Yves Noack

Plant-induced weathering of a basaltic rock: Experimental evidence

Abstract The active role of higher plants in the weathering of silicate minerals and rocks is still a question for debate. The present work aimed at providing experimental evidence of the important role of a range of crop plants in such processes. In order to quantitatively assess the possible effect of these diverse plant species on the weathering of a basaltic rock, two laboratory experiments were carried out at room temperature. These compared the amounts of elements released from basalt when leached with a dilute salt solution in the presence or absence of crop plants grown for up to 36 days. For Si, Ca, Mg, and Na, plants resulted in an increase in the release rate by a factor ranging from 1 to 5 in most cases. Ca and Na seemed to be preferentially released relative to other elements, suggesting that plagioclase dissolved faster than the other constituents of the studied basalt. Negligible amounts of Fe were released in the absence of plants as a consequence of the neutral pH and atmospheric pO2 that were maintained in the leaching solution. However, the amounts of Fe released from basalt in the presence of plants were up to 100- to 500-fold larger than in the absence of plants, for banana and maize. The kinetics of dissolution of basalt in the absence of plants showed a constantly decreasing release rate over the whole duration of the experiment (36 days). No steady state value was reached both in the absence and presence of banana plants. However, in the latter case, the rates remained at a high initial level over a longer period of time (up to 15 days) before starting to decrease. For Fe, the maximum rate of release was reached beyond 4 days and this rate remained high up to 22 days of growth of banana. The possible mechanisms responsible for this enhanced release of elements from basalt in the presence of plants are discussed. Although these mechanisms need to be elucidated, the present results clearly show that higher plants can considerably affect the kinetics of dissolution of basalt rock. Therefore, they need to be taken into account when assessing the biogeochemical cycles of elements that are major nutrients for plants, such as Ca, Mg, and K, but also micronutrients such as Fe and ‘nonessential’ elements such as Si and Na.

Water-rock interactions in tropical catchments: field rates of weathering and biomass impact

The knowledge of dissolution and precipitation rates of rocks and minerals is important to understand the effect of natural as well as human influences on the soil system. The combination of mass-balance calculations and water sample geochemistry allows us the computation of weathering rates for small watersheds. The basaltic area of Ribeirao Preto (Parana, Brazil) provides an opportunity to study solute acquisition by meteoric waters in basaltic terranes, to quantify the water-basalt interaction, to address the impact of biomass on the weathering and to estimate the age of the weathering processes affecting the basalts under humid tropical climate conditions. The mass-balance approach is extended to Si and trace elements as well as neoformed minerals. The computations are showing that the transformation rates calculated with a biomass factor are 1.3–5 times faster than the rates achieved for the model not taking into account the biomass. The rates of release of Si (log k, k in mol m−2s−1 Si) computed for augite and labradorite have the same magnitude: −12.6 and are two orders of magnitude smaller than the experimental rates. The rate of precipitation of kaolinite (log k, k in mol m−2s−1 Si) is − 15.2. The mean rate of lowering of the weathering front has a value of around 40 ± 10 mm kyr−1 under humid tropical climate. A mean age of 700 ± 200 kyr is calculated for the weathering processes affecting the basalts of the Parana basin.

Sr isotopic evidence for ion-exchange buffering in tropical laterites from the Paraná, Brazil

Sr isotopes have been measured in laterite sequences and in associated waters from the region of Ribeirao Preto (Parana basin, Brasil), in order to characterize the water-rock interactions corresponding to the successive stages of weathering of these continental flood basalts, We analyzed detailed sections of concentric rims of alteration in ball-shaped spheroids developed around cores of fresh basalts. Waters were also analyzed on the different types of facies in this region: unaltered basalts, thick weathered profiles of lateritic material, and surrounding sandstones, The results show that a major part of the initial Sr is removed from the basalt during the earliest stages of weathering, within the narrow grey rims surrounding the unweathered core. Radiogenic Sr is also leached from hydrothermal minerals located in basalt fractures or disseminated in the basalt groundmass at this early stage. Very high water/rock ratios (of at least 100,000) are calculated for this process, from the Sr-87/Sr-86 in the related waters. A general trend of increasing Sr-87/Sr-86 ratios is observed in the increasingly weathered facies (inner yellow cortexes, red kaolinitic soils and matrices) and in their related waters. The very high Sr-87/Sr-86 ratios of the red kaolinitic horizons probably result from isotopic exchange with radiogenic rainwaters or sandstone waters, and also from the presence of residual or neoformed radiogenic minerals. Budget calculations, as well as experimental leaching of kaolinite samples, suggest that significant amounts of radiogenic Sr can be leached from these facies during weathering, Thus, the kaolinitic facies of tropical laterite profiles are able first to trap strontium from surficial waters, then to provide radiogenic Sr to the waters that weather the fresh basalts at the weathering front. Therefore, the thick weathered horizons must be considered as a natural cation-exchange layer, that play an active role in the weathering process.

