Cadmium anomalies in Jurassic carbonates (Bajocian, Oxfordian) in western and southern Europe
Thèse de doctorat : Université de Neuchâtel, 2006 ; 1858.
The trace metal cadmium (Cd) is a highly toxic element, with adverse effects both on human health as well as on ecosystem equilibriums. Its transfer into the environment occurs by the mediation of natural and anthropogenic processes. Important natural sources comprise volcanic emissions and the biogeochemical weathering of Cd-enriched rocks, such as phosphorites or organic-rich deposits. This... PlusAjouter à la liste personnelle
- The trace metal cadmium (Cd) is a highly toxic element, with adverse effects both on human health as well as on ecosystem equilibriums. Its transfer into the environment occurs by the mediation of natural and anthropogenic processes. Important natural sources comprise volcanic emissions and the biogeochemical weathering of Cd-enriched rocks, such as phosphorites or organic-rich deposits. This study considers the Cd concentrations in Jurassic limestone, in particular of Bajocian (Middle Jurassic) and Oxfordian (Late Jurassic) age in western and southern Europe. In the Jura Mountains area, carbonate rocks related to these time periods have been shown to contain surprisingly high Cd concentrations, especially since most carbonate rocks are known to be strongly depleted in Cd relative to the average crust. Soils that are developed from bedrocks of these geological ages equally show a strong tendency to be Cd-enriched to anomalously high levels. One of the specific aims of this study is to trace the geographical and stratigraphical distribution of Cd enrichments in Jurassic carbonates, in western and southern Europe, and consequently to develop a predictive tool to identify the presence of Cd-enriched soils related to Jurassic rock substrata in the investigated areas. A further goal is to reconstruct the sedimentary and environmental conditions that have led to Cd enrichments in Jurassic limestone. Ten sections in carbonate successions of Middle or Late Jurassic age have been analyzed in Switzerland, France, Spain and Italy. Additionally, a specific study aiming at quantifying the relationships between Jurassic limestone and associated soils has been conducted in the Lower Burgundy area (France). Our results from the analysis of carbonate sections suggest that for both periods Cd enrichments correspond to a widespread phenomenon in western and southern Europe, unrelated to the specificities of the carbonate facies. Two major features are nevertheless distinguished: - a general increase in Cd values is observed in both open-marine and platform settings, and particularly well marked in basin environments; this shift towards more elevated values is interpreted as a witness of general changes in the cadmium cycle at least within the western Tethyan realm, and perhaps on a wider geographical scale; - shallow-water sections additionally display major enrichments, with Cd values frequently above 1 μg/g, restricted to narrow stratigraphic intervals. These very high concentrations are attributed to a specific mechanism of enrichment. Cd enrichments in Jurassic shallow-water calcareous rocks are proposed to be related to (1) the quantity of Cd available in seawater, and (2) important biological activity and organic matter production on the carbonate platform margin. The general increase of Cd contents in both deep and shallow-water carbonates is tentatively linked to general environmental change, and especially to intense volcanic processes, which may have led to an increase in the availability of Cd in the environment. The results of our investigations with regards to Bajocian and Oxfordian limestone and associated soils in the Lower Burgundy area confirm that a causal link exists between Cd anomalies in soils and high Cd contents in the associated carbonate bedrock. Mean enrichment factors calculated for Cd in the soil relative to the associated carbonate substratum vary from 4.6 to 5.7, those calculated between soil and rock inside restricted geographical areas being around 5.5 – 5.7. The widespread presence of Cd-enriched carbonates in investigated sections of Middle and Late Jurassic age suggests that these enrichments may occur on a larger geographical scale within western and southern Europe. This would also imply that corresponding soils have the potential to be naturally enriched in Cd, in particular for soils related to the weathering of shallow-water limestone.