Reconstruction of the eruptive sequence of the Deccan Traps, India : climatic and environmental consequences

In this thesis, we attempt to constrain the date, timing and duration of flood basalt volcanism in the particular case of the Deccan traps of India, and to model the climatic impact of such massive volcanism. With this goal in mind, we have sampled 12 new sections of the traps along 300km (from North to South) of the Western Ghat escarpment. Sections were selected based on previously available petrologic, geochemical and volcanological data. Using the paleomagnetic analysis we have been able to recognize groups of statistically identical directions in a number of flows, either superimposed in the same section, or laterally separated by up to 130km. We hypothesize, based on the characteristics of recent secular variation and the typical sizes of the 95% confidence intervals of the remanence directions, that these directional groups represent mega-flows or cooling units that cooled within a period of decades to at most a century. We estimate that the total amount of time represented by the flows could be less than 10,000 years, and that most of the time (400 to 500. 000 years) is represented by less than 10 thick bole levels. Therefore we have established an absolute age undistinguishable from that of the Cretaceous-Tertiary boundary (although this boundary cannot yet be precisely located within the section) at 64. 7±0. 6Ma. In order to start modeling the climatic impact of such flood basalt sequences, we have first used as a “building block” the 1783 fissure eruption at Laki in Iceland (one of the largest historical basaltic eruptions). Using the LMDZ-INCA model, we replicate the observed (semi-global) dispersal over much of northern hemisphere of the unusual haze that was observed at the time and are able to understand the spread and intensity of the mortality crisis which ensued. A single Deccan pulse could have reached 10 times the flux of Laki, and for up to 100 times longer. And there were tens of pulses suggesting the causal consequences of the climatic impact of flood basalts regarding mass extinctions.