GEOCHEMISTRY OF GLACIAL-ASSOCIATED SEDIMENTS OF THE NEOPROTEROZOIC OF SOUTH CHINA

A series of samples was taken from Neoproterozoic strata of southern China that span the two glacial episodes. Whole rock chemistries and S isotopes exhibit extreme values that may reflect the peculiar environment of the Earth at this time. Sampling was from strata exposed at two Mn mines, Tanganshan and Xiangtan. Published S isotope data is also available from related deposits at Minle.

Tanganshan and Xiangtan samples show unusually high uranium contents, ranging up to 800 ppm. S isotopes are extraordinarily heavy in both deposits, averaging +30 permil at Tanganshan and +44 permil at Xiangtan. Both the Mn ores and the host black shales exhibit the U and S isotope anomalies. Both lithologies are also high in organic C (3 to 4%) and in Y (100-300 ppm average). Xiangtan ores and shales are strongly enriched in Ba (up to 15,000 ppm) whereas those at Tanganshan are relatively low (100-800 ppm).

Sulfide S values for this time period tend to be exceptionally heavy worldwide, which must reflect peculiar conditions at the earth's surface at this time. A possible cause is post-glacial ventilation of a stagnant ocean that was formed during the lower glacial "Snowball I"" episode. Sea level rise and deep-water overturn associated with deglaciation would have brought Mn, Fe, and heavy sulfate S into shallow waters. Supporting this mechanism is the extreme concentration of uranium, which could be produced by very slow deposition during maximum flooding. The difference in S isotope values and Ba between the two localities could be a response to either differences in communication with the open ocean or differences in surface water productivity.