University of Nebraska, United States

Many microfossil species typical of the latest Cretaceous are commonly found in lowermost Paleocene sediments fueling the debate about whether they represent genuine survivors or fossils reworked from underlying Cretaceous sediments. Thirty eight of 41 planktic foraminifer species present in the latest Cretaceous at ODP Site 1049 also occur in the earliest Paleocene above a layer of ejecta from the Yucatan impact structure. We believe nearly all these Cretaceous taxa were reworked from older Cretaceous sediments for four reasons: 1) large individual foraminifera typical of the Cretaceous do not record a dramatic cooling event seen in oxygen isotope records of Paleocene planktic foraminifera and fine-fraction carbonate. Instead, the Cretaceous species have both oxygen and carbon isotope ratios similar to their conspecifics in the late Maastrichtian. 2) Typical Cretaceous taxa found in Paleocene sediments and in the ejecta bed are size-sorted; there is a curious absence of small specimens unlike the size distribution present in upper Maastrichtian foraminifer assemblages. 3) In contrast, species known to be indigenous to the Paleocene are uniformly minute. 4) Large specimens of foraminifera typical of the Cretaceous are almost always filled with matrix unlike the enclosing Paleocene ooze and contain nannofossil taxa that became extinct well before the close of the Cretaceous. Apparently most of the larger specimens of typical Cretaceous taxa did not survive the boundary. If only the small specimens are genuine survivors, then the K-T extinction must have been extremely severe with no more than three species of planktic foraminifera surviving the boundary event in the western Atlantic.