Carbon Isotope Ratios Across Cambrian/Ordovician Boundary

High-resolution chemostratigraphy of the Cambrian-Ordovician GSSP: Enhanced global correlation tool

Authors:

Azmy et al

Abstract:

The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) represents the Global Stratotype Section and Point (GSSP) for the Cambrian–Ordovician boundary on Laurentia. The formation consists of the Martin Point (lower) and the Broom Point (upper) members, which constitute a thick (~ 170 m) deep subtidal to slope marine carbonate sequence. Preservation of the micritic carbonates of the Green Point Formation was evaluated by multiple petrographic and geochemical screening tools. The δ13C and δ18O values of near-primary micrites range from − 4.7 ‰ to + 1.7‰ (VPDB) and from − 8.7 ‰ to − 5.5 ‰ (VPDB), respectively, with no significant correlation (R2 = 0.002). Similarly, the δ13C values show no significant correlation with Mn/Sr ratios or total REE contents, which suggest that the investigated carbonates retain their near-primary δ13C signatures that can be utilized to construct a high-resolution carbon-isotope profile for the GSSP. The δ13Ccarb profile about the GSSP trends generally positive into Bed 22 of the Green Point Formation. The top of the C. intermedius Zone marks the onset of a negative δ13Ccarb shift to about 6.0 ‰ culminating in the upper part of the zone. Subsequently, the δ13Ccarb profile trends positive with a ‘double switch-back’ about the boundary point and then continues on to the highest value in the uppermost part of Bed 23. In addition, the Ce*/Ce values approximately between 0.8 and 1.0, through the GSSP suggest generally dysoxic waters before, during, and immediately after the boundary. The high-resolution chemostratigraphic results, curve, and trends covering the GSSP should facilitate correlation of the Cambrian–Ordovician boundary sections/sequences from other locations.