Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the Sr-87/Sr-86 ratio of seawater

The impact of the Deccan Traps on chemical weathering and atmospheric CO2 c onsumption on Earth is evaluated based on the study of major elements, stro ntium and Sr-87/Sr-86 isotopic ratios of the main rivers flowing through th e traps, using a numerical model which describes the coupled evolution of t he chemical cycles of carbon. alkalinity and strontium and allows one to co mpute the variations in atmospheric pCO(2), mean global temperature and the Sr-87/Sr-86 isotopic ratio of seawater, in response to Deccan trap emplace ment. The results suggest that the rate of chemical weathering of Deccan Tr aps (21-63 t/km(2)/yr) and associated atmospheric CO consumption (0.58-2.54 x 10(6) mol C/km(2)/yr) are relatively high compared to those linked to ot her basaltic regions. Our results on the Deccan and available data from oth er basaltic regions show that runoff and temperature are the two main param eters which control the rate of CO2 consumption during weathering of basalt s, according to the relationship: f = R-f x C(0)exp[-Ea/R(1/T-1/298)] where f is the specific CO2 consumption rate (mol/km(2)/yr), R-f is runoff (mm/yr), C-0 is a constant (= 1764 mu mol/l), Ea represents an apparent act ivation energy for basalt weathering (with a value of 42.3 kJ/mol determine d in the present study), R is the gas constant and T is the absolute temper ature (K-o). Modelling results show that emplacement and weathering of Decc an Traps basalts played an important role in the geochemical cycles of carb on and strontium. In particular, the traps led to a change in weathering ra te of both carbonates and silicates, in carbonate deposition on seafloor, i n Sr isotopic composition of the riverine flux and hence a change in marine Sr isotopic composition. As a result, Deccan Traps emplacement was respons ible for a strong increase of atmospheric pCO(2) by 1050 ppmv followed by a new steady-state pCO(2) lower than that in pre-Deccan times by 57 ppmv, im plying that pre-industrial atmospheric pCO(2) would have been 20% higher in the absence of Deccan basalts. pCO(2) evolution was accompanied by a rapid warming of 4 degreesC, followed after 1 Myr by a global cooling of 0.55 de greesC. During the warming phase, continental silicate weathering is increa sed globally. Since weathering of continental silicate rocks provides radio genic Sr to the ocean, the model predicts a peak in the Sr-87/Sr-86 ratio o f seawater following the Deccan Traps emplacement. The amplitude and durati on of this spike in the Sr isotopic signal are comparable to those observed at the Cretaceous-Tertiary boundary. The results of this study demonstrate the important control exerted by the emplacement and weathering of large b asaltic provinces on the geochemical and climatic changes on Earth. (C) 200 1 Elsevier Science B.V. All rights reserved.