Direct measurements of chemical composition of shock-induced gases from calcite: an intense global warming after the Chicxulub impact due to the indirect greenhouse effect of carbon monoxide
Introduction
Shock waves generated by hypervelocity impacts of large asteroids or comets on planets induce devolatilization of the impactors and crustal materials, and thereby a large amount of gases are released into the atmosphere. Since the released gases, such as H2O, SO2, and CO2, significantly may affect the global environment including the biosphere through the global warming (e.g., O'Keefe and Ahrens, 1989), sunlight shielding (e.g., Sigurdsson et al., 1992, Pope et al., 1994), and acid rain (e.g., Pope et al., 1994), the shock-induced devolatilization has been considered to play important roles in the atmospheric evolution and mass extinctions in Earth's history. In especially, it is now widely accepted that the Cretaceous–Paleogene (K–P) extinction at 65 Ma is caused by the Chicxulub impact event. Thus, studying the shock-induced devolatilization is critical for understanding the mechanisms responsible for the mass extinction.
Here we focused on hypervelocity impacts on carbonate rocks which occupy 15–20% by volume of Earth's sedimentary rocks. Since carbonate rocks are distributed widely on the surface of the Earth, the Earth has experienced a large number of impacts on carbonate rocks represented by the Chicxulub impact (Grieve and Robertson, 1979, Hildebrand et al., 1991). Carbonate rocks are thermodynamically unstable at high temperatures induced by shock heating (Tyburczy and Ahrens, 1986) and are devolatilized to form carbon-bearing gas species in the impacts (Boslough et al., 1982). Previous studies have discussed the prolonged global warming caused by injection of shock-induced CO2 from carbonates by assuming the following decomposition reaction;CaCO3 → CaO + CO2(e.g., O'Keefe and Ahrens, 1989, Pierazzo et al., 1998, Ivanov et al., 1996). In the Chicxulub impact, the increase in surface temperature due to the greenhouse effect of shock-induced CO2 has been estimated to be ~ 1–2 °C (Pope et al., 1997, Pierazzo et al., 1998).
In this study, we examine the importance of production of CO in shock-induced devolatilization of carbonate rocks. Considering the thermodynamic stability of CO among carbon-bearing gases at high-temperature conditions (e.g., Kress and McKay, 2004), CO would be produced by the devolatilization of carbonate rocks via the following reaction as well as CO2;CaCO3 → CaO + CO + O.
If CO is released into the atmosphere, the abundances of CH4 and tropospheric O3 will increase through photochemical reactions in the troposphere (e.g., Intergovernmental Panel on Climate Change (IPCC), 2007). Consequently, CO has more intense indirect radiative forcing than CO2 owing to strong greenhouse effect of CH4 and tropospheric O3 (Daniel and Solomon, 1998), although CO itself has little direct radiative forcing. Therefore, the injection of shock-induced CO can rise the surface temperature higher than the previous estimates considering CO2 only.
Despite such importance in the post-impact environmental effect, the chemical composition (i.e., CO/CO2 ratio) of the gas species produced by devolatilization of carbonate rocks has been poorly constrained by laboratory experiments. This is because direct measurements of shock-induced gases have been difficult in previous studies using single-stage powder guns and 2-stage light-gas guns. In the previous studies, gun debris and CO2-rich combustion gases flowing into a target chamber may have mixed with the shock-induced gases (Tyburczy and Ahrens, 1986). Boslough et al. (1982) investigated the chemical composition of the shock-induced gases from calcite using a gas-recovery system with a liquid nitrogen trap. Although they have showed that CO may be dominant in the shock-induced gases at shock pressure of ~ 19 GPa, the chemical composition of the shock-induced gases has not been investigated systematically. Since the chemical composition of shock-induced gases from carbonate rocks has been uncertain, it has been difficult to assess the possible environmental influence of large impacts on carbonate targets that have occurred on the Earth.
In this study, we conduct direct measurements of the chemical composition of shock-induced gases from calcite using both a laser gun that is free from gun debris and CO2-rich combustion gases (Ohno et al., 2008) and an isotopic labeling technique. First, we describe the experimental system of the shock-induced devolatilization using a laser gun in Section 2. The chemical composition of the shock-induced gases as a function of peak shock pressure is shown in Section 3. Based on our experimental data, we calculate radiative forcings of shock-induced gases produced by the Chicxulub impact using a tropospheric one-box photochemical model and discuss the environmental effect in Section 5.
Section snippets
Experimental
We conduct shock-induced devolatilization experiments with a laser gun described by Ohno et al. (2008) and use isotopic-labeled calcite (Ca13CO3) samples as the targets for identification of the shock-induced gases. In this section, we first describe the principle and experimental conditions of the laser gun method. Then, the chemical property and preparation of the target are discussed. Finally we describe our experimental procedure.
A schematic diagram of the experimental system is shown in
Detection of shock-induced 13CO and 13CO2 from calcite
Fig. 2 shows the time variations of QMS signals of m/z = 4 (He), 29 (13CO), and 45 (13CO2) in the shot onto the 13C-labeled calcite. The signal intensities of m/z = 29 and 45 increased rapidly by 2 to 3 orders of magnitude immediately after the impact, while that of m/z = 4 did not change significantly during the experimental run. These results indicate that the gas species of m/z = 29 and 45 are produced by the impact. Fig. 2 also shows the results of a blank shot with dotted lines. In the blank shot,
Mechanism for the CO production
Considering the thermodynamical stability of CO, a high temperature in target materials is required for the production of shock-induced gases with CO/CO2 ~ 2. Using the Gibbs free energy minimization method, we estimate the equilibrium temperature of gas species required for the stable existence of comparable amount of CO with that of CO2. The results show that the temperature of gas species becomes more than 6000 K in the range of peak shock pressure in our experiments. Such a high temperature
Implications for the environmental change after the Chicxulub impact
For evaluating the importance of the injection of CO gas on the environmental change after the Chicxulub impact, we estimate the radiative forcing of the shock-induced CO by incorporating our experimental data into a simple tropospheric photochemical model. As described in Section 1, the presence of CO in the troposphere enhances the abundances of CH4 and O3. If the abundance of CO increases in the troposphere, it consumes OH radical and thereby inhibits the loss of CH4 through the reaction
Conclusion
We investigated the chemical composition (CO/CO2) of shock-induced gas species from calcite in order to estimate the increase in surface temperature after the Chicxulub impact. Based on gas analyses of the direct measurements of shock-induced gases using both the laser gun method and isotope labeling technique, we have shown that gaseous CO as much as ~ 2 times that of CO2 is produced by shock-induced devolatilization of calcite. These experimental results suggest that gaseous CO has been
Acknowledgements
K.K. would like to thank K. Saiki of the Univ. of Tokyo who made him use the XPS and provided useful comments. Y.S. would like to thank N.O. Ogawa and N. Ohkouchi of the JAMSTEC for advice on pre-treatment technique of metal foil for avoiding organic contaminations. This research was partly supported by the Grand in Aide from the Japan Society for the Promotion of Science.
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