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

摘要

The impact of the Deccan Traps on chemical weathering and atmospheric CO_2 consumption on Earth is evaluated based on the study of major elements, strontium and ~(87)Sr/~(86)Sr isotopic ratios of the main rivers flowing through the traps, using a numerical model which describes the coupled evolution of the chemical cycles of carbon, alkalinity and strontium and allows one to compute the variations in atmospheric pCO_2, mean global temperature and the ~(87)Sr/~(86)Sr isotopic ratio of seawater, in response to Deccan trap emplacement. The results suggest that the rate of chemical weathering of Deccan Traps (21-63 t/km~2/yr) and associated atmospheric CO_2 consumption (0.58-2.54 * 10~6 mol C/km~2/yr) are relatively high compared to those linked to other basaltic regions. Our results on the Deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of CO_2 consumption during weathering of basalts, according to the relationship: f = R_f * C_0exp [-Ea/R(1/T-1/298)] where f is the specific CO_2 consumption rate (mol/km~2/yr), R_f is runoff (mm/yr), C_0 is a constant (= 1764 μmol/l), Ea represents an apparent activation energy for basalt weathering (with a value of 42.3 kJ/mol determined in the present study). R is the gas constant and T is the absolute temperature (°K). Modelling results show that emplacement and weathering of Deccan Traps basalts played an important role in the geochemical cycles of carbon and strontium. In particular, the traps led to a change in weathering rate of both carbonates and silicates, in carbonate deposition on seafloor, in Sr isotopic composition of the riverine flux and hence a change in marine Sr isotopic composition. As a result, Deccan Traps emplacement was responsible 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, implying 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 ℃, followed after 1 Myr by a global cooling of 0.55 ℃. During the warming phase, continental silicate weathering is increased globally. Since weathering of continental silicate rocks provides radiogenic Sr to the ocean, the model predicts a peak in the ~(87)Sr/~(86)Sr ratio of seawater following the Deccan Traps emplacement. The amplitude and duration 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 basaltic provinces on the geochemical and climatic changes on Earth.