Local Structures of Se and Br in Cretaceous–Paleogene Boundary Sediments and Concentration Effects of Ferrihydrite Originating From Impact Ejecta

ABSTRACT

The local structures and chemical states of Se and Br atoms in the Cretaceous–Paleogene (KPg) boundary sediments from Stevns Klint in Denmark were studied to obtain information on the concentration processes of both elements using Se and Br K‐edge x‐ray absorption fine structure (XAFS) spectroscopy. The Se K‐edge x‐ray absorption near‐edge structure (XANES) spectra, radial structural functions (RSFs) around Se, and Se–O distance of 1.73(1) Å revealed that Se in the KPg boundary sediments exists as a ferric selenite structure intimately associated with the ferrihydrite. Se was incorporated into ferrihydrite through precipitation and diagenesis caused by the sedimentation of Fe‐rich impact ejecta. A high concentration of Se occurs through a process similar to that of As and Sb in the KPg sediments. The XANES spectra, RSFs and Br–C distance of 1.87(2) Å showed that Br in the KPg boundary sediments is covalently bonded with carbon in highly polymerised aromatic groups. The stability and long‐term persistence of organobromine in the KPg boundary sediments are attributed to the strong C–C and C–Br bonds in the polymerised aromatic groups. Because no iodine enrichment was observed, Br–C bond formation in the KPg boundary sediments likely had little biological involvement. The formation of Br–C bonds in organobromine compounds may be promoted by the redox activity of the abundant Fe and other transition metals. The existence of abundant Fe from impact ejecta leads to unusually high concentrations of As, Se and Sb by coprecipitation and of Br by its redox activity, causing Br–C bonding.