Isotope composition (δ34S, δ18O) of the Middle Triassic-Early Jurassic sulfates in eastern Iberia

Abstract

This study combines published isotope data (223 samples) with new data (153 samples) on the δ34S and δ18O composition of Ca-sulfate rocks (anhydrite and secondary gypsum) accumulated in eastern Iberia during the Middle Triassic to Early Jurassic. The δ34S mean values of the evaporite units reveal the following trends: a decrease from the lower Anisian (~18‰) to the Carnian-Norian interval (~15‰); an increase at the Rhaetian (~16‰); a decrease in the Hettangian-Sinemurian interval (~14‰); and finally an increasing trend from the Sinemurian-Pliensbachian (~16‰) to the Pliensbachian-Toarcian (~19‰). This δ34S isotope assemblage together with the generalized low standard deviations observed for the different evaporite units is consistent, and suggests that the isotope composition of these Ca-sulfates are primary signatures related to feeding marine water sulfate. Depending on the evaporite units, these δ34S mean values are, nevertheless, ~1‰ to ~3‰ lower than those reported for coeval Triassic evaporites of the Germanic basin and the Alp-Apennine domains. In eastern Iberia, the δ18O values are more variable and also display wider scatters than the corresponding δ34S values. Of particular interest in our study are: (i) the low δ34S values for the Anisian in comparison with those in the Germanic basin; (ii) the shift to heavier values in the Rhaetian, recorded for first time in the evaporites of eastern Iberia; (iii) the low values observed for the Hettangian, which suggest that the change to the heavier values of the Jurassic (~19‰) occurred after the end-Hettangian in eastern Iberia; and (iv) the affinity of the δ34S values recorded for the Hettangian-Sinemurian interval to those known for the Canadian domain.