Trace and Major Element Chemistry Across the Cretaceous/Tertiary Boundary at Stevns Klint

Details Trace and Major Element Chemistry Across the Cretaceous/Tertiary Boundary at Stevns Klint Trace and Major Element Chemistry Across the Cretaceous/Tertiary Boundary at Stevns Klint

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.228g&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 228

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INAA measurements of samples obtained by high-resolution stratigraphy on a mm scale reveal considerable variations in element concentrations across the boundary with their respective maxima stratified in distinct sublayers (Graup et al., 1992). These results suggest that measurements of bulk boundary samples a few cm thick may be inappropriate as concentration variations and element ratios would be leveled out pretending a single geochemical signal. Having investigated a sample comprising sublayers B, C, and D (Fig. 1), Alvarez et al.(1980) acknowledge that "no information is available on the chemical variations within the boundary." This kind of information is given below and shown in Fig. 1 (sublayers A and B are drafted in double scale). From the main lithologic characteristics of Maastrichtian to Paleocene sediments (Schmitz, 1988; Graup et al., 1992) it is readily deduced that Eh and pH conditions in the marine environment changed from oxic-mildly alkaline with normal carbonate sedimentation (Q-M) to anoxic-(mildly) acid with deposition of pyrite spherules (A3), organic material, and clay minerals in the Fish Clay (A-D), followed by a restoration of oxic-alkaline conditions depositing the Cerithium limestone (E- I). The element distribution across the boundary obviously mirrors these alternating environmental conditions: compounds soluble under acid and reducing conditions like Ca-carbonate and Mn are strongly depleted in the Fish Clay (Fig. 1A), whereas compounds stable and insoluble under these conditions are highly enriched (Fig. 1B). The opposite holds true for the calcareous sediments. Across the boundary, enhanced element concentrations are not evenly distributed but appear to be stratified with maximum concentrations in three distinct sublayers for the following elements: (1) A1 (hard clay): peak concentrations for REE (La 72 ppm) and U (45.5 ppm) as compared to 13 ppm La and 2 ppm U in sublayer A2 immediately above. (2) A3 (pyrite spherules): peak concentrations for Fe, Co, Ni, Au, and all chalcophiles. The trace elements correlate well with Fe across the boundary. (3) B (organic-rich marl): peak concentrations for Ir (87.6 ppb), Re (96 ppb, but 113 ppb in C), and organic carbon (2.3%). Ir correlates well with organic carbon (data from Schmitz, 1988), to a lesser extent with Re, and, possibly, Os, but is not correlated with Ni, Co or Au (Graup et al., 1992). Despite large variations in absolute concentrations and, therefore, also of ratios for elements with differing chemical behaviour, there are some pairs of chemically closely related elements (siderophiles as well as chalco- and lithophiles), the ratios of which remain fairly constant over the whole boundary range. Examples shown in Fig. 1A: Ni/Co (average 7.6/std.dev. 1.2) and La/Yb (12.9/2.4). Although Eh,pH conditions vary widely, these elements are not fractionated from each other because of their closely similar geochemical behaviour. The high concentrations of Ir, Ni, and chalcophile elements making up the K/T geochemical anomaly should be indicative of an external component added to the marine environment. The elements introduced were subsequently precipitated according to their chemical properties and changing Eh,pH conditions resulting in stratification of peak concentrations. The constancy of certain element ratios indicates an extended period of availability for this external component. REFERENCES: Alvarez L.W., Alvarez W., Asaro F., and Michel H.V. (1980) Science 208, 1095-1108. Graup G., Palme H., and Spettel B. (1992) Lunar Planet. Sci.(abstract) 23, 445. Schmitz B. (1988) Geology 16, 1068-1072.

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