Fast back-reactions of shock-released CO2 from carbonates:: An experimental approach

This work aims at investigating the processes leading to the liberation of CO2 and SO2/SO3 in the atmosphere after large meteorite impacts into sediments. Firstly, we review reactions and thermodynamic conditions to produce CO2 from carbonates and SO2/SO3 from sulfates. We show that decomposition of the carbonates and sulfates only occurs during shock pressure release. Secondly, we examine mineralogical and chemical data of natural impact breccias where pure CaO (lime) is always lacking and where secondary carbonates and probably sulfates occur abundant. This observation evidences the importance of back-reactions of CO2 and SO2/SO3 with the initially produced CaO. Third, we explore the kinetics and thermodynamics of the reactions involving CaO and CO2. We have performed 32 degassing and back-reaction experiments with fine-grained. chemically precipitated calcite, and with coarse-grained natural calcite, dolomite, and magnesite. Experiments with calcite confirm that residual CaO is highly reactive in the presence of CO2 in the 573-973 K interval: within less than 200 s, some 40 to 80% of CaO has back-reacted into CaCO3. These high reaction rates suggest that much of the impact produced CO2, may be highly transient. Scanning electron microscope observations show that these high reaction rates are enhanced by the exceptionally porous structure of the residual CaO. The kinetics of the CaO + CO2 reaction are explained by a gas-solid reaction model, in which the reaction rates are controlled by gas mass transfer through the porous CaO. the CO2-CaO surface interactions, and the diffusion of CO2 through CaCO3. Similar experiments conducted with dolomite and magnesite show that residual Mg-oxides do not react significantly at the 1000 s time scale and may, therefore, survive as witness of degassing in impact breccias. Published kinetic modeling of SO2/SO3 back-reactions with hot CaO to CaSO4 indicates typical conversion rates of around 50% after 1200 s. Hence back-reactions play also a crucial role in limiting the total amount of sulfur oxides released by an impact event into the Earth's atmosphere and stratosphere. At low temperatures, residual CaO should react with water to yield Ca(OH)(2) (another very efficient CO2 pump), or dissolve in natural waters strongly increasing the pH. This pH effect is, globally compensated by the acid species (H2CO3, H2SO4) produced from liberated CO2 and SO2/SO3. Our experimental data, and the assessment of existing literature indicate that the amount of chemically active gases that have been released into the atmosphere by the Chicxulub impact event are most likely overestimated. Copyright (C) 2001 Elsevier Science Ltd.