Résultat de la recherche

Filtres

111
3
3
21
Réinitialiser les filtres

Paramètres

Voici les éléments 1 - 3 sur 3
  • Publication
    Accès libre
    The transfer of Cadmium from rock to soil and the associated vegetation cover under natural conditions at the Swiss Jura Moutains
    (2010)
    Quezada Hinojosa, Raul Percy
    ;
    Föllmi, Karl
    ;
    Matera, Virginie
    ;
    Adatte, Thierry 
    ;
    Verrecchia, Eric 
    ;
    Baize, Denis
    As a result of soil-surveying studies conducted in the Swiss and French Jura Mountains during the early 1990's, anomalous cadmium (Cd) concentrations were identified in soils developed mostly on Bajocian and Oxfordian limestone. Measured Cd concentrations exceed in most of the cases the Swiss official tolerance guideline concentration for non-polluted soils established at 0.8 mg⋅kg-1. Several research works have confirmed the geogenic origin of Cd in soils derived mainly from the weathering of a Cd-rich carbonate substrata. Cd is a highly toxic trace element and the pedogenic / physicochemical conditions leading to its transfer from rocks to soils and its potential bioavailability to plants are in need of a detailed geochemical assessment. The aim of the present work is to complete the geochemical database by studying rock-soil-plant interactions with regard to this element under natural conditions in two specific sites. A first study of rock-soil interaction was carried out determining Cd-bearing phases in a soil developed on top of a road-cut section outcropping at the SW-facing slope of the Schleifenberg hill (canton Basel-Land, Switzerland). This section consisting of an oblique succession of Bajocian oolitic carbonate includes several horizons which are anomalously enriched in Cd (0.03–4.90 mg⋅kg-1). Cd contents in this soil are in the 0.3–2.0 mg⋅kg-1 range. Vertical pedogenetic processes (weathering of underlying bedrock) as well as lateral colluvial limestone (weathering of uphill carbonates) are responsible for the origin of Cd in the soil. Half of the Cd still resides in the carbonate fraction, while the Cd released from the weathered carbonates is associated either with organic matter (over 10%) or with Fe and Mn-oxyhydroxides (approximately 30%). Adsorption of a low percentage of Cd on clays is of less importance since Pb, Zn, Cu and Cr ions will compete with Cd to gain adsorbed sites on clays. No exchangeable Cd phase was found and this, together with the buffer capacity of this calcareous soil, suggests that the amount of mobile Cd is quite negligible, which also greatly reduce the amount of bioavailable Cd. Where developed on steep slopes, the soil will hardly accumulate and colluviums will constantly renew it. A second study regarding the transfer and distribution of geogenic Cd in the soil was conducted on six closely spaced soil profiles at the site called Le Gurnigel (canton Neuchâtel, Switzerland). The soils consist mainly of cambisols and cambic-neoluvisols showing an important allochthonous, aeolian fraction. Cd concentrations generally increase down the soil profiles, showing maxima (up to 16.3 mg⋅kg-1) near the soil-bedrock interface. Most Cd resides in the carbonate and organic fractions in topsoils, whereas the amorphous oxyhydroxides fraction becomes the most important Cd-bearing phase in the middle and in subsoils. Cd, Zn and Cr are positively correlated with comparable distributions in the soil profiles suggesting a common bearing phase such as Fe oxyhydroxides for these three elements. A complex transfer pattern of Cd starts with the release of Cd from the underlying bedrock, and then transferred into oxide, hydroxide, carbonate and organic phases. Additionally, the lateral advection of Cd-rich soils formed on steep slopes acts as a local allochthonous input of Cd to these soils, which in turn is transferred from the topsoil towards the deeper horizons by biological and pedogenic processes. The amount of readily exchangeable and therefore potentially bioavailable Cd is low in these soils (on average 0.2 mg⋅kg-1) provided that the pH remains above 5. Under stronger acidic and oxidizing conditions, Cd bound to organic matter may be mobilised and the bioavailability of Cd would range between 3.3–5.4 mg⋅kg-1 in cambisols and reach up to 1.7 mg⋅kg-1 in deeper cambic-neoluvisols. Soil-plant interactions were studied at the Le Gurnigel analyzing six local plants chosen for their ubiquity in the studied soil profiles. Cadmium accumulation was separately determined in roots and shoots. Three herbs, two graminoids and a tree were used for this purpose. They showed that the accumulation of Cd varies from one species to another and even between plants from the same family. Global levels of Cd in the selected vegetation are in the 2–6 mg·kg-1 range, thus exceeding the official limit value of Cd concentration tolerated in vegetal food for animals established at 1 mg·kg-1. The different behaviours were compared as a function of the variability of Cd in soils. A rise in the concentration of Cd in the soil progressively reduces the transport of Cd toward the shoots reducing also the yield production and increasing the accumulation of Cd in roots. Transfer coefficients from soil / rhizosphere to plant are inversely proportional to the total Cd concentration in soils and do not depend on species identity but instead on soil type. Sequential chemical extractions revealed that variations of Cd distribution between distant soil and rhzosperic soil occur mainly in the first three Cdbearing phases due principally to the incorporation of roots exudates that modify pH and redox conditions of the rizhosphere. High levels of Cd (up to 9 mg·kg-1) were found in shoots of three of the studied plants and may represent a mid-term hazard for animals and human health since these plants are used either for grazing of cattle or for medical purposes. The phenomenon of natural enrichment of soils with geogenic Cd and its progressive accumulation in vegetation covers is suspected to have a widespread occurrence elsewhere, as a function of frequent outcrops of Cd-enriched carbonates of Bajocian and Oxfordian age in western and southern Europe principally in France, Spain and Italy.
