Basaltic volcanism and mass extinction at the Permo-Triassic boundary: Environmental impact and modeling of the global carbon cycle

in Earth And Planetary Science Letters (2005), 234(1-2), 207-221

The Siberian Traps represent one of the most voluminous continental flood basalt provinces on Earth. The mass extinction at the end of the Permian was the most severe in the history of life. In the ... [more ▼]

The Siberian Traps represent one of the most voluminous continental flood basalt provinces on Earth. The mass extinction at the end of the Permian was the most severe in the history of life. In the present paper, these two major concurrent events that occurred are analysed and a geochemical model coupled with an energy balance model is used to calculate their environmental impact on atmospheric CO2, oceanic delta(13)C, and marine anoxia. The latitudinal temperature gradient is reduced relative to today, resulting in warmer temperatures at high latitudes. The warmer climate and the presence of fresh basaltic provinces increase the weatherability of the continental surfaces, resulting in an enhanced consumption of atmospheric CO2 through weathering. First, the eruption of the Siberian traps is accompanied by a massive volume of C-13 depleted CO2 degassed from the mantle and added to the ocean through silicate weathering, thus lowering marine delta(13)C. Second, the rapid collapse in productivity induces a strong decrease in the global organic carbon burial. This too tends to increase the proportion of light carbon in the ocean. These two effects can explain the low delta(13)C values across the PT boundary, and methane release need not be invoked to explain the delta(13)C fluctuations. It is proposed that the phosphorus cycle, which drives primary production in the model, plays an important role on the recovery of productivity and the delta(13)C variations. (c) 2005 Elsevier B.V All rights reserved. [less ▲]

Basalt weathering laws and the impact of basalt weathering on the global carbon cycle

in Chemical Geology (2003), 202(3-4), 257-273

This study attempts to characterise the chemical weathering of basalts and to quantify the flux of carbon transferred from the atmosphere to the ocean during this major process at the surface of the Earth ... [more ▼]

This study attempts to characterise the chemical weathering of basalts and to quantify the flux of carbon transferred from the atmosphere to the ocean during this major process at the surface of the Earth. To this aim, we have compiled different published chemical compositions of small rivers draining basalts. Basaltic river waters are characterised by relatively high Na-normalized molar ratios (Ca/Na: 0.2-3.9; HCO3/Na: 1-10; Mg/Na: 0.15-6) in comparison with those usually observed for river draining silicates. The data also show the climatic influence on basalt weathering and associated CO2 consumption. Runoff and temperature are the main parameters controlling the chemical weathering rate and derived CO2 consumption during basaltic weathering. From these relationships and digital maps, we are able to define the contribution of basalts to the global silicate flux. Taking account of this result, we estimate that the CO2 flux consumed by chemical weathering of basalts is about 4.08 x 10(12) mol/year. The fluxes from the islands of Indonesia and regions of central America represent around 40% of this flux. The global flux of CO2 consumed by chemical weathering of basalts represents between 30% and 35% of the flux derived from continental silicate determined by Gaillardet et al. [Chem. Geol. 159 (1999) 3]. Finally, it appears that volcanic activity not only acts as a major atmospheric CO2 source, but also creates strong CO2 sinks that cannot be neglected to better understand the geochemical and climatic evolution of the Earth. (C) 2003 Elsevier B.V. All rights reserved. [less ▲]

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

in Earth And Planetary Science Letters (2001), 188(3-4), 459-474

The impact of the Deccan Traps on chemical weathering and atmospheric CO2 consumption on Earth is evaluated based on the study of major elements, strontium and Sr-87/Sr-86 isotopic ratios of the main ... [more ▼]

The impact of the Deccan Traps on chemical weathering and atmospheric CO2 consumption on Earth is evaluated based on the study of major elements, strontium and Sr-87/Sr-86 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 Sr-87/Sr-86 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 consumption (0.58-2.54 x 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 CO2 consumption during weathering of basalts, 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 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-o). 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 degreesC, followed after 1 Myr by a global cooling of 0.55 degreesC. 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 Sr-87/Sr-86 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. (C) 2001 Elsevier Science B.V. All rights reserved. [less ▲]