Assessing human exposure to aluminium, chromium and vanadium through outdoor dust ingestion in the Bassin Minier de Provence, France

The Western part of the “Bassin Minier de Provence”, a former coal mining area, is still occupied by old polluting industries such as a coal-fired power plant and an alumina factory. The identified pollution sources that raise more concern in the population are the emission of gases and dusts, as well as the storage of raw and transformed materials. In 2011, a preliminary survey was carried out in the area as the first step to an exposure and health risk-assessment study. This first survey intends to assess human exposure through ingestion and health risk associated with potentially harmful elements (PHEs) in ground-level dusts collected in recreational areas used by children. Dust samples were taken at 19 sites distributed across the study area, depending on the location of public parks, public gardens, playgrounds and schools. Pseudo-total concentrations of 53 elements were determined by ICP-MS. Bioaccessible concentrations were estimated using the unified bioaccessibility method. This study presents the results obtained for Al, V and Cr, which seem to be related with industry and show similar distribution patterns. PHEs presumably related to traffic or other urban pollution sources are not discussed in this study. The highest total concentrations occur in dusts near the alumina plant that have significant amounts of Al mineral phases (gibbsite and alumina). However, in these dusts only small fractions of the elements under study are in bioaccessible forms. The highest bioaccessible fractions occur in dusts collected near the coal-fired power plant. Further investigation is required to assess potential pathways of exposure and health risk in this area.

Polymetallic pollution from abandoned mines in Mediterranean regions: a multidisciplinary approach to environmental risks

Abandoned mines are a recurrent problem for nearby communities in Mediterranean regions because mine tailings represent a major source of polymetallic contamination. Metal contaminants are emitted in mining areas and dispersed by wind and water erosion in the surroundings. The goal of this literature review was to identify the specific features of polymetallic contamination arising from abandoned mines in the Mediterranean regions. Mediterranean climate conditions and local geochemical context are the most important factors that control the metal-bearing particle dispersion toward the different compartments of ecosystems. Acid mine drainage, as an important source of damage to the environment, is limited to a certain extent by the predominance of carbonate rocks in the Mediterranean regions. In opposite, aeolian contamination is specific to the semiarid conditions of the Mediterranean climate. In this context, impacts on different compartments such as agricultural soils and edible plants or human populations were underlined. The analysis of environmental laws and regulations of North and South Mediterranean countries shows that one of the main differences is the lack of identification and definition of mining waste as a public concern in the latter countries. In order to limit the transfer of contaminants from mining waste to the different components of the environment, phytostabilization of mine tailings was considered as the more adapted green technology even in the Mediterranean region where water access is limited. Finally, this review of polymetallic pollution from abandoned mines in Mediterranean regions enabled to identify priority actions for future research.

Sr isotopic tracing in a lagoonal system: : example of surficial carbonate sediments in the Nador lagoon (northeastern Morocco)

The Nador lagoon is the largest one in Morocco and along the Mediterranean. It is located on the northeastern coast near the Rif chain (North Morocco), and it is surrounded by volcanic and sedimentary rocks of various lithofacies. The watershed has an area of about 2200 km(2) and is drained by a dense river network. The lagoon dimension is of 115 km(2) (25 x 7.5 km) with a depth not exceeding 8 m. The island barrier is 25 km in length and 350 m in average width. The carbonate fraction is the dominant component of the present-day sedimentation in the Nador lagoon. It represents 13 to 48% of total sediments. Its distribution within the lagoon reflects the nature of marine and continental watershed. The carbonate fraction is composed, in decreasing order of importance, of calcite, magnesium calcite, dolomite and aragonite. The Sr isotopic signatures of lagoonal carbonates sediments show that they are dominated by marine biogenic sediments deposited, and by chemical precipitation. In addition, other carbonates of external origin and less radiogenic Sr isotopic are also present, coming from aerosols and ancient carbonates and transported by streams and rivers. From northwest to southeast, the Sr concentration of these lagoon carbonates increases, showing a slow renewal of waters in the southeastern zone because of its remoteness from communication with sea.