Sulfur and oxygen isotope compositions of Upper Triassic sulfates from Northerm Apennines (Italy): palaeogeographic and hidrogeochemical implications.

Emilia-Romagna in the Northern Apennines were analyzed for sulfur and oxygen isotope compositions, yielding ?34S and ?18O values of 15.5±0.4? and 10.8±1.2?, respectively (mean ±99% confidence intervals). Combining these values with those of other Burano Formation sulfate deposits along the Apennine chain, mean for ?34S and ?18O values are obtained (15.2±0.2? and 10.9±0.5?, respectively). These isotopic signatures are interpreted as preserved primary features, despite the fact that the Burano Formation underwent anchizone to epizone metamorphism during the Apennine orogenesis. An overall ?18O value of 10.9±1.5? (mean ± pooled standard deviation), obtained by combining consistent sets of data from Italy and Spain, closely approaches that of gypsum deposited from the Tethys ocean during the Late Triassic. In addition, reviewing the isotope data published on Late Triassic evaporite sulfates from the Mediterranean area and abroad, several ?34S values appear to be lower than the inferred primary isotopic signature, and seemly decrease from East to West in the Mediterranean region, suggesting a similar trend for the Tethys ocean sulfate. Possibly, 34S-depleted sulfate entered the ocean through oxidation of volcanic SO2 emitted in the atmosphere and degassed from the seafloor during the development of Late Triassic rifting. On the other hand, positive shifts of ?34S and ?18O values also occur, defining a common trend that may be related to synsedimentary biological effects or post-depositional metasomatic-metamorphic effects, the latter affecting particularly the ?18O signature. Therefore, the ?34S and ?18O signatures of evaporite sulfate may provide a like "slide-rule" diagram to distinguish between isotopic effects related to biological or abiological processes, thus contributing to the reconstruction of paleoenvironments and paleogeographic settings. Based on the ?34S-?18O "slide-rule", the isotopic composition of sulfate dissolved in spring and stream waters of northern Tuscany was interpreted in terms of origin of the sulfate and modifying processes in solution. It was concluded that sulfate in springs derives from Upper Triassic evaporite existing locally at depth (Burano Formation), whereas sulfate in streams is manifestly a mixture of Burano Formation sulfate with supergene sulfate from oxidation of sulfide in the rocks. In sulfurous springs, both sulfur and oxygen isotope fractionations with respect to the source sulfate signatures may be ascribed to bacterial effects. However, the oxygen isotope exchange of sulfate with water should have been a very minor process as supported by the nearsurface temperature values estimated by sulfate-water oxygen isotope thermometry.