The Cretaceous-Tertiary boundary Fiskeler at Stevns Klint, Denmark: the geochemistry of the major trace metals

Abstract

Trace metals in the four discrete layers of the Cretaceous-Tertiary boundary (Fiskeler) at Stevns Klint (Denmark) are relatively well studied, yet much remains to be learned about them. Therefore, an integrated study of the trace (meteoritic Ir, partly meteoritic Cr/Ni/Co/Au and terrestrial Zn/Cu/La/Ce/Nd/Sm/Eu/Tb/Yb/Ta/Th) metals in the basal black marl of Fiskeler and in its (carbonate, HCl-soluble, smectite, HCl-insoluble, silicate and kerogen) fractions was undertaken. The mineralogy of the marl is comparatively simple, authigenic calcite (mainly derived from planktonic marine algae: coccoliths), detrital Mg-smectite and quartz being the principal components, with lesser amounts of kerogen, Fe3+-oxides, pyrite, ilite and feldspar. Selective leaching procedures were used to establish geochemical associations and specific mineralogical locations of the trace metals. The results identified the main locations of the major trace metals (Cr/Ni/Co/Zn/Cu/Au), They occur mainly in the smectite and, to a lesser extent, in the biogenic calcite (Ni/Co/Zn) and kerogen (Ir/Au). The trace metal data are in accordance with the hypothesis that substantial proportions of Cr/Ni/Co/Zn/Cu/Ir/Au were probably contained in the detrital smectite arriving at the site of the deposition. In a general discussion of the results, a geochemical model describing the incorporation of trace metals in the smectite is presented, based on the weathering/trasport of the clay by (impact-induced) acid surface waters and the adsorption of trace metal ions by the smectite particles/colloids in the acidic solution of nearby oxic soil. The distribution/enrichment patterns of Cr/Ni/Co/Zn of the basal black marl and higher Fiskeler layers indicate, on the other hand, that notable proportions of these metals were incorporated into the smectite structure during the formation of the clay. The trace metal data are also considered in light of previously published paleoecological and geochemical information for Fiskeler. It is proposed, for example, that the local (impact-induced) superacid rainfall and wildfires played an important role in providing appropriate humics (i.e. appropriate organics of decaying land green plants) having porphyrin stuctures for the formation of kerogen (enriched with Cu2+-porphyrins). In addition, it has been suggested that the biogenic calcite with high Ni is probably a product of the metabolic uptake of boundary seawater (enriched with this metal) by the coccoliths. The metal enrichment of the seawater was created by a sudden and high influx of (apparently mainly meteoritic) Ni: airborne and laterally redeposited (by the acid surface/river waters) from a nearby soil. Lastly, the lateral distribution of kerogen (enriched with Cu2+-porpyhyrins) supports an earlier interpretation presented by Hultenberg(1,2) that the alleged (Nye Klov/Dania) boundaries in northwestern Denmark represent erosion and subsequent redeposition of Fiskeler in eastern Denmark.