SULFUR ISOTOPIC FRACTIONATION IN K-T BOUNDARY ROCKS FROM THE CHICXULUB CRATER (YAXCOPOIL-1): A LABORATORY STUDY

The chemical speciation, concentration and stable isotope systematics of sulfur in the K-T boundary rocks from the CSDP drill core point to a complex interplay of depositional processes and post-impact related alteration. Sequential extraction of sulfur species reveals trends not shown by whole-rock isotopic compositions. Overall, the impact interval is depleted in total carbon and sulfur concentrations compared to the overlying and underlying sedimentary units. Differences in the average and median sulfate values observed between the Upper and Lower impact interval are inferred to reflect a perturbation of the sulfur isotope systematics during the impact and post-impact alteration. Several studies have pointed to a 3 to 4‰ enrichment in δ³⁴S of sulfate relative to contemporaneous seawater through the Upper impact interval.

A set of laboratory experiments were conducted in a Parr Oxygen Bomb to determine what isotopic fractionation might result from simulating an open system with an abundance of oxygen available at the time of sediment and rock vaporization. The shallow marine carbonate setting observed in the Yaxcopoil-1 core was synthesized using a mixture of carbonate and sulfate. A maximum enrichment of 4.5‰ in δ³⁴S was observed as the system was degassed. This provides a second mechanism, beyond bacterial sulfate reduction, for the enrichment of δ³⁴S in the secondary sulfates observed in the Upper impact interval of Yaxcopoil-1 core.