Biotic effects of the Chicxulub impact, the K-T event and sea level
change upon planktic foraminifera were evaluated in a new core and
outcrops along the Brazos River, Texas, about 1000 km from the Chicxulub
impact crater on Yucatan, Mexico. Sediment deposition occurred in a
middle neritic environment that shallowed to inner neritic depths near
the end of the Maastrichtian. The sea level fall scoured submarine
channels, which were infilled by a sandstone complex with reworked
Chicxulub impact spherules and clasts with spherules near the base. The
original Chicxulub impact ejecta layer was discovered 45-60 cm below the
sandstone complex, and predates the K-T mass extinction by about 300,000
years.
Results show that the Chicxulub impact caused no species extinctions or
any other significant biotic effects. The subsequent sea level fall to
inner neritic depth resulted in the disappearance of all larger (>150 mu
m) deeper dwelling species creating a pseudo-mass extinction and a
survivor assemblage of small surface dwellers and low oxygen tolerant
taxa. The K-T boundary and mass extinction was identified 40-80 cm above
the sandstone complex where all but some heterohelicids, hedbergellids
and the disaster opportunistic guembelitfids went extinct, coincident
with the evolution of first Danian species and the global delta(13)C
shift. These data reveal that sea level changes profoundly influenced
marine assemblages in near shore environments, that the Chicxulub impact
and K-T mass extinction are two separate and unrelated events, and that
the biotic effects of this impact have been vastly overestimated. (C)
2008 Elsevier B.V. All rights reserved.