SUPERHEAVY S ISOTOPES FROM GLACIAL-ASSOCIATED MN ORES OF THE NEOPROTEROZOIC OF SOUTH CHINA: OCEANIC ANOXIA OR SULFATE LIMITATION?

Black-shales and Mn carbonate ores interbedded with glacial deposits from the Neoproterozoic of southern China exhibit extremely heavy values of pyrite d³⁴S that may reflect the peculiar environment of the Earth at that time. d³⁴S averages +30 permil at Tanganshan and +44 permil at Xiangtan, with no distinction between the shale and carbonate lithologies, unlike younger deposits, which show much lighter d³⁴S in the shales than in the Mn ores. Trace element patterns of both the shales and the Mn ores are entirely similar to those of Phanerozoic deposits. REE patterns of the Mn ores lack the positive Eu anomaly that characterizes Archean Fe-Mn accumulations, but have a moderate positive Ce anomaly on NASC normalized plots. The sum of the REE is higher than in other Mn deposits, but lower than in modern deep-sea crusts.

d³⁴S values of sulfide S from many localities in this time period 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 d³⁴S values we observed reflect peculiar conditions in the world oceans at this time rather than purely local effects. A possible cause is post-glacial ventilation of a stagnant ocean that was formed during a "Snowball Earth" episode. Deglaciation would have produced a sea level rise and deep-water overturn of a previously anoxic ocean that would have brought Mn, Fe, and isotopically heavy sulfate S into shallow waters. Some evidence contrary to this scenario comes from the similarity in whole rock geochemistry of the host rocks and Mn ores to typical black shales and Mn carbonates from younger rocks. Moreover, the absence of a positive Eu anomaly in the REE patterns suggests that the oceans were oxidizing enough to precipitate abundant Fe oxides at mid-ocean ridges and the presence of a positive Ce anomaly in Mn-rich sediments suggests generally oxidizing oceanic conditions. Because all of the other chemical systems look normal, a better explanation of the heavy S is an ocean with highly variable, but generally low dissolved sulfate.