The Mo- and U-isotope signatures in alternating shales and carbonate beds of rhythmites: A comparison and implications for redox conditions across the Cambrian-Ordovician boundary

The Green Point Formation (GPF) of the Cow Head Group in western Newfoundland (Canada) includes the Global Boundary Stratotype Section and Point (GSSP) for the Cambrian-Ordovician (epsilon-O) boundary on Laurentia. This formation is composed primarily of thick, deep subtidal to slope deposits (rhythmites) of alternating lime mudstones and shale interbeds. Here, we employ isotopic and elemental data of Mo and U of these shales and carbonates to better understand their depositional environments and the open ocean redox conditions at the epsilon-O transition. The variability of delta Mo-98(auth) (-0.40 to +3.16 parts per thousand) indicates variable aqueous sulfide concentrations ([H2S](aq)) in bottom waters or porewaters during deposition of the shales. A significant positive correlation of shale [Mo](auth) with [U](auth) coupled with low Mo concentrations compared to Phanerozoic euxinic shales further points to weakly reducing (similar to dysoxic/suboxic to anoxic) benthic conditions with H2Saq primarily confined to porewaters. For the lime mudstones, the delta Mo-98 variability (-0.30 to +2.34 parts per thousand) suggests variable [H2S](aq) in porewaters during early diagenesis. Low Mo concentrations ([Mo](carb) = 0.25 +/- 0.80 mu g/g, 2 sigma) and a poor correlation of [Mo](carb) with delta Mo-98(carb) indicate weak sulfidic ([H2S](aq )< 11 mu M) porewater conditions and possibly dysoxic to suboxic bottom environments. Thus, the delta Mo-98(carb) may not track the delta Mo-98 of basin waters because of low [H2S](aq) in porewaters and insignificant accumulation of authigenic Mo during early diagenesis. This explanation is consistent with a poor correlation between delta Mo-98(carb) and the delta U-238 of basin waters (estimated from delta U-238(carb)). A cautious examination of Mo-EF/U-EF, delta Mo-98(auth), and delta U-238(auth) of the shales suggests that the GPF was likely deposited in a basin with topographic barriers. During deposition of the lower and middle part of the studied interval, the barriers in combination with sea-level fluctuations may have caused partial isolation of the local basin from the open ocean. Rapid sea-level rises in the earliest Ordovician greatly increased the connectivity between the local basin and the open ocean, resulting in remarkable increases in Mo-EF/U-EF, [Mo](auth), and [U](auth) of the shales. The delta U-238 of the earliest Ordovician open oceans, inferred from delta U-238(carb), exhibited perturbations and an average value of -0.78 +/- 0.33 parts per thousand (2 sigma), reflecting oceanic redox oscillations and expanded ocean euxinia compared to the modern oceans.

Automated summary

The Green Point Formation in Western Newfoundland is important for understanding the Cambrian-Ordovician boundary and the depositional environments of lime mudstones and shale interbeds. Isotopic and elemental data of Mo and U are used to study the redox conditions and sulfide concentrations in the bottom waters and porewaters of the formation. The study reveals variable sulfide concentrations and weakly reducing bottom conditions, as well as fluctuations in porewater sulfide concentrations during early diagenesis.