The Weathering-Modified Iridium Record of a New Cretaceous—Palaeogene Site at Lechówka Near Chełm, SE Poland, and Its Palaeobiologic Implications

Abstract

In the light of integrated biostratigraphic and geochemical data, a complete shallow-marine succession across the Cretaceous—Palaeogene (K—Pg) boundary, with the critical boundary clay coupled with a burrowed siliceous chalk (“opoka” in Polish geological literature), possibly equivalent of the basal Danian Cerithium Limestone in Denmark, has been discovered at Lechowka near Chelm, SE Poland. An extraterrestrial signature marking the K—Pg boundary is confirmed by anomalously high amounts of iridium (up to 9.8 ppb) and other siderophile elements (especially Au and Ni), as well as by an elevated Ir/Au ratio consistent with a chondrite meteoritic composition. The major positive iridium spike surprisingly occurs in Maastrichtian marls, 10 cm below the boundary clay interval, which can be explained by diagenetic mobilisation and re-concentration of the impact-derived components. Thus, intensively infiltrating, humic acid-rich ground waters during the long-lasting Palaeogene weathering in tropical humid regimes ...