DID A VOLCANIC MEGA-ERUPTION CAUSE GLOBAL COOLING DURING THE LATE ORDOVICIAN?

The 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 (DKB), is believed to have been larger than the largest recent and sub-recent volcanic eruptions (e.g., Toba, Pinatubo). As these recent eruptions have been shown to have caused a lowering of global surface temperature, the Deicke eruption could have led to the observed cooling event (and associated faunal turnover) during the Sandbian-Katian of Laurentia. In this scenario, due to its large size, the volcanic eruption triggered a long-term global cooling event by initiating a positive ice-albedo feedback loop.

We tested this hypothesis by estimating sea surface temperatures around the DKB. To this end, we established a high-resolution oxygen isotope of conodont apatite spanning the DKB in the Carimona Formation of Minnesota. We measured taxa specific δ¹⁸O values for Drepanoistodus suberectus, Panderodus gracilis, and Polyplacognathus ramosus.

We find that the δ¹⁸O values for D. suberectus directly above and below the DKB are 20.1‰ and 19.9‰ respectively. However, 20 cm below the DKB the δ¹⁸O value for D. suberectus is 19.3‰. Similarly, δ¹⁸O values for two samples with P. gracilis directly below the DKB are 19.9‰ and 19.8‰. Again, 20 cm below the DKB the δ¹⁸O value for P. gracilis is lighter (19.2‰) than the samples immediately below the DKB. Based on their paleoecological distribution, these taxa most likely had a pelagic lifestyle. Taken together, this would suggest that the cooling event of the surface ocean (cooling by ~2.5°C) preceded the volcanic eruption, this does not support a sustained cooling due to an ice-albedo feedback hypothesis.

δ¹⁸O values for P. ramosus are consistently heavy (ranging from 19.6‰ to 19.9‰), including the samples with light δ¹⁸O value for D. suberectus and P. gracilis. Based on its distribution, this taxon most likely had a benthic or nektobenthic lifestyle or that it followed a food resource with this mode of life. This suggests that bottom water temperatures were relatively cool during this episode. The diminishing difference between the pelagic and benthic taxa may indicate that SST was lowered through a different mechanism (e.g., local ocean upwelling) rather than global cooling.