[show abstract][hide abstract] ABSTRACT: Traditionally, iridium has been considered an element of low mobility, but its behavior is still debated. Ir concentration in a soil affected by a catastrophic mining spill in 1998 that covered the soil with a layer of tailings offers the opportunity to analyse an exceptional Ir-bearing horizon 10 years after deposition. This has enabled comparisons with the values of past Ir-bearing horizons associated to impact-related mass-extinction events. Iridium concentration in the tailings (0.349 ppm) was 5-fold higher than the anomaly in the K–Pg at The Moody Creek Mine section (the highest values obtained from terrestrial sections). The oxidative weathering of the tailings caused the release of Ir and infiltration into the soil. Iridium distribution in depth indicates redistribution throughout the profile in relation to the change in the physico-chemical properties of the soil. With regard to the background concentration in the soil (0.056 ppm), anomalous values of Ir (0.129 ppm) can be detected to 11 cm below the layer of tailings. The correlation analysis between the Ir concentration and the main properties and constituents of the soils indicated a significant correlation with sulfur, iron, clay content, and pH. Selective extractions were made to study the forms in which Ir can be mobilized in the soil. The residual/insoluble fraction was >90% of the total Ir concentration in soil. Soluble-in-water concentration of Ir (1.5% of total) was detected in the uppermost 2–3 cm of the soil, which were directly affected by the leaching of acidic waters coming from the oxidation of the pyrite tailings. Iridium retention in the affected part of the soil reached 9% of the total Ir concentration; this retention could be related to the amorphous iron forms dissolved by the oxalic-oxalate extraction. However, according to our research, original Ir abundance could be secondarily modified, and then a direct analysis of the iridium values recorded in sediments could induce misinterpretations. The comparison between the actual example and the fossil record belonging to terrestrial settings, can be considered as a valuable approach, especially when Iridium data were used by researchers to interpret the impact-related mass-extinction events in the past.
[show abstract][hide abstract] ABSTRACT: Biotic recovery after the Cretaceous-Palaeogene (K-Pg) impact is one unsolved question concerning this mass extinction event. To evaluate the incidence of the K-Pg event on biota, and the subsequent recovery, a recent environmental disaster has been analysed. Areas affected by the contamination disaster of Aználcollar (province of Sevilla, southern Spain) in April 1998 were studied and compared with the K-Pg event. Several similarities (the sudden impact, the high levels of toxic components, especially in the upper thin lamina and the incidence on biota) and differences (the time of recovery and the geographical extension) are recognized. An in-depth geochemical analysis of the soils reveals their acidity (between 1.83 and 2.11) and the high concentration of pollutant elements, locally higher than in the K-Pg boundary layer: values up to 7.0 mg kg(-1) for Hg, 2030.7 mg kg(-1) for As, 8629.0 mg kg(-1) for Pb, 86.8 mg kg(-1) for Tl, 1040.7 mg kg(-1) for Sb and 93.3-492.7 p.p.b. for Ir. However, less than 10 years after the phenomenon, a rapid initial recovery in biota colonizing the contaminated, 'unfavourable', substrate is registered. Nesting of the ant Tapinoma nigerrima (Nylander) has taken place through the tailing layer, with arranged particles from inside the soils showing similar values in pollutant elements as the deep soils. This agrees with recent ichnological evidence of a rapid colonization of the K-Pg boundary layer, classically interpreted as an inhabitable substrate, by organisms with a high independence with respect to substrate features (i.e. Chondrites trace makers). The dramatic consequences of the K-Pg boundary impact and the generalized long-time recovery interpreted after the event (in the order of 10(4)-10(5) years) could have been overestimated due to the absence of a high-temporal resolution in the range of 10(2)-10(3) years.