Abstract: The K-T Boundary Bolide Impact vs. Global Biotic Effects

Gerta Keller

Recent developments in Cretaceous/Tertiary (K/T) boundary studies indicate that (1) the mass extinction associated with this event is not the result of a single catastrophe, (2) extinctions occurred over an extended time period and were selective rather than random within organismal groups as well as between different groups, and (3) the biotic effects were most severe and sometimes limited to tropical-subtropicaI regions, while high-latitude faunas and floras escaped virtually unscathed. The growing global body of high-resolution faunal and floral data upon which these observations are based now provide the empirical database and the opportunity to test various K/T catastrophe scenarios, whether volcanic or bolide-impact based. All scientists agree that a major environmental change o curred across the K/T boundary; they disagree on the cause, whether impact or volcanism, and on the biotic effects of this environmental change. Here, I will address the latter. Ultimately, it is the biotic consequences that determine how significant a bolide impact or massive volcanism at K/T boundary time was for life on Earth.

Comparison of stable isotope records and species extinction and evolution patterns of plantic foraminifera across the K/T boundary in the most complete high- and low-latitude sections (El Kef, Agost, Caravaca, Brazos River, El MimbraI, Nye Klov, and ODP Site 738) reveals major differences in extinction patterns. No significant species extinctions or major faunal abundance changes coincide with the K/T boundary in high latitudes, and nearly all Cretaceous species survived and thrived well into the early Tertiary. The same lack of species extinctions has been observed in siliceous microfossils, palynomorphs, and invertebrate faunas. In contrast major species extinctions occurred in low latitudes in some organismal groups (e.g., planktic foraminifera) where two-thirds of the Cretaceous t xa disappeared at or below the K/T boundary. But only tropical and subtropical, large, complex, and ornamented taxa disappeared. Stable isotopic data indicate that these taxa were predominantly deeper water dwellers living at or below the thermocline. Only surface dwellers survived. All of the disappearing taxa were already rare, endangered species in the late Maastrichtian ocean, comprising only between 5 and 17% of the planktic foraminiferal population. Stable isotope data mirror this high-low latitude extinction pattern, showing a major drop in surface productivity in low latitudes, but only minor or no carbon-13 changes in high latitudes. Although it is possible that the extinction of some or all of these taxa was related to the climatic changes associated with a K/T boundary bolide mpact, a direct cause-and-effect relationship can not be demonstrated.

Neither the current impact, volcanism, or gradual environmental change scenarios alone can explain these high- and low-latitude differences in species extinction and isotope records. Any successful scenario will have to account for the fact that extended and selective extinction patterns are present in various organismal groups, that these extinction patterns are restricted to low latitudes, and that no significant extinctions occurred in any organismal groups in high latitudes. These observations may be best explained by a multiple-cause scenario including long-term climatic and environmental changes accelerated by a catastrophic event at K/T boundary time.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994