S and O (SO4) isotopes, simultaneous modeling, and environmental significance of the Nijar messinian gypsum, Spain

Abstract

The Messinian evaporites in Nijar Basin, southeastern Spain, consist of coarse-grained primary selenites and laminated fine-grained gypsums. These gypsums, which are similar to those deposited in other Mediterranean Messinian basins, have rather variable δ34S and δ18O (SO4) (+21 to +24‰ for δ34S, +10 to +17‰ for δ18O) that are generally greater than the normal Tertiary marine values (21–22‰ for δ34S and 13‰ for δ18O). Moreover, the variation of δ18O (SO4) is larger than that of δ34S. Continental input and reservoir effect did not play important roles for these isotope variations because these factors should have resulted in lower δ34S and δ18O (SO4) than the normal marine values. Redox reactions of sulfur species in brine should have been responsible for the isotope variations.To interpret the variable δ34S and δ18O (SO4), we quantitatively modeled δ34S and δ18O (SO4) of dissolved sulfate during redox reactions in two marine evaporative settings: in sediment pores in a shallow-water pan, and in a free-brine column in a deep-water basin. Modeled δ34S and δ18O of dissolved sulfate during reduction of sulfate yield small variations, especially for O isotopes in a shallow-water pan. However, modeled δ34S and δ18O of dissolved sulfate during reduction and reoxidation cycling processes in a deep-water basin show a progressive increase of up to 10‰ for both δ34S and δ18O (SO4) above their normal marine values. Importantly, the modeled pathways for the deep-water basin setting are consistent with the Nijar data, whereas those for pore waters in shallow-water pans are a poor fit to the Nijar data. The modeled results consistent with trace elements, Sr isotope data, and selenite fabrics suggest that these Yesares selenites have formed in deep marine brines.The modeling results also demonstrate that δ18O (SO4) can have larger variation than δ34S because the incorporation of water oxygen and probably dissolved free oxygen increases δ18O of the reoxidized sulfate, whereas reoxidation of sulfide to sulfate decreases the total δ34S (SO4). The modeled results could provide important geological implications for isotope and environment interpretations for ancient evaporites. For example, redox reactions of sulfur species during sulfate precipitation could modify normal marine δ34S and δ18O (SO4) more efficiently than freshwater contributions and reservoir effects. Therefore, to establish δ34S and δ18O (SO4) age curves, redox processes for ancient evaporites should be considered.