Back to Science Version Page
McLean, D. M., 1982, abstract in Russell, D. A., and Rice, G., eds., K-Tec II Cretaceous-Tertiary Extinctions and Possible Terrestrial and Extraterrestrial Causes: Syllogeus Series 39, National Museums of Canada (Proceedings, May 1981 workshop), p. 143-144.
Deccan Volcanism and the Cretaceous-Tertiary Transition Scenario: A Unifying Causal Mechanism
Dewey M. McLean
The Cretaceous-Tertiary (K-T ) transition Deccan flood basalt volcanism, one of the greatest outpourings of lavas from the Earth in geological history, was synchronous with, and accounts for via fluctuation of the carbon cycle, the K-T transition: (1) marine CaCO3 dissolution and "clay" events, and the apparently linked extinctions of calcareous microplankton, (2) drops in d13C and d18O values, and (3) via "greenhouse" conditions, the extinctions of the dinosaurs by heat-infertility linkage. Theorized to represent mantle plume activity originating near Earth's core-mantle boundary, this volcanism provides a mechanism for conveying iridium and osmium from the core (where they presumably exist in cosmical proportions) to be distributed via volcanic exhalations. Carbon gases are among the most abundant volcanic emanations, and possibly reflect degassing of Earth's interior of material trapped during Earth's accretion. Deep origin volcanism spanning the 5 million years of the Deccan volcanism (65 to 60 Myr ago) would have brought vast amounts of carbon gases to Earth's surface. Volcanic CO2 and reduced carbon in igneous rocks have low d13C values (7 and 19 to 28, respectively), accounting for the drop in d13C values across the K-T contact. Build up of atmospheric CO2 would result in "greenhouse" conditions and the drop in d18O values (a warming signal) across the K-T contact. Acidic volcanic gases injected into the oceans would cause CaCO3 dissolution; the holding of CaCO3 components in solution would allow accumulation of a concentrated clay layer (e.g. the Danish fish clay) on the Cretaceous erosional (corrosional) surface. Lowered marine pH would also trigger calcareous microplankton extinctions; they were most severe in the Tethyan region in proximity to the Deccan volcanism. "Greenhouse" conditions would have affected reptilian reproduction according to size. Larger organisms have relatively small surface-volume ratios; during warming, organisms above a critical size (10 kilograms) would have retained excessive body heat, causing degeneration of internal germinal tissues in males and, in females, disruption of calcium metabolism and hormonal systems. The abrupt terminations of geological ranges of many taxa at the K-T contact seemingly reflect hiatus control of ranges associated with the shallow marine CaCO3 dissolution event, and terrestrial hiatuses and facies control associated with the terminal Cretaceous regression of epeiric seas off the continents, that have created the illusions of simultaneous global range terminations. In all likelihood, the K-T extinctions spanned hundreds of thousands, if not millions, of years.