Superheavy S isotopes from glacier-associated sediments of the Neoproterozoic of south China: Oceanic anoxia or sulfate limitation?

Abstract

Black shales and Mn carbonates interbedded with glacial deposits from the Neo- proterozoic of southern China exhibit extremely heavy values of pyrite S isotopes that may reflect the peculiar environment of Earth at this time. δ 34 S averages +30‰ at Tanganshan and +44‰ at Xiangtan, compared with typical values of 0‰ to +5‰ found in younger deposits. Furthermore there is no distinction between the shales and the Mn carbonate ores in the Neoproterozoic, unlike the younger deposits, which show much lighter δ 34 S in the shales than in the Mn ores (the spread is 25‰). Most other chemical parameters are very similar to both the younger Mn deposits and those from the Paleoproterozoic. The exception is the rare earth elements (REE). All Neoproterozoic Fe ores and most Neoproterozoic Mn ores lack the positive Eu anomaly that characterizes Archean and Paleoproterozoic Fe-Mn accumulations. On the other hand, Neoproterozoic Mn deposits have positive Ce anomalies on North American Shale Composite (NASC) normalized plots, in contrast to other MnCO 3 ores. The ΣREE is also higher than in other Mn deposits, but lower than in modern deep-sea crusts. Sulfide S values in all Neoproterozoic shales tend to be exceptionally variable and to often show much heavier values than can be found in marine strata from the Phanerozoic. Therefore the anomalous δ 34 S values we observed reflect peculiar conditions in the world oceans at this time rather than purely local effects. Times of enrichment of seawater sulfate in 34 S do not correspond to periods of glaciation, so the likely cause of the S isotope patterns is not worldwide glaciation, but a generally low level of dissolved sulfate S in the Neoproterozoic oceans that allowed modest increases in the amounts of S removed as pyrite to drive down the oceanic S reservoir enough to produce strong Rayleigh reservoir effects. The abundance worldwide of Sturtian-age Mn and Fe deposits indicates an increase in Fe flux to the oceans that would have been sufficient to depress SO 4 2- levels severely and to result in residu- al dissolved S extremely enriched in 34 S. REE evidence indicates that most of this enhanced Fe and Mn flux came from diagenetic remobilization of detrital oxides