Stable isotope ratios, iridium concentrations, and mineralogy of sediments at and around the Cretaceous/Tertiary boundary at ODP Hole 119-738C

High-resolution stratigraphic evidence of an apparently complete carbonate-rich Cretaceous/Tertiary boundary interval in Section 119-738C-20R-5 from the southern Kerguelen Plateau is summarized and interpreted. The change of the calcareous nannoflora and of the planktonic foraminifers is spread over a laminated interval of about 15 cm thickness. The base of this laminated interval lies in uppermost Maestrichtian chalks, 2 cm below a distinct 2-mm-thick "gray clay" layer, which shows the highest iridium enrichment (18 ppb) measured in this section. No shocked quartz or microspherules, characteristic of an impact, were found. No change in the clay mineralogy, which could be expected for a large volcanic or impact event, could be identified. Elevated metal and iridium concentrations (> 1.6 ppb) occur already in the bioturbated uppermost Maestrichtian chalks several centimeters below the "gray clay" and decrease above the iridium peak gradually over a laminated 12-cm-thick interval to background values of 0.1-0.3 ppb Ir. Application of bio- and magnetochronology shows that the accumulation rates of carbonates and clays, but not of the metals, decreased dramatically at the Cretaceous/Tertiary boundary and thus, the lack of dilution may have led to the observed metal concentrations.