Geochimie et teneurs isotopiques des systèmes saisonniers de dissolution de la calcite dans un sol sur craie

Abstract

In a soil developed on the Cretaceous chalk of the Eastern Paris basin, calcite dissolution begins at the surface. The soil water is rapidly saturated in calcite. Calcite dissolution follows two different pathways according to seasonal pedoclimatic conditions. During winter: the soil is only partly saturated in water and the CO 2 partial pressure is low (Ca 10 −3 atm.). As a consequence total inorganic dissolved carbon (TIDC) is a hundred times the carbon content of the gaseous phase. Equilibrium is usually observed between the two phases. It is a closed system. The measured carbon 14 activity (87,5%) and 13C content ( δ tidc 13C = −12,2%0 ) of the drainage water are very close to theoretical values calculated for an ideal mixing system between gaseous and mineral phases (respectively characterized by the following isotopic values: δ G 13C = −21,5%0 ; A G 14C = 118% ; δ M 13C = +2,9%0 ; A M 14C = 28% ). During spring and summer: the soil moisture decreases, the input of biogenic CO 2 induces an increase of the soil CO 2 partial pressure (Ca from 3.10 −3 atm to 7.10 −3 atm). The carbon content of the gaseous phase is higher by an order of magnitude compared to winter conditions. Therefore the aqueous phase is undersaturated in CO 2 with respect to the latter. This disequilibrium occurs as a result of unbalanced rates of CO 2 dissolution and CO 2 effusion toward atmosphère. It is an open system. The carbon isotopic ratio of the aqueous phase is regulated by that of the gaseous phase, as demonstrated by the agreement between measured and calculated isotopic compositions (respectively δ L mes = from −9,4%0 to −11,5%0, δ l calc = from −9,8%0 to −13,9%0 A L mes = 119%, A L calc = from 119% to 125%). The solutions originating from both systems (open and closed) move downwards without significant mixing together. It has also been observed that no significant variation of the TIDC isotopic composition occurs during precipitation of secondary calcite.