Computer Science
Scientific paper
Dec 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983gecoa..47.2079d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 47, Issue 12, pp.2079-2090
Computer Science
Scientific paper
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: 13 C = 4.9 18 O PDB + 15.9 End members of this correlation are the orange rounded nodules ( 13 C - + 8%., 18 O - -1.5 ) and the transparent angular polycrystalline nodules ( 13 C - -13; 18 O - -6). Partially dissolved nodules have formed under periglacial climatic conditions. Crystallisation would have occurred under the following (equilibrium) environmental conditions: 18 O SMOW (soil solution) -7, 13 C ( gaseous CO 2 ) - -5.2, t - -2° C . Soil solution was enriched in 18 O by evaporation and atmospheric CO 2 was enriched in 13 C as compared to present day. Transparent polycrystalline nodules are compatible with present day environmental conditions: 18 O (soil solution) ranging from -9 to -4 and 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 p CO 2 appears between the root zone and deep soil horizons. The degassing accounts for an increase of about 2 in 13 C of the total dissolved inorganic carbon and of the related solid carbonate. Evaporation is the main driving force for secondary calcite precipitation.
Dever L.
Durand R.
Fontes Jean-Charles
Vachier P.
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