Etude pedogenetique et isotopique des neoformations de calcite dans un sol sur craie: Caracteristiques et origines

Abstract

Cryoturbated facies are found at the boundary between soil horizons and Cretaceous chalk. Several types of secondary calcite appear in soil horizons: orange coloured and rounded (partially dissolved) nodules, deeply coloured angular aggregates, transparent isolated rhombs and polycrystalline nodules, needles. The carbon and oxygen isotope compositions of these calcites are correlated: δ 13 C = 4.9 δ 18 O PDB + 15.9 End members of this correlation are the orange rounded nodules ( δ 13 C − + 8%., δ 18O − −1.5‰ ) and the transparent angular polycrystalline nodules ( δ 13 C − −13; δ 18 O − −6). Partially dissolved nodules have formed under periglacial climatic conditions. Crystallisation would have occurred under the following (equilibrium) environmental conditions: δ 18 O SMOW (soil solution) −7, δ 13 C ( gaseous CO 2) − −5.2, t − −2° C. Soil solution was enriched in 18O by evaporation and atmospheric CO 2 was enriched in 13C as compared to present day. Transparent polycrystalline nodules are compatible with present day environmental conditions: δ 18 O (soil solution) ranging from −9 to −4 and δ 13 C (soil CO 2) ranging from −24.5 to −23. These nodules crystallize between May and October at soil temperatures ranging from 10 to 25°C, from evaporated soil solutions. Angular coloured aggregates may form under present day winter conditions for temperatures between 0 and 10°C. However they may also result from present accretion of fragments of periglacial nodules. All recent secondary calcite results from CO 2 degassing and evaporation of soil solutions. Degassing is controlled by the gradient of CO 2 partial pressure within the soil profile. During winter this gradient is low and the resulting calcite precipitation is not significant. During summer a large difference in pCO 2 appears between the root zone and deep soil horizons. The degassing accounts for an increase of about 2‰ in δ 13 C of the total dissolved inorganic carbon and of the related solid carbonate. Evaporation is the main driving force for secondary calcite precipitation.