Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry
Résumé
CO$_2$ exchange between terrestrial ecosystems and the atmosphere is key to understanding the feedbacks between climate change and the land surface. In regions with carbonaceous parent material, CO$_2$ exchange patterns occur that cannot be explained by biological processes, such as disproportionate outgassing during the daytime or night-time CO$_2$ uptake during periods when all vegetation is senescent. Neither of these phenomena can be attributed to carbonate weathering reactions, since their CO$_2$ exchange rates are too small. Soil ventilation induced by high atmospheric turbulence is found to explain atypical CO$_2$ exchange between carbonaceous systems and the atmosphere. However, by strongly altering subsurface CO2 concentrations, ventilation can be expected to influence carbonate weathering rates. By imposing ventilation-driven CO$_2$ outgassing in a carbonate weathering model, we show here that carbonate geochemistry is accelerated and does play a surprisingly large role in the observed CO$_2$ exchange pattern of a semi-arid ecosystem. We found that by rapidly depleting soil CO$_2$2 during the daytime, ventilation disturbs soil carbonate equilibria and therefore strongly magnifies daytime carbonate precipitation and associated CO$_2$ production. At night, ventilation ceases and the depleted CO$_2$ concentrations increase steadily. Dissolution of carbonate is now enhanced, which consumes CO$_2$ and largely compensates for the enhanced daytime carbonate precipitation. This is why only a relatively small effect on global carbonate weathering rates is to be expected. On the short term, however, ventilation has a drastic effect on synoptic carbonate weathering rates, resulting in a pronounced diel pattern that exacerbates the non-biological behavior of soil-atmosphere CO$_2$ exchanges in dry regions with carbonate soils.
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