ABSTRACT: Cretaceous/Tertiary Boundary Impact Hypothesis and the Fossil Record

KELLER, GERTA, Princeton University, Princeton, NJ

Can the fossil record support a Cretaceous/Tertiary (K/T) boundary bolide impact hypothesis that calls for the sudden and catastrophic destruction of nearly all life on Earth? Can the fossil record support a hypothesis of multiple bolide impacts that led to the destruction of nearly all life on Earth over a short time span? The answer is no to both questions based on painstaking centimeter-scale studies of microfossils in numerous marine sections.

The K/T boundary event did not cause the sudden extinction of nearly all planktic foraminiferal species as commonly claimed in support of the presumed devastating effects of a large bolide impact; nor did it result in global catastrophic extinctions in other microfossil groups including nannofossils, radiolaria, diatoms, dinoflagellates, pollen, and spores. In most microplankton groups the fossil records show a significant decline in species diversity during the latest Maastrichtian followed by an additional and generally more sudden decline in diversity at the K/T boundary. Even among planktic foraminifera, the microfossil group believed to have been most severely affected by the K/T catastrophe, about one-third of the species survived into the Tertiary. An attempt has been made to e plain this extended pattern of species extinctions by multiple impacts (comet shower) over a short time period. However, despite high resolution studies of numerous sections worldwide there is neither geochemical (Ir, microtektites), mineralogical (shocked quartz), or fossil evidence to support this hypothesis.

The pattern of species extinctions among microplankton is complex, beginning during the final 100,000 to 300,000 years of the Cretaceous, accelerating across the K/T boundary, and reaching maximum negative conditions a few tens of thousands of years into the Tertiary accompanied by low surface productivity. Return to a more stable ecosystem and increased surface productivity does not occur until about 250,000 to 350,000 years after the K/T boundary. Within this transition interval habitats of deep and intermediate dwelling planktic foraminiferal species are selectively eliminated and by K/T boundary time only surface dwellers survive. This implies the disruption of the water-mass stratification, change in thermocline,

and lowered surface productivity. No single cause is likely to account for these prolonged and dramatic faunal and environmental changes. However, the onset of this faunal turnover appears to be related to the latest Maastrichtian climatic cooling and sea level regression that was followed by a rapid transgression across the K/T boundary and probably expansion of the oxygen minimum zone through the earliest Tertiary. Other environmental changes, including volcanism and/or a bolide impact, may have accelerated the demise of a Cretaceous fauna already on decline. The microfossil record unequivocally shows that a large bolide impact does not have to lead to the catastrophic environmental consequences hypothesized by the K/T boundary impact theory.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)