Isotopic composition of the major contributors to atmospheric sulfur

Abstract

Only a few S isotope data from atmospheric prei5pitates are available. These results demonstrate the possibility t o discriminate between sulfur burdens frdm different natural and/or anthropogenic sources. The a3*S patterns of the major suppliers of atmospheric sulfur are discussed., Their 6 ranges overlap so completely that we cannot use S isotope data of atmospheric sampIes to calculate the net contribution rates from the individual sources at a global scale. For selected areas, however, such conclusions can frequently be drawn. The most reliable results are t o be expec5ed from areas with only two (at maximum three) major sulfur suppliers with well known S isotopic composition and large 6 difference between the individual sources. Limitations are given mainly by the complex origin of atmospheric sulfur from a variety of different sources (especially in highly industrial regions) and by the broad 6 ranges even in relatively "uniform" suppliers. Furthermore the fate of the sulfur compounds after emission to atmosphere may be quite complex. Additional fractionation processes efface the "fingerprint" character of the S isotope composition. In some cases these difficulties may be overcome, when the S isotope values are correlated with other geochemical data.. . Preface Fundamentals of S isotope fractionation The various papers of this issue dealing with atmospheric sulfur compounds give a n idea of the complexity of the atmospheric sulfur cycle. Methods are warranted that enable a clear discrimination between sulfur burdens coming from the different natural and anthropogenous sources, and some preliminary sulfur isotopic investigations are claimed to play this role. The present paper gives a critical review of the possibilities and limitations of this method. Tracing back atmospheric sulfur to its terrestrial or marine sources is possible only from a detailed knowledge of the S isotopic composition of the source materials in question. A large amount of data already exists for this purpose, but the information is so widely dispersed in literature that a main purpose of the present article must be to summarize our knowledge of the S isotopic distribution patterns of typical source materials and of the isotope fractionation effects encountered in the mechanisms of sulfur transfer to atmosphere. Tellus XXVI (1974), 1-2 b Sulfur has 4 stable isotopes with mass numbers 32, 33, 34, and 36. I n stable S isotope work the abundance ratio between the two main isotopes 34S and is determined, and publiph-ed values generally are given in the "delta notation" where R means the abundance ratio 34S/32S. Spl stands for unknown' sample and m for the troilite sulfur of iron meteorites, the isotope ratio of which is thought to be similar to that of mean terrestrial sulfur. Isotope fractionation comes ftqm small differences in the behaviour of the isotopic species due to their mass difference. Physical frac-tionation processes (diffusion, evaporation) are of no account in sulfur isotope geochemistry, and most of the extended natural variation in S isotopic composition comes from differences in chemical reaction rates. These differences