Abstract

We analyzed in situ the trace-element distributions in the pristine 2-cm-thick spherule-rich Cretaceous-Paleogene (K-Pg) event bed at Ocean Drilling Program (ODP) Leg 207 Site 1259C (Demerara Rise, western Atlantic) using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) with a spot diameter of 235 μm, corresponding to a sample volume of ∼8.6 × 105 μm3. Unparalleled so far, this high resolution pinpoints the fallout of the Chicxulub projectile to the uppermost 2 mm of the K-Pg bed. Most of the Ni is not of extraterrestrial origin, as indicated by low, typical upper-crustal Ni/Cr ratios. Very low Zr/Hf and Nb/Ta ratios are confined exclusively to the K-Pg bed, indicating that storms and flooding spilled highly fractionated supracrustal material off the Guiana craton to reach the seafloor contemporaneously with the now-altered glass spherules. Chicxulub ejecta volumetrically dominate the K-Pg bed but contributed only minimally to the trace-element budget, as indicated by the very low rare earth element concentrations. At the top of the ejecta layer, together with the fallout of Pt from the impactor (∼0.1 ppm), a pronounced Pb enrichment (up to 1500 ppm) occurs, which is interpreted to derive from the Guiana craton. Both elements obviously were contained in submicrometer-size particles that could only reach the seafloor attached to larger particles, the sedimentation of which may have occurred up to a year after the Chicxulub impact event. Our high-resolution analysis of the K-Pg event bed allows us, for the first time, to differentiate among the contributing components in detail, as well as assess the settling time frame of this geologically important boundary deposit.

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