Did a volcanic mega-eruption cause global cooling during the Late Ordovician?

The Late Ordovician Taconic orogeny was associated with volcanic eruptions along the subduction zones of the Iapetus Ocean. One of these eruptions, which led to the deposition of the Deicke K-bentonite Bed, is believed to have been larger than the largest recent and subrecent volcanic eruptions (e.g., Toba, Pinatubo). The Deicke eruption has been proposed to have led to a cooling event and associated faunal turnover during the Sandbian-Katian of Laurentia based in part on the observed lowering of global surface temperature after recent mega-eruptions. We tested for a geologically resolvable climatic perturbation associated with the Deicke eruption by estimating changes in ocean temperatures from the oxygen isotope ratios of single-species separates of conodont apatite from a section of the Carimona Member of the Platteville Formation in southeastern Minnesota, United States, that includes the Deicke K-bentonite. In contrast to predictions of models invoking more or less direct volcanic forcing for Ordovician climate trends, we found no obvious or consistent change in temperature at or above the bentonite, but did see evidence of cooling ( approximately 4 degrees C) among presumed nekto-benthic taxa in the 0.7 meters of the section below the bentonite. Thus, at least for the study area, there is no evidence that the Deicke eruption induced a significant cooling event.