Résumé
Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extremely positive delta S-34(pyrite) values together with higher delta S-34(pyrite) than coeval delta S-34(CAS)) are mostly interpreted as resulting either from secondary postdepositional processes or from multiple redox reactions between sulfate and sulfide in stratified sulfate-poor environments. We report here the first observation of strongly positive delta S-34 values for both dissolved sulfate and sulfide (average delta S-34(diss.sulfate) value of 34.6 parts per thousand and delta S-34(diss.sulfide) values of 36.7 parts per thousand) compared to the present-day seawater delta S-34(diss).(sulfate) (similar to 21 parts per thousand), with a negative apparent fractionation between sulfate and sulfide (Delta S-34(diss.sulfate-diss.sulfide) similar to -2.1 +/- 1.4 parts per thousand), in the sulfate-poor (m) modern thalassohaline lacustrine system Dziani Dzaha (Mayotte, Indian Ocean). Overall, surface sediments faithfully record the water column isotopic signatures including a mainly negative Delta S-34(sed.sulfate-sed.sulfide) (-4.98 +/- 4.5 parts per thousand), corresponding to the definition of superheavy pyrite documented in the rock record. We propose that in the Dziani Dzaha this superheavy pyrite signature results from a two-stage evolution of the sulfur biogeochemical cycle. In a first stage, the sulfur cycle would have been dominated by sulfate from initially sulfate-rich marine waters. Overtime, Raleigh distillation by microbial sulfate reduction coupled with sulfide burial in the sediment would have progressively enriched in S-34 the water column residual sulfate. In a second still active stage, quantitative sulfate reduction not only occurs below the halocline during stratified periods but also in the whole water column during fully anoxic episodes. Sulfates are then regenerated by partial oxidation of sulfides as the oxic-anoxic interface moves downward. These results demonstrate that the atypical superheavy pyrite isotope signature does not necessarily require postdepositional or secondary oxidative processes and can result from primary processes in restricted sulfate-poor and highly productive environments analogous to the Dziani Dzaha.