Sulfur isotopic compositions across terrestrial Cretaceous-Tertiary boundary successions

Isotopic compositions of sulfur and concentrations of sulfur and carbon have been measured for sedimentary rocks across terrestrial Cretaceous-Tertiary (K-T) boundary successions that originated from flood-plain and backswamp environments. Organic carbon contents are relatively constant below the boundary (26 cm for the Dogie Creek section and 9 cm for the Brownie Butte section), but change abruptly at the K-T boundary. At the K-T event, a high input of sulfate to the freshwater wetlands might have resulted from the melt ejecta and/or acid rain. We interpret the low ratio of organic C to nonorganic S at the melt ejecta layer and sulfur isotopic data as consistent with this hypothesis; however, additional analyses across a thicker interval are necessary to rule out alternative hypotheses. Maruoka, T., Koeberl, C., Newton, J., Gilmour, I., and Bohor, B.F., 2002, Sulfur isotopic compositions across terrestrial Cretaceous-Tertiary boundary successions, in Koeberl, C., and MacLeod, K.G., eds., Catastrophic Events and Mass Extinctions: Impacts and Beyond: Boulder, Colorado, Geological Society of America Special Paper 356, p. 337–344. INTRODUCTION Most sulfides present in sediments originate from H2S produced by sulfate-reducing bacteria (e.g., Berner, 1984). This process is accompanied by a fractionation of the sulfur isotopes that results in sulfide being S depleted with respect to the parent sulfate. The extent of isotopic fractionation between sulfate and sulfide is dependent on the kinetics of the sulfate reduction reaction (e.g., Kaplan and Rittenberg, 1964; Chambers and Trudinger, 1979), which, in turn, is affected by the environment of deposition. Consequently, the isotopic composition of sulfide can be used as a proxy for environmental conditions such as the temperature of water, sulfate abundance, and types of electron source. Holser et al. (1988) observed that dS values of sulfides and sulfate in marine sediments have not changed much since the end of the Cretaceous. However, there have been relatively few studies of sulfur across the Cretaceous-Tertiary (K-T) boundary. Schmitz et al. (1988) reported dS values of 31.5‰ in metal-rich pyrite spherules that occur in the basal Fish Clay from the marine K-T succession at Stevns Klint, Denmark. Heymann et al. (1998) observed native sulfur in a spherule-bearing unit from marine K-T successions around the Gulf of Mexico with dS values of 24.9‰. Both these dS values are similar to those observed in Late Cretaceous sedimentary rocks, and suggest that the geological record provides no indication of major perturbations in the microbiological utilization of sulfur across the K-T boundary. However, Kajiwara and Kaiho (1991, 1992) observed a temporary and marked increase in pyrite dS values across the K-T boundary in the T. Maruoka et al. 338 marine succession at Kawaruppu, Hokkaido, Japan, and suggested an anoxic depositional environment immediately after the K-T boundary as an explanation for the variation in dS values. Holmes and Bohor (1994) observed an increase in dS values in the terrestrial K-T boundary claystone unit at the Sugarite site in the Raton Basin, New Mexico (Pillmore and Flores, 1987). The dS value increases by 3.5‰ and the dC value of the organic matter is also 2‰–3‰ heavier in the boundary claystone unit (fireball and melt ejecta layer) than in the surrounding coal. They suggested impact-derived carbon and sulfur from vaporized marine target rocks as the origin of the isotopically heavier S and C in the boundary claystone. The terrestrial environment is suitable for the detection of an impact-derived sulfate input because the concentrations of sulfate in terrestrial environments are generally low relative to marine environments, and the perturbation effects are minimal in terrestrial environments. However, it is important to distinguish isotopically heavy S originating from vaporization of target rocks from that associated with environmental change caused by the K-T event. In this study we determined S contents and dS values in sedimentary rocks across two K-T boundary sequences and used these data to examine paleoenvironmental conditions of terrestrial successions across the K-T boundary from the Western Interior of the United States.