n° 396 - September 2001

One explanation for dinosaur extinction around the Cretaceous-Tertiary boundary 65 million years ago implicates a volcanic eruption in India whose dust and CO₂ emissions altered atmospheric chemistry and climate.

CNRS researchers from the "Laboratoire de Mécanismes de Transfert en Géologie" (LMTG, Geological Transfer Mechanisms Laboratory) in collaboration with the "Institut de Physique du Globe de Paris"(Paris Geophysical Institute) and the "Laboratoire de Physique Atmosphérique et Planétaire de Liège" (Laboratory for Planetary and Atmospheric Physics of the University of Liège) used a numerical model to show that thick basalt lavas, the Deccan Traps, copiously deposited in northwest India at that time, caused such changes. Results indicate that for over one million years, a balance was regulated by a source and sink system, respectively volcanic outgassing and silicate chemical weathering, acting as a CO₂-consuming pump.

Basalts are the silicate rocks most susceptible to weathering and the Deccan Traps have shrunk by two-thirds in 65 million years. Researchers derived a law for CO₂ consumption in basalt weathering from data taken from rivers draining the Deccan Traps and other basaltic regions. Runoff and atmospheric temperature were the main factors controlling soil and rock
solubility and hence CO₂ consumption rate.

The model predicted oceanic carbon and strontium cycle variations. The emplacement of the Deccan Traps was responsible for a strong increase of atmospheric CO₂ by 1050 ppmv (triple the present rate) and a global temperature increase of 4°C. Subsequent continental weathering over 1.5 million years was enough for excess atmospheric to be reabsorbed, with consequent global cooling of 0.55 °C. Moreover, this important CO2 outgassing caused a sharp decrease in carbonate sedimentation (-45% in only 20,000 years) via a strong sea-water acidification.
This pause is observed in marine carbonates at the Cretaceous/Tertiary boundary.

This CO₂ peak and global warming support the idea that, whatever caused dinosaur extinction, volcanic eruption and the formation of the Deccan Traps greatly amplified that crisis.