Origin of sulphur in soil and water in a Precambrian terrain, S. India

Gneisses and charnockites are major members of the bedrock of the southern-central part of the Indian peninsula. The charnockite is commonly weathered into Vertisols containing gypsum concretions in amounts commercially extractable. Sulphur in the concretions as well as sulphur in atmospheric deposition and in groundwater was investigated by means of sulphur isotopic analysis. The isotopic ratios in the gypsum fall in the same range as those in atmospheric deposition suggesting that the sulphate is of atmospheric origin brought in by the strong monsoonal winds. The deposition contains, along with the seawater sulphate, a reduced sulphur component possibly emanating from DMS or H2S from the Indian westcoast. The concretions are likely to be of a fossil character. The latest arid period on the peninsula occurred during the peak of the last glaciation which could be one possible period of accumulation. Another major sulphate source in the studied area is ammonium sulphate used as nitrogenous fertilizer. The mobilization of sulphur through bedrock weathering is negligible.