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

Abstract

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 (Є-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 Є-O transition. The variability of δ98Moauth (−0.40 to +3.16‰) 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 (~dysoxic/suboxic to anoxic) benthic conditions with H2Saq primarily confined to porewaters. For the lime mudstones, the δ98Mo variability (−0.30 to +2.34‰) suggests variable [H2S]aq in porewaters during early diagenesis. Low Mo concentrations ([Mo]carb = 0.25 ± 0.80 μg/g, 2σ) and a poor correlation of [Mo]carb with δ98Mocarb indicate weak sulfidic ([H2S]aq < 11 μM) porewater conditions and possibly dysoxic to suboxic bottom environments. Thus, the δ98Mocarb may not track the δ98Mo 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 δ98Mocarb and the δ238U of basin waters (estimated from δ238Ucarb). A cautious examination of MoEF/UEF, δ98Moauth, and δ238Uauth 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 MoEF/UEF, [Mo]auth, and [U]auth of the shales. The δ238U of the earliest Ordovician open oceans, inferred from δ238Ucarb, exhibited perturbations and an average value of −0.78 ± 0.33‰ (2σ), reflecting oceanic redox oscillations and expanded ocean euxinia compared to the modern oceans.