LOW SULFATE AND ANOXIA IN A MID-PROTEROZOIC MARINE BASIN

We report on the iron chemistry and S-isotopic record of mid-Proterozoic (ca. 1500-1400 Ma) shales from the Roper Group, northern Australia. The Roper Group is a thick (>1500 m), predominately marine siliciclastic succession preserved over an area of 145,000 km2. The ramp-like succession, formed in an intracratonic basin in contact with the global ocean, contains six major depositional sequences that record episodic flooding followed by basin filling and shoreline progradation. Facies are laterally persistent and repeat vertically in successive sequences, allowing environmental variations in seawater chemistry to be distinguished from long term secular changes. Our samples were collected from four drill cores; a total of 117 samples includes 37 from inner shelf shales, 37 from distal shelf facies, and 43 from basinal environments.

Our data shows that FeHR/FeT and DOP for shelf shales fall within the “oxic” quadrant defined by FeHR/FeT<0.38 and DOP<0.45. In contrast, basinal sediments are dominated by high FeHR/FeT (>0.38) and DOP (>0.45) values, indicating euxinic bottom waters. The relatively low FeHR/FeT and DOP in some basinal shales of the Roper Group could result from the decreased transfer of dissolved Fe from shelf sediments or a high flux of terrestrial clastics. Alternatively, it may be that anoxia was established at depths greater than the shelf/basin boundary. Regardless, high DOP and FeHR/FeT together provide compelling evidence for the presence of predominantly euxinic deep waters beneath oxic surface waters at the time Roper sediments were deposited.

The total range of S-isotope variation among Roper pyrites is about 70‰. In the Roper Basin, different sedimentary facies along the onshore-offshore gradient exhibit consistent differences in d34S value, with the most negative values occurring in basinal shales and the most positive in inner shelf samples. We argue that facies-dependent signature from the Roper Group may have resulted from S-isotopic fractionation in an euxinic basin with low sulfate concentration. Further biogeochemical research carried out in the illuminating framework of sequence stratigraphy should clarify the relationships between seawater chemistry and changes in the Proterozoic biosphere.