| Autor: |
L. Dever, R. Durand, P. Vachier, J.Ch. Fontes |
| Rok vydání: |
1983 |
| Předmět: |
|
| Zdroj: |
Geochimica et Cosmochimica Acta. 47:2079-2090 |
| ISSN: |
0016-7037 |
| DOI: |
10.1016/0016-7037(83)90033-9 |
| Popis: |
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: δ13C = 4.9δ18OPDB + 15.9 End members of this correlation are the orange rounded nodules (δ13C − + 8%., δ18O − −1.5‰) and the transparent angular polycrystalline nodules (δ13C − −13; δ18O − −6). Partially dissolved nodules have formed under periglacial climatic conditions. Crystallisation would have occurred under the following (equilibrium) environmental conditions: δ18OSMOW (soil solution) −7, δ13C (gaseousCO2) − −5.2, t − −2°C. Soil solution was enriched in 18O by evaporation and atmospheric CO2 was enriched in 13C as compared to present day. Transparent polycrystalline nodules are compatible with present day environmental conditions: δ18O (soil solution) ranging from −9 to −4 and δ13C (soil CO2) 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 CO2 degassing and evaporation of soil solutions. Degassing is controlled by the gradient of CO2 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 pCO2 appears between the root zone and deep soil horizons. The degassing accounts for an increase of about 2‰ in δ13C of the total dissolved inorganic carbon and of the related solid carbonate. Evaporation is the main driving force for secondary calcite precipitation. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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