Cretaceous-Paleogene boundary Fish Clay at Hojerup (Stevns Klint, Denmark): trace metals in kerogen

Abstract

Geochemical analyses of trace metals (Ir, Ni, Co, Cr, Zn, Au and Pb) and rare earth elements (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) in kerogen of the black marl at the Cretaceous-Paleogene boundary Fish Clay at Hojerup have been undertaken. Substantial proportions of the trace metals and rare earths were probably contained in terrestrial humic substances (the kerogen precursor) arriving at the marine sedimentary site. This is in accord with a previous hypothesis that kerogen is mainly derived from humic acids of an oxic soil in of the adjacent coastal areas of eastern Denmark. It is also suggested that humics were transported mainly through fluvial transport into the site of the deposition of the Fish Clay. The local weathering/leaching of the impact-ejecta fallout on the land surface or local terrestrial rocks by impact-induced? acid surface waters perhaps played an important role in providing trace metals and rare earths for these humic substances. Apparently, chondritic Ir, An, Ni, Co, Cr and chondritic and non-chondritic Zn originated from the impact fallout; Pb and rare earth elements were most likely sourced by the rocks exposed in the coastal areas of eastern Denmark.