  • Publication
    Accès libre
    Biogeochemical changes during the Cenomanian-Turonian Oceanic Anoxic Event (OAE 2)
    (2006)
    Mort, Haydon
    ;
    Adatte, Thierry 
    Understanding how nutrients behave during sub-oxic environments is crucial in predicting future changes in the ocean-climate system. This study focuses on the role of phosphorus, in its different sedimentary reservoirs, during the Cenomanian-Turonian anoxic event (OAE 2), which occurred during the late Cretaceous (~93.5 Ma). In the geological record this period in the Earth’s history is represented by the widespread occurrence of sediments containing elevated organic matter content and a large positive 13C excursion. Within these lithologies are biological and geochemical signatures that indicate a significant depletion in the amount of oxygen present in the water column. Phosphorus is a macronutrient, used during primary productivity. After the life cycle of the organism, the subsequent destruction of its organic matter on the sea floor releases phosphate at the sediment-water interface. The phosphate can then follow a number of possible biogeochemical pathways, including its mineralization and sorption into authigenic minerals and oxyhydroxides. The path it follows is largely dependant on the concentration of oxygen in the surrounding water. During periods of oxygen deficiency the oxygen required for the degradation of organic matter is derived by reducing electron donors (e.g. MnO2, Fe(OH)3 and SO43-), which are also binding sites for the phosphorus in the water. The reduction in the number of binding sites causes the sediments retention ability for phosphorus to decrease resulting a net increase in the concentration of phosphate in the water column. This phosphate is then available again for primary producers, should it be recycled into the photic zone. By studying the behaviour of phosphorus we attempt infer switches in the mode of sediment accumulation and to consequently reconstruct the oxidation history of OAE 2. To do this a sequential extraction technique (SEDEX) is used to identify the various types (or phases) of phosphorus found in five exposures distributed across the Tethyan Realm (Europe and North Africa) and the Western Interior Seaway (North America). Other proxies are used to consider of regional influences in our data (e.g. 13C, mineralogy, hydrogen and oxygen indices, planktonic foraminiferal biostratigraphy). Phosphorus mass accumulation rates (P MARs) show a distinct trend, similar in each of the five sections. A generalised observation is as follows; (1) an peak in P MARs at the start of the increase in 13C values; (2) a sharp decrease in inorganic-P phases during the isotope excursion itself and a smaller reduction in organic-P. Two sections actually see an enrichment of organic-P during this time. The Corg/Preactive molar ratios (Redfield ratio) begin to increase at this point as does the quantity of organic matter in the sections; (3) A return to ‘background’ values at or just after the start of the isotope plateau. This is interpreted in the following way; (1) an environmental perturbation caused a rapid increase in the amount of phosphate in the ocean, which resulted in the initial increase in P MARs. At present, the most plausible it that increase was caused by a maximum flooding event that caused the rapid reworking of nutrients stored on previously dry land allowing them to settle on the bottom of the ocean but also to become bioavailable. (2) The decrease in inorganic-P was caused by the desorption of phosphate from oxyhydroxides and the reduction in phosphorus authigenesis. We suggest that this phosphate became recycled into the upper water column to further stimulate productivity, which, in-turn, consumed more O2 on the sea floor. Organic-P either increased later (as is seen in sections with a deep paleodepth) or did not decrease very much because of decelerated bacterial degradation of organic matter due to oxygen deficient conditions. The increase in organic matter content after P MARs start to decrease is seen as evidence to support this hypothesis (3) Pre-excursion MAR values are low considering how much phosphate was in the water column suggesting that phosphorus recycling was sustaining the 13C plateau. Intense and sustained primary productivity consumed bottom water oxygen but simultaneously increased atmospheric O2 concentrations. It is possible that atmospheric O2 eventually acted as a negative feedback when the partial pressure of free O2 in the atmosphere became greater than in the mixing layer of the ocean, starting the oxidation of nutrients in the ocean. We present tentative evidence for this with a P curve from an Egyptian section. The preservation of organic matter in black shales would have dramatically reduced the concentration of CO2 in the atmosphere, cooling the climate leading to a reduction in humidity, precipitation, and continent-ocean nutrient fluxes.