CHICXULUB IMPACT AND MASS EXTINCTION

The Lilliput effect marks morphologic and intraspecies size reductions in response to environmental stresses commonly associated with the aftermath of mass extinctions. This study shows that the Lilliput effect is a universal biotic response associated with greenhouse warming, mesotrophic or restricted basins, shallow marginal settings and volcanically active regions during the late Maastrichtian. Sedimentary sequences analyzed from Tunisia, Egypt, Texas, Argentina, the South Atlantic and Indian Ocean reveal that the biotic stress response appears uniform, regardless of the cause, varying only with the degree of biotic stress. Overall, late Maastrichtian environments span a continuum from optimum conditions to the catastrophic (mass extinctions) with a predictable set of biotic responses relative to the degree of stress induced by oxygen, salinity, temperature and nutrient variations as a result of climate and sea level changes and volcanism. Early stages of biotic stress result in diversity reduction and the elimination of large specialized species (k-strategists) leading to morphologic size reduction via selective extinction/disappearances and intraspecies dwarfing of survivors. Later stages of biotic stress result in the complete disappearance of kstrategists, intraspecies dwarfing of r-strategists and dominance by low oxygen tolerant small heterohelicids. At the extreme end of the biotic response are volcanically influenced environments, which cause the same detrimental biotic effects as observed in the aftermath of the K–T mass extinction, including the disappearance of most species and blooms of the disaster opportunist Guembelitria.  PDF

Impact stratigraphy is an extremely useful correlation tool that makes use of unique events in Earth's history and places them within spatial and temporal contexts. The K-T boundary is a particularly apt example to test the limits of this method to resolve ongoing controversies over the age of the Chicxulub impact and whether this impact is indeed responsible for the K-T boundary mass extinction. Two impact markers, the Ir anomaly and the Chicxulub impact spherule deposits, are ideal because of their widespread presence. Evaluation of their stratigraphic occurrences reveals the potential and the complexities inherent in using these impact signals. For example, in the most expanded sedimentary sequences: (1) The K-T Ir anomaly never contains Chicxulub impact spherules, whereas the Chicxulub impact spherule layer never contains an Ir anomaly. (2) The separation of up to 9 m between the Ir anomaly and spherule layer cannot be explained by differential settling, tsunamis, or slumps. (3) The presence of multiple spherule layers with the same glass geochemistry as melt rock in the impact breccia of the Chicxulub crater indicates erosion and redeposition of the original spherule ejecta layer. (4) The stratigraphically oldest spherule layer is in undisturbed upper Maastrichtian sediments (zone CF1) in NE Mexico and Texas. (5) From central Mexico to Guatemala, Belize, Haiti, and Cuba, a major K-T hiatus is present and spherule deposits are reworked and redeposited in early Danian (zone P1a) sediments. (6) A second Ir anomaly of cosmic origin is present in the early Danian. This shows that although impact markers represent an instant in time, they are subject to the same geological forces as any other marker horizons—erosion, reworking, and redeposition—and must be used with caution and applied on a regional scale to avoid artifacts of redeposition. For the K-T transition, impact stratigraphy unequivocally indicates that the Chicxulub impact predates the K-T boundary, that the Ir anomaly at the K-T boundary is not related to the Chicxulub impact, and that environmental upheaval continued during the early Danian with possibly another smaller impact and volcanism.  PDF

Cretaceous volcanic activities (LIPs and CFBPs) appear to have had relatively minor biotic effects, at least at the generic level. Major biotic stress during the Cretaceous was associated with OAEs and related to nutrient availability largely from weathering, greenhouse warming, drowning of platform areas, and volcanism. The biotic effects of OAEs were often dramatic at the species level, causing the extinction of larger specialized and heavily calcified planktonic foraminifera (rotaliporid extinction) and nannoconids (nannoconid crises), the temporary disappearances of other larger species, and the rapid increase in r-selected small and thin-walled species, such as the low oxygen tolerant heterohelicids and radially elongated taxa among planktic foraminifera and thin walled nannofossils. Biotic diversity increased during cool climates, particularly during the late Campanian and Maastrichtian, reaching maximum diversity during the middle Maastrichtian. High biotic stress conditions began during greenhouse warming and Deccan volcanism about 400 ky before the K-T boundary; it reduced abundances of large specialized tropical planktic foraminiferal species and endangered their survival. By K-T time, renewed Deccan volcanism combined with a large impact probably triggered the demise of this already extinction prone species group.

Evidence from NE Mexico, Texas, and the Chicxulub crater itself indicates that this 170 km-diameter crater predates the K-T boundary by ∼300,000 years and caused no species extinctions. The Chicxulub impact, therefore, can no longer be considered a direct cause for the K-T mass extinction. However, the K-T mass extinction is closely associated with a global Ir anomaly, which is considered too large, too widespread, and too concentrated in a thin layer to have originated from volcanic activity, leaving another large impact as the most likely source. This suggests that a second still unknown larger impact may have triggered the K-T mass extinction.  PDF