S-34-ENRICHED SULFATE IN THE BELCHER GROUP, NWT, CANADA - EVIDENCE FOR DISSIMILATORY SULFATE REDUCTION IN THE EARLY PROTEROZOIC OCEAN

Sulphate molds and halite casts have been reported from a dolomite-mudstone sequence near the base of the approximately 1.9 Ga Belcher Group (Bell and Jackson, 1974). By the in vacuo Kiba technique (Ueda and Sakai, 1983) we have obtained delta-S-34 values for trace amounts of sulphate and sulphide in three samples of dolomite containing sulphate molds from the Kasegalik Formation. For sulphate, the delta-S-34 values are +26.9 parts per thousand to +28.9 parts per thousand, averaging +27.8 parts per thousand, and are independent of sulphate concentration (37 p.p.m. to 584 p.p.m.) and the sulphide/sulphate ratio. The delta-S-34 values are considered to represent the composition of seawater in which the sediments were deposited. This adds to the fragmentary record of the isotopic composition of early Proterozoic seawater.