XAFS study of Sb and As in Cretaceous-Tertiary boundary sediments: an index of soiling of the global environment with dust and ashes from impact ejecta falls

摘要

The local structure around antimony (Sb) atoms in Cretaceous-Tertiary (K-T) boundary sediments from Stevns Klint in Denmark was studied using Sb K-edge X-ray absorption fine structure (XAFS) spectroscopy to obtain information about the chemical state and coordination environment. We also performed arsenic (As) K-edge XAFS measurements. The Sb K-edge X-ray absorption near edge structure (XANES) spectrum of K-T boundary sediments was compared with those of various kinds of reference Sb minerals, such as Sb sulfides and Sb~(5+) complex oxides, and soils containing ferrihydrite (schwertmannite), Sb_2O_3, and Sb_2O_5. The XANES pattern and threshold energy of K-T boundary sediments are similar to those of ferrihydrite (schwertmannite) soil samples. There is no chemical shift in the threshold energies among K-T boundary sediments, Sb~(5+) oxide complex minerals, and Sb_2O_5. In As XAFS analyses, the threshold energy of K-T boundary sediments is approximately similar to those of As~(5+) minerals, and the XANES pattern of K-T boundary sediments is almost similar to those of ferrihydrite (schwertmannite) soil samples. The oxidation states of Sb and As of K-T boundary sediments are estimated to be Sb~(5+) and As~(5+), respectively. Sb and As in K-T boundary sediments are coordinated with oxide ions, and Sb and As exist in the same local structure positions as Sb and As in ferrihydrite (schwertmannite). The XANES spectra and radial structure function for Sb atoms also showed that Sb in K-T boundary sediments is stored in a SbO_6 octahedral coordination environment. The Sb-O interatomic distance in the K-T boundary sediments sample is 1.99(1) A. Abundant ferric hydroxides occur in K-T boundary sediments. Sb is considered to be coprecipitated with As and Fe ions, and Sb and As in K-T boundary sediments are incorporated in low crystalline ferrihydrite (schwertmannite) throughout precipitation and sedimentation. The environment at K-T boundary sediments resembles that of soil contaminated by Sb and As in local areas at the present age. However, in an unusual environment, such as widely distributed K-T boundary sediments in the world, unusually high concentrations of Sb~(5+) and As~(5+) could become an index of the soiling of the global environment with dust and ashes derived from asteroid impact ejecta falls.