GOLDSCHMIDT CONFERENCE TOULOUSE 1998 Geochemical characterization of the K-T boundary at Beloc/Haiti

Major environmental changes at the K-T boundary caused by the impact of an asteroid or some other extraterrestrial body or by extensive volcanic activity play the leading roles in the discussion of the worldwide environmental stress in the ocean (a Review is given in Glasby and Kunzendorf 1996) resulting in a marked impact on ocean chemistry, ocean productivity and sea level drop. Geochemical characterization of the sediment layers of the K-T boundary can help to reconstruct the story of this time period. ~13C values of plantonic skeletons reflect a drastic decrease in the primary productivity (Hsu and McKenzie 1985) with no corresponding change in value for benthic skeletons. This was interpreted as being due to an almost complete reduction in oceanic plankton production and the elimimation of the surface to bottom carbon isotope gradient of the oceans by mixing within a few decades. Oxygen isotope data indicate a cooling of 2-3 ~ up to 6 ~ (Sarkar et al., 1992) followed by a rapid warming (10 ~ Late Cretaceous i character-ized by a regression (over 100 m during 0.5 my) followed by an equally rapid rebound after the event (Haq et al., 1987). Concerning the worldwide Ir anomaly at this boundary, it can be calculated that this Ir-rich layer accumulated at an average rate of 5.977 my Ir/cm 2 ky over a period of 18.000 years which is somewhat less than the residence time of Ir in seawater (Glasby and Kunzendorf 1996). Work has been done on the complex pattern of trace elements, pyrite is the major carrier for Ni, Co, As, Sb, and Zn. This idea is supported by the similarity in the contents of Au, Pt, Ni and Co with those of black shales (e.g. Schmitz 1985). Ir is most probably absorbed on fine particles during the volcanic event (Hansen et al., 1988). Glasby and Kunzendorf (1996) showed that the elements reach peaks in the Fish clay grading upwards in the sequence Fe-Co-Zn-As-Sb, Ir-Cr, Au, REE and U. Thompson et al. (1993) showed that although the sediment was deposited under anoxic conditions the peaks do suggest he influence of diagenetic processes on the element distribution