Sulfur Concentration and Isotopic Composition across the Terrestrial Cretaceous-Paleogene Boundary in Montana, USA

The Cretaceous-Paleogene mass extinction (~66 Ma) is hypothesized to have been caused by environmental effects of the Chicxulub asteroid impact and/or Deccan Traps volcanism. Both events potentially released massive amounts of sulfur into the atmosphere and thus increased sulfate aerosol production, leading to significant global cooling and acid rain. Sulfur released by each of these events has a characteristic isotopic composition, and therefore may be distinguishable in sulfur isotope records.

Little is known about the terrestrial sulfur record across the Cretaceous-Paleogene boundary (KPB), excluding a narrow ~20-thousand year (ka) range across the boundary. While the Chicxulub impact coincides with the KPB, the 20-ka interval excludes most of the million year period of Deccan Traps (DT) eruptions, so whether DT-derived sulfate aerosols are present remains unknown. To characterize changes to the sulfur cycle across the KPB, we have measured sulfur abundance and isotopic composition of sediment for several hundred thousand years across the KPB in the Hell Creek region of Montana, significantly extending the terrestrial sulfur isotope record. This study locality also enables comparison of the sulfur isotope record to other chemostratigraphic records, as well as assimilation into a high-resolution geochronologic framework.

The majority of our δ 34 S values fall within the range expected for volcanic sulfur (0-8 ‰), suggesting that volcanism is the dominant sulfur source. Local volcanism associated with the Laramide Orogeny most likely also deposited sulfur in Montana during this time interval. Tephra layers deposited by these local eruptions have consistently higher sulfur concentrations, and in some cases higher δ 34 S values (3-8 ‰), confirming this hypothesis. The dominant source of diagenetic sulfur is potentially the underlying marine Bearpaw Fm., however we do not observe higher (10+ ‰) δ 34 S values indicating reprecipitation of marine sulfates. We also do not observe any significantly negative (<-4 ‰) δ 34 S values, suggesting that postdepositional microbial sulfate reduction in the water column is negligible. However, individual episodes of DT sulfur deposition are not readily apparent. Increasing our sampled range beyond the known DT eruptive interval will allow us to test for a pervasive DT influence.