The biogeography of export productivity across the Cretaceous-Paleogene boundary as inferred from sediment mixing and biogenic barium

The impact at the Cretaceous-Paleogene (KPg) boundary is thought to have precipitated a sudden decrease in global export productivity, in addition to widespread extinction. A global decrease in oceanic export productivity gains support from the collapse in δ13C gradients between surface-dwelling and deep-ocean foraminifera and decreased sedimentation rates. However, a growing body of evidence suggests a milder, spatially heterogeneous perturbation. Here we investigate the magnitude and spatial distribution of export production across the KPg boundary using two proxies, the depth of sediment mixing and the accumulation of biogenic barium. In the modern ocean, the depth of sediment mixing is correlated to the amount of organic matter reaching the sea floor as a function of water depth and export production. We calculate the relative depth of sediment mixing by fitting mixing models to the shape of iridium anomalies. In all 10 sites examined globally, export productivity is indicated by the presence of sediment mixing. The depth of mixing inferred from a subset of these sites records an inter-site range in mixing depths greater than the range measured in the modern ocean. Biogenic barium (measured by Ba/Al ratios) indicates a spatially heterogeneous effect of boundary events on export production as well. While biogenic barium decreases at most sites studied, the magnitude of this decline varies among sites, with one site in the North Pacific failing to record a sustained depression. Together, our results suggest that the decrease in export productivity following the KPg boundary event may have been a regional, albeit widespread, phenomenon.