  We have identified iridium in an ~5 m-thick section of pelagic sediment cored
in the deep sea floor at Site 886C, in addition to a distinct spike in iridium
at the K-Pg boundary related to the Chicxulub asteroid impact. We distinguish
the contribution of the extraterrestrial matter in the sediments from those of
the terrestrial matter through a Co-Ir diagram, calling it the
"extraterrestrial index" fEX. This new index reveals a broad iridium anomaly
around the Chicxulub spike. Any mixtures of materials on the surface of the
Earth cannot explain the broad iridium component. On the other hand, we find
that an encounter of the solar system with a giant molecular cloud can aptly
explain the component, especially if the molecular cloud has a size of ~100 pc
and the central density of over 2000 protons/cm^3. Kataoka et al. (2013, 2014)
pointed that an encounter with a dark cloud would drive an environmental
catastrophe leading to mass extinction. Solid particles from the hypothesized
dark cloud would combine with the global environment of Earth, remaining in the
stratosphere for at least several months or years. With a sunshield effect
estimated to be as large as -9.3 W m^-2, the dark cloud would have caused
global climate cooling in the last 8 Myr of the Cretaceous period, consistent
with the variations of stable isotope ratios in oxygen (Barrera and Huber,
1990; Li and Keller, 1998; Barrera and Savin, 1999; Li and Keller, 1999) and
strontium (Barrera and Huber, 1990; Ingram, 1995; Sugarman et al., 1995). The
resulting growth of the continental ice sheet also resulted in a regression of
the sea level. The global cooling, which appears to be associated with a
decrease in the diversity of fossils, eventually led to the mass extinction at
the K-Pg boundary.

Earth and Planetary Astrophysics