Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

Abstract. CO₂ 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, CO₂ exchange patterns occur that cannot be explained by biological processes, such as disproportionate outgassing during the daytime or nighttime CO₂ uptake during periods when all vegetation is senescent. Neither of these phenomena can be attributed to carbonate weathering reactions, since their CO₂ exchange rates are too small. Soil ventilation induced by high atmospheric turbulence is found to explain atypical CO₂ 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 CO₂ outgassing in a carbonate weathering model, we show here that carbonate geochemistry is accelerated and does play a surprisingly large role in the observed CO₂ exchange pattern of a semi-arid ecosystem. We found that by rapidly depleting soil CO₂ during the daytime, ventilation disturbs soil carbonate equilibria and therefore strongly magnifies daytime carbonate precipitation and associated CO₂ production. At night, ventilation ceases and the depleted CO₂ concentrations increase steadily. Dissolution of carbonate is now enhanced, which consumes CO₂ 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 CO₂ exchanges in dry regions \mbox{with carbonate soils}.