dominated throughout the entire section. Therefore, the changes in the TOC, N, δ¹⁵N_org, and δ²³⁸U profiles across the boundary are not as sharp as would be expected by a local change from oxic shallow-water to dysoxic/anoxic deep-water settings.