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Sulfur isotope composition can provide evidence concerning genesis of sulfur-bearing minerals (Jensen, 1967; Ault and Kulp, 1960). Sulfur isotope analyses were therefore carried out on evaporites from Sites 225, 227, and 228 to shed light on possible relationships between base metal sulfides of the Atlantis II Deep (Bischoff, 1969: Hackett and Bischoff, 1973) and adjacent sedimentary sulfur minerals. The two most abundant isotopes of sulfur, S 3 4 and S 3 2 , are fractionated in natural processes, and maximum fractionation occurs when sulfate sulfur is reduced to sulfide. The reduction is kinetically slow and is usually mediated by bacteria. Kaplan et al. (1963) have shown that reduction of sulfate by bacteria in marine sediments produces sulfides which are enriched in S 3 2 and have a relatively large range of δ S 3 4 values, often extending to values as low as -30 or -40 °/oo. Sulfides associated with magmatic-type hydrothermal deposits, which may represent mantle derived sulfur, have a small range of δ S 3 4 averaging near 0 °/0o (Jensen, 1967). Kaplan et al. (1969) and Hartman and Nielsen (1966) have determined that sulfur isotope composition of the base metal sulfides in the Atlantis II Deep follows neither of the above patterns, but displays a small range of +3.1 to +9.8, averaging +5.7 % o . This isotopic composition is significant because it is analogous to the composition of many ancient stratiform and stratibound sulfide deposits (Stanton, 1972; Sangster, 1968). These values cannot be easily explained because the only available sources of large quantities of sulfur are seawater or evaporite deposits. Sulfate reduction by bacteria should produce sulfides of negative sulfur isotopic composition, and inorganic reduction would require high temperature (Kajiwara, 1971). Kaplan et al. (1969) concluded that the most likely source of sulfur is the thick section of Tertiary evaporites to the south of the Atlantis II Deep. They suggested that both metals and sulfides, which were obtained by reaction of CaSC»4 or sulfate-rich brine with organic matter in shales are introduced into the Atlantis II Deep with the hot brine. The present study of anhydrite and sulfides from Sites 225, 227, and 228 (Figure 1) is an attempt to clarify the source and the mode of introduction of sulfur into the Re*d Sea deposits. Sites 225 and 227 are located in the axial valley of the Red Sea near but not in the hot brine area. Site 228 is on the western flank of the axial valley approximately 150 miles south of the Atlantis II Deep. METHODS

Sulfur Isotope Studies Of Evaporites and Shales from Sites 225, 227, and 228 in the Red Sea