Cadmium as a tracer of volcanism at the Cretaceous-Paleogene boundary

Major volcanic eruptions like the Deccan have long been invoked as causes of global mass extinctions. Deccan volcanism erupted ~ 600,000 km³ over the span of ~ 700 ka contemporaneously with the end-Cretaceous mass extinction. It has been difficult to establish a correlation between eruption size and extinction intensity because the frequency of eruptions and extent of degassing of individual flows is not well constrained. For example, the eruption of the same amount of lava by high frequency, low effusion flows is less likely to result in a mass extinction than by eruption of low frequency, high effusion flows. This is due to volcanic eruptions outgassing climate perturbing gases SO₂ and CO₂. When in the atmosphere, SO₂ and CO₂ could lead to a global climatic catastrophe capable of driving the extinction observed in planktic foraminifera that preceded the end-Cretaceous mass extinction by 200 ka. To determine the role Deccan volcanism played on the extinction it is critical to constrain the eruptive and effusive rates of Deccan eruptions. In addition to SO₂ and CO₂, volcanoes also emit volatile metals, e.g., Cd, Re, Hg, that form aerosols preserved in contemporaneous sediments. Trace metals, such as Cd, accumulate in sediments, where the excess Cd reflects the intensity of volcanic emissions. In such instances, high frequency, low effusion rate eruptions result in low Cd enrichments, whereas low frequency, high effusion rate eruptions result in high Cd enrichments in sediments. To constrain the eruption tempo of the Deccan, we have performed measurements of elemental abundances on the stratigraphically well-preserved KPg section at Elles, Tunisia. In this report, we conducted a high-resolution study of ~ 90 samples covering ~ 20 ka above the KPg boundary to ~ 350 ka below the KPg boundary. Elemental compositions for ~ 50 elements of Elles sediments were obtained by solution ICP-MS. In some samples, particularly sediments from 100 ka period preceding the boundary, Cd enrichments were eight times that of the upper continental crust (UCC). A lack of correlation between Cd and TOC, Zn, P₂O₅, and Mo below the boundary suggest the Cd enrichments are not from an influx of biogenic detritus nor from organic burial. Above the boundary, normal shale Cd values representing 25 ka are interpreted here to represent the period between the Ambenali and Poladpur phases. Cadmium as a tracer relates foram-based chronology with the intensity of the Deccan eruption.