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/Ir/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 withCu2+-porpyhyrins) supports an earlier
interpretation presented by Hultenberg1,2 that the alleged (Nye Kl?v/Dania)
boundaries in northwestern Denmark represent erosion and subsequent
redeposition of Fiskeler in eastern Denmark.
Trace metal enrichment observed in soils around a coal fired power plant in South Africa
