FACIES-DEPENDENT SULFUR ISOTOPIC FRACTIONATION IN A MID-PROTEROZOIC MARINE BASIN

Canfield (1998) proposed that the cessation of iron formation deposition ca. 1850 Ma accompanied the expansion of sulfidic deep waters, not, as traditionally interpreted, the complete oxygenation of marine water columns. This hypothesis predicts that redoxclines should be recognizable within the sedimentary successions of mid-Proterozoic basins. Low to moderate oxygen levels might also be reflected by marine sulfate concentrations much lower than today’s. Superbly preserved siliciclastic rocks of the ca. 1500 Ma Roper Group, northern Australia, permit these predictions to be tested. Stratigraphically, the Roper succession consists of six major sequences stacked one atop another, permitting one to sample repeatedly along a paleoenvironemtnal depth gradient from coastal facies to basinal deposits formed below storm wave base. Fe speciation data for Roper shales from inner and distal shelf facies show low FeHR/FeT ratios (0.03-0.18, with a mean of 0.07) and low DOP values (mean=0.12), indicating oxygenated bottom waters. In contrast, basinal shales in the Roper Group display geochemical signatures typical for euxinic environments: FeHR/FeT>0.38 and DOP>0.45. Sulfur isotopic abundances in early diagenetic pyrites within the Roper succession display a marked facies dependence that reflects the Roper redoxcline. Only in the most distal basinal shales – those associated with euxinic conditions – can one find evidence of extensive S-isotopic fractionation. S-isotopic values as low as –20.7‰ suggest biological fractionation of about 40‰, near the maximum expected for bacterial sulfate reduction. In contrast, inner shelf pyrites show little evidence of extensive fractionation; rather, S-isotopic values as high as 49.7‰ strongly suggest Raleigh distillation in strongly sulfate-depleted pore waters. In between, most shallow basinal to inner shelf samples indicate fractionation of ca. 10‰ of less, consistent with bacterial sulfate reduction under conditions of sulfate limitation. In combination with previously reported data from the antecedent McArthur and Tawallah basins, Roper geochemistry indicates that in northern Australia, at least, marine sulfate and, by implication, oxygen concentrations were consistently low through some 250 million years of post-BIF Proterozoic history.