Iridium marks the spot

Between the reddish-brown, silty claystone and the gray limestone shown in this sediment core is a thin layer of clay, located at approximately the 34 cm mark in the image. Geologists identify that layer with the Cretaceous–Paleogene (K–Pg) boundary. At that geologic marker in time 66 million years ago, an estimated 75% of Earth’s species became extinct, including the last dinosaurs. Two international collaborations in 2016 collected the sediment core from part of the Chicxulub crater buried underneath Mexico’s Yucatán peninsula. Steven Goderis of Vrije Universiteit Brussel in Belgium and his colleagues recently analyzed the core’s geochemistry. At the K–Pg boundary they found iridium in unearthly concentrations, which lends additional support to a long-held hypothesis: that an asteroid impact is responsible for the extinction event.

The researchers measured the concentrations of various elements in and around the K–Pg boundary layer of clay. They found the highest concentrations of iridium and other highly siderophile, or iron-loving, elements in the thin clay layer. Those elements aren’t usually found at such high concentrations in Earth’s crust and mantle, so their presence close to the surface indicates the deposition of meteorite material. Perhaps most telling, the high iridium concentrations match those of some 350 other places colocated in time, which suggests a meteorite impact with global environmental ramifications. (S. Goderis et al., Sci. Adv. 7, eabe3647, 2021, doi:10.1126/sciadv.abe3647 ; photo courtesy of J. Morgan et al., in Proceedings of the International Ocean Discovery Program, vol. 364, International Ocean Discovery Program, 2017.)