Middle Proterozoic Ocean Chemistry: Evidence from the McArthur Basin, Northern Australia

Abstract

We report on the iron-sulfur systematics and S-isotopic compositions of upper Paleoproterozoic carbonaceous shales from the Wollogorang and lower Reward formations, McArthur Basin, northern Australia. Iron speciation in these rocks is similar to that of modern euxinic Black Sea sediments and distinct from sediments depositing from oxygenated water, providing persuasive evidence of euxinic depositional environments in a mid-Proterozoic marine basin. High values of Degree of Pyritization (DOP) also indicate anoxic bottom-water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides. The isotopic composition of the sulfides is, however, quite enriched in S-34 and for the lower Reward Formation similar to contemporaneous seawater sulfate. These S-34-enriched sulfides are consistent with extensive, or even complete, sulfate depletion within the euxinic basins in which the sediments were deposited. Based on analogy with modern euxinic environments and further constrained by model results, middle Proterozoic seawater sulfate concentrations were probably within the range of 0.5 to 2.4 mM. Taken together, our results indicate a relatively low-sulfate late Paleoproterozoic ocean and sulfidic deep waters in the McArthur seaway, a condition that could have encompassed much larger regions of the ocean.