Hagstrum, J.T. and Abbott, D., 2002, December. Evidence for a large bolide impact in the Proto-Pacific Ocean preceding the Chicxulub impact by about 2 million years. In AGU Fall Meeting Abstracts (Vol. 1, p. 0297).


Although the Cretaceous-Tertiary (K-T) transition is generally accepted as having been caused by a single large asteroid impact at ~65 Ma near Chicxulub on the Yucatán Peninsula, a `comet shower' or multiple-impact hypothesis has also been proposed to explain multiple extinction pulses in latest Cretaceous time. The contributory effects of contemporaneous Deccan volcanism and rapid sea level changes also remain controversial. We have discovered spherule layers several meters below the K-T boundary (Chicxulub impact layer) in giant piston core GPC-3 and in DSDP drill cores 576-8-1 (43-45 cm) and 596-3-6 (142-144 cm) from the northern and southern Pacific Ocean, respectively. We have also found a spherule layer ~8 cm below the peak K-T Ir anomaly in core 596-3-4 (50-51 cm); this layer contains the farthest known spherules from the Chicxulub crater (>10,000 km at 65 Ma). Corliss and Hollister [Nature, 282, 1979, p. 707-9] initially reported small (~20 μm) cristobalite spherules in core GPC-3 within a zone of disrupted layering, between ~2 m below the K-T boundary and the core bottom (~2 m thick), but considered them of volcanic origin. We have found larger spherules and mineral crystals (up to >200 μm) dispersed within this disturbed zone. The spherule layers found ~5.5 m below the peak K-T Ir anomaly in hole 576, and ~3.8 m below the peak K-T Ir anomaly in hole 596 are relatively undisturbed. Hole GPC-3 is the easternmost on Mesozoic crust in the Pacific Ocean, and we interpret the pre-K-T spherules and mineral crystals as ejecta and vapor-phase condensates, respectively, from an oceanic impact site farther east on crust now subducted beneath western North America. Apparently, the pre-K-T spherule layers are related to an earlier large impact because of the size of the condensed particles (>100 μm) in the GPC-3 core, and because some of the particles have chemical compositions (Fe-Ti-C-O) that are not of volcanic origin. Disruption of the sediments and dispersal of the spherules and mineral grains in core GPC-3 were probably caused by megatsunami waves associated with nearby impact. Such waves (initially up to 4 km high) might account for an abrupt anomaly in the seawater 87Sr/86Sr ratio, also preceding the K-T boundary by several m.y., because the waves likely washed vast amounts of 87Sr-rich continental soils into the oceans. In addition, erosion of shallow continental margins or deposition of higher-energy sediments by megatsunami waves left a stratigraphic record that may be erroneously interpreted as indicating a regression-transgression pulse in sea level near the time of impact.