Sulphur isotopic variations in relation to sulphur pollution of Lake Erie

The background sulphate concentration has been estimated to be 6.4 mg. 1-1, implying that the anthropoenous sources account for 60 to 70% of sulphate concentration in Lake Erie water. From 1850 AD, the sulphate contribution due to pollution increased at the rate of 2 mg.1-1 per decade, but has levelled off (at 16 mg.1-1) during the last decade. An annual sulphur budget for the lake is presented and the storage time for sulphate (2.3a) shown to be effectively the same as the water flushing time of 2.6a. The data presented show that the sulphur inputs from the various sources may be identified on the basis of their delta34S values; the use of sulphur isotopic variations to trace the movement (and mixing patterns) of the different water masses requires a further study. Only a small fraction (< 2%) of the annual sulphur inputs into the lake is retained in the sediments. Dissolved sulphate in the interstitial waters (in contact with the sediments) generally disappears below 5 cm from the mud-water interface; the presence of large populations of viable sulphur cycle bacteria therefore indicates extensive microbial recycling of sulphur at the lower depths. The sediment samples analysed show that elemental-S, organic-S and iron sulphides (hydrotroilite-mackinawite-greigite and framboidal pyrite) are present in proportions that vary with the geographic location and age of the sediments. Isotopic exchange across the mud-water interface occurs with a net loss of either 34S (anaerobic Central Basin sedimants) or 32S (aerobic Eastern Basin sediments). The distribution and isotopic composition of sulphur in in the sediments of the Western Basin would suggest that the hypolimnion of the basin had been anaerobic some time in the past. (author)