Bilans élémentaires holocènes et altération des minéraux dans trois sols forestiers du Québec méridional

Abstract

<jats:title>Résumé</jats:title><jats:p>Nous avons calculé les flux élémentaires holocènes et le taux d’altération historique dans trois sols forestiers par la méthode du bilan de masse de Brimhall<jats:italic>et al</jats:italic>. (1991 a, b). Les sols (Hermine : HER ; Rouyn-1 : RN1 ; Rouyn-4 : RN4) se distinguent par leur matériel d’origine et leur niveau d’exposition aux dépôts atmosphériques de métaux traces. L’indice de changement volumétrique (ICV) calculé avec Zr est positif (expansion) dans RN1 et dans les horizons B de HER et est positivement corrélé à la concentration en C organique et en oxydes de Al et Fe. L’analyse des profils permet de dégager trois groupes d’éléments. Un premier groupe (Ca, K, Mg, Mn, Na, Si) montre des concentrations qui diminuent de l’horizon C vers la surface du profil en réponse à l’intensification de l’altération des minéraux. Le second groupe (Al, Fe, P, Ti, Cr, Ni) présente des zones d’enrichissement dans les horizons B et des pertes dans le Ae. Ce patron mixte reflète l’effet combiné de la formation d’oxydes (Al, Fe, Ti) et de la rétention des éléments par ces oxydes (P, Cr, Ni). Le troisième groupe (Cd, Cu, Pb, Zn) affiche des flux fortement positifs dans les horizons de surface en réponse à la contamination atmosphérique des sols et à la bioconcentration des métaux traces. Les taux d’altération holocènes moyens des éléments majeurs correspondent très bien aux taux publiés pour une variété de sols forestiers de milieux comparables. Le classement des éléments en ordre de mobilité relative décroissante est : Mn &gt; Mg &gt; Ca &gt; Na &gt; K &gt; P &gt; Si &gt; Al &gt; Fe &gt; Ti.</jats:p>

Long-term base cation weathering rates in forested catchments of the Canadian Shield

“…Chemical changes at the profile scale are interpreted as the results of mineral weathering, which is one of the main processes responsible for the development of soil profiles (Johnson and Watson-Stegner, 1987;Brimhall et al, 1991b). Moreover, soil profiles are progressively deformed during pedogenic development, because of processes such as weathering, eluviation, illuviation, bioturbation and the incorporation of organic matter leading to volumetric changes (collapse/dilation) in weathered horizons (Brimhall et al, 1991b;Anderson et al, 2002;Courchesne et al, 2002). The neoformation of amorphous solid phases (Al and Fe oxyhydroxides), the growth of plant roots or the integration of organic substances generally produce horizon dilation.…”

“…The basic assumptions are that: 1) the actual chemical composition of the C horizon is identical to the composition of the initial parent material; 2) the total chemical composition of the parent material is uniform throughout its profile at the initiation of pedogenesis and; 3) additions of material to the soil surface through, for example, aeolian deposition, as well as losses of material through physical surface erosion are minimal, so that the present-day vertical variation in total chemical composition reflects the cumulative effects of mineral weathering in the soil profile over time. This estimation method has limitations because some assumptions notably those relative to the initial uniformity of the parent material and to the equivalence between the chemical composition of the unweathered parent material and the present C horizons cannot always be verified (Courchesne et al, 2002;Schaller et al, 2009). Despite these limitations, the soil profile mass balance has been considered a reliable method to estimate long-term weathering losses at pedogenic time scale (Ouimet, 2008), and was used in a number of studies (Kirkwood and Nesbitt, 1991;Taylor and Blum, 1995;Courchesne et al, 2002;Schaller et al, 2009).…”

“…This estimation method has limitations because some assumptions notably those relative to the initial uniformity of the parent material and to the equivalence between the chemical composition of the unweathered parent material and the present C horizons cannot always be verified (Courchesne et al, 2002;Schaller et al, 2009). Despite these limitations, the soil profile mass balance has been considered a reliable method to estimate long-term weathering losses at pedogenic time scale (Ouimet, 2008), and was used in a number of studies (Kirkwood and Nesbitt, 1991;Taylor and Blum, 1995;Courchesne et al, 2002;Schaller et al, 2009).…”

“…This method is based on the geochemical behavior of conservative elements such as Ti and Zr which are considered immobile during weathering processes (Cann, 1970;Floyd and Winchester, 1975). It takes into account pedogenic processes, including changes in soil chemical composition and volumetric soil expansion or contraction (Brimhall et al, 1991a(Brimhall et al, , 1991bCourchesne et al, 2002). The basic assumptions are that: 1) the actual chemical composition of the C horizon is identical to the composition of the initial parent material; 2) the total chemical composition of the parent material is uniform throughout its profile at the initiation of pedogenesis and; 3) additions of material to the soil surface through, for example, aeolian deposition, as well as losses of material through physical surface erosion are minimal, so that the present-day vertical variation in total chemical composition reflects the cumulative effects of mineral weathering in the soil profile over time.…”

Modelling the formation and evolution of soils, towards an initial synthesis

“…The method is based on a thorough knowledge of the parent material and on the hypothesis of the conservation of an element considered as invariant. Finally, even though such models are based on analysis at the scale of a soil profile, subsequent elaborations enable the taking into account of outside input from erosion or as eolian deposits (Brimhall et al, 1988(Brimhall et al, , 1991Courchesne et al, 2002). Moreover, the spatial variability of the soil cover is modelled by introducing the initial heterogeneity of the parent material.…”