The sedimentological signature of impact spherules and its relation to ejecta transport mechanisms during the Chicxulub asteroid impact (Cretaceous/Paleogene boundary)

The Chicxulub impact is the most likely cause of the Cretaceous/Paleogene boundary (KPB) mass extinction and the only impact event in Earth's history with a globally traceable ejecta bed. Although the impact spherules are thought to represent molten or vaporized material, precise genesis and transport mechanisms remain underconstrained. Here, we show that the morphology, size, internal structure, and distribution of the spherules within the KPB deposits are related to origin and transport processes. The study of thousands of spherules from KPB deposits in Colombia, the USA, and Spain has revealed the presence of three fractions related to three different distribution processes, each with a distinctive sedimentological signature. The coarser fraction (“aspergo deposit,” from the Latin aspergo, meaning splash) comprises rotational and agglutinated/irregular forms (size > 2 mm) containing abundant vesicles, unmelted inclusions, and schlieren, implying an origin from molten materials and transport following ballistic trajectories. The intermediate fraction (“pyrocloud deposit” from the Greek pyr, meaning fire) represents a mixture of molten and condensed droplets, including spheres, rotational, and agglutinated/irregular forms (size 0.3–2 mm), transported by the rapid expansion of a fast-moving, high-temperature turbulent cloud. The globally distributed finer fraction (“fireball layer”) is composed exclusively of spheres (size <0.3 mm) condensed from a vapor plume after the Chicxulub impact. These observations provide valuable insights into ejecta distributions during massive asteroid impacts and enhance our understanding of the Chicxulub impact and its aftermath.