Iridium bearing Carbon Black at the Cretaceous-Tertiary Boundary

Carbon black has been observed to be the staining agent of the 3.5 m grey chalk underlying the Fish Clay in the Stevns Klint section. We show that carbon black extracted from the grey chalk and Fish Clay has a carbon isotope value significantly different both from the underlying Upper Cretaceous carbon black and from that of Upper Cretaceous charcoal. Carbon black stained bryozoan skeletons in the grey chalk are often seen to encrust on non-stained bryozoan skeletons and vice versa in the same stratigraphic level. This indicates that carbon black deposition took place in pulses during the ea. 50,000 year long grey chalk time interval. The carbon black-stained bryozoan skeletons are found to be rich in Ir as well as in other elements, and we demonstrate that the Ir-carrying phase is not a silicate, but rather the carbon black itself. A volcanic origin is suggested for the Ir-bearing carbon black.