Multi-Sulfur Isotopes in Cretaceous-Tertiary Boundary Samples from the Western Interior-Search for Photochemical Effects

To assess environmental perturbations induced by the impact event at the Cretaceous–Tertiary (K–T) boundary, and to search for photochemical effects in the dust while suspended in the atmosphere after the impact event, such as observed in ice core volcanism and in the Earth's Archean, concentrations and isotopic compositions of sulfur were determined in sedimentary rocks that span the terrestrial K–T boundary at Dogie Creek, Montana, Berwind Canyon, and Brownie Butte, Wyoming, in the Western Interior of the United States. The boundary clays at both sites are not bounded by coals. Earlier carbon and δ³⁴S isotopic studies have shown that organic carbon contents are relatively constant below the boundary, but that there are abrupt changes at the K-T boundary. The constant sedimentation, inferred by the constancy of the organic carbon concentration, continued before the K-T boundary and ended with the K-T event. At the K-T event, the high input of sulfate to the freshwater wetlands might have resulted from the melt ejecta and/or acid rain, as is suggested by the low ratio of organic C to non-organic S at the melt ejecta layer. The anoxia of the water in the wetlands may have caused a maximum of sulfur concentration accompanied by minimum δ³⁴S. Our new measurements of all sulfur isotopes δ³³S,δ³⁴S,δ³⁶S from the fireball layer reveal light δ³⁴S values (-1.4 to -5.8 per mil), consistent with previous measurements. Interestingly, the sample from Berwind Canyon is slightly mass independently fractionated (0.08 per mil below normal mass fractionation). From measurements of volcanic debris, Precambrian sulfur, Martian sulfates, and laboratory experiments, this is interpreted as deriving from short wavelength UV photolysis in the stratosphere; if this is confirmed, the results may be utilized to resolve the stratospheric chemical alteration of the impact debris and the consequences for global photochemistry.