Chemical weathering of the volcanic soils of Isla Santa Cruz (Galápagos Islands, Ecuador)

We present a study on weathering of volcanic soils using 43 profiles (131 horizons) sampled in Santa Cruz Island (Galapagos Islands). Several weathering indices, based on chemical composition, are used. Since the geological material is highly homogeneous the intensity of weathering is mostly related to climatic conditions controlled by topography. There is a gradient of increasing weathering from the arid conditions predominant in the coast to elevations of 400-500 m a.s.l. where much more humid conditions prevail

Silicate rock weathering and atmospheric/soil CO2 uptake in the Amazon basin estimated from river water geochemistry: seasonal and spatial variations

Using the data of the CAMREX project (1982–1984) on the water geochemistry of the Amazon river and its main tributaries, it was possible to assess the silicate rock weathering processes and the associated consumption of atmospheric/soil CO2, taking into account seasonal and spatial variations. This study confirms the important role of the Andes in the fluvial transport of dissolved and particulate material by the Amazon, and it shows for the first time that the silicate weathering rate and atmospheric/soil CO2 consumption are higher in the Andes than in the rest of the Amazon basin. The seasonal variations exhibit the significant role of runoff as a major factor controlling silicate weathering processes and show that the chemical erosion rates vary greatly from low discharge to high discharge. The average weathering rate estimated for the whole Amazon basin (15 m/My) is comparable to other estimations made for other tropical–equatorial environments. A comparison between physical and chemical weathering rates of silicate rocks for the Amazon basin and for each tributary basin show that in the Andes and in the Amazon trough, the soil thicknesses are decreasing whereas in the Shield the soil profiles are deepening

Using [delta] ph as a geochemical index of illite neoformation in saprolite

Sal pH is routinely measured for agronomic purposes. When the difference between KCI pH and H2O pH, or ApH, yields positive values, it is used by soil scientists as a classification criterion for identifying anionic subgroups according to the Soil Taxonomy or geric properties according to the WRB. Negative values have not been granted much attention. Here we focus on the occurrence of highly negative ApH values in the weathering zone of profiles developed on gneiss in semiarid Northeast Brazil and semiarid South India and interpret them as proxies of a geochemical weathering process involving the neoformation of illite. Detailed optical, chemical and mineralogical characterizations involving scanning electron microscopy coupled with X-ray element mapping demonstrate the neoformation of illite inside plagioclase feldspar crystals after their partial dissolution. This study thus reveals that meteoric weathering is capable of producing Mite not only from mica, Le., by a transformation process, but also within non-alkali feldspar by a neoformation process. The ApH is shown to be a good proxy for detecting such weathering signatures because the recently neoformed Mate flakes, which present a significant compositional deficit in K, reveal their presence by a detectable uptake of K from the KCI solution. This finding changes the perspective over the origin of illite in continental environments, which has most commonly been attributed to hydrothermal processes. (Texte intégral

Space weathering: Laboratory analyses and in-situ instrumentation

Simulations of space weathering using laser irradiation are exploited to study the formation of sub-microscopic iron. A variety of magnetic techniques are evaluated to characterise this iron and are considered for in-situ instrumentation

Cryptoendolith colonization of diverse substrates (1): cultivation and characterization

We are investigating whether cryptoendolithic microorganisms are able to colonize diverse substrates through biogenic weathering. This first part of the study involves the cultivation and characterization of microbial consortia from Antarctic sandstone habitats

Sensitivity of the global carbonate weathering carbon-sink flux to climate and land-use changes

The response of carbonate weathering carbon-sink flux (CCSF) to its environmental drivers is still not well understood on the global scale. This hinders understanding of the terrestrial carbon cycle. Here, we show that there is likely to be a widespread and consistent increase in the global CCSF (ranging from + 9.8% (RCP4.5) to + 17.1% (RCP8.5)) over the period 1950–2100. In the coming years the increasing temperature might be expected to have a negative impact on carbonate weathering. However, the increasing rainfall and anticipated land-use changes will counteract this, leading to a greater CCSF. This finding has been obtained by using long-term historical (1950–2005) and modeled future (2006–2100) data for two scenarios (RCP4.5 and RCP8.5) for climate and land-use change in our CCSF equilibrium model. This study stresses the potential role that carbonate weathering may play in the evolution of the global carbon cycle over this century

Photochemical weathering and contemporary volatile loss on Mars

In an earlier series of papers by the author it was proposed that photochemical weathering of Fe(2+) in magnetite and in mafic silicates may be occurring in the contemporary surface environment with a resultant loss of O2 from the atmosphere. Morris and Lauer challenged the photochemical weathering model, proposing that oxidation by radiant heating rather than UV photoelectron emission induced oxidation may have dominated in the authors experiments. Subsequent laboratory studies of photochemical weathering of magnetite described here support the authors original proposal that UV illunimation can indeed drive the oxidation of magnetite under contemporary Martian surface conditions. The negative results of the Morris and Lauer study can now be explained

Structure of supercritically dried calcium silicate hydrates (C-S-H) and structural changes induced by weathering

The nanostructure of supercritically dried calcium silicate hydrates was researched. This particular drying procedure was used to avoid nanostructure modifications due to conventional drying processes. Thus, in this study, the as-precipitated cementitious C-S-H structure was obtained for the first time. A specific surface area 20 % larger than conventionally dried C-S-H was measured. Given the importance of this nanostructured phase for the properties of hydrated cements, especially when in contact with CO2-rich environments, the supercritically dried C-S-H was weathered for 2 weeks. The structural effects of this weathering process on the C-S-H were researched and calcium carbonate microcrystal precipitation or the presence of silica by-product are reported. Calcite and aragonite polymorphs were observed, as well as nanoporous silica forming globular arrangements. In addition, 2 weeks of weathering was not enough to carbonate the entire C-S-H sample.Junta de Andalucía TEP11

Impact of nitrogenous fertilizers on carbonate dissolution in small agricultural catchments: Implications for weathering CO2 uptake at regional and global scales

The goal of this study was to highlight the occurrence of an additional proton-promoted weathering pathway of carbonate rocks in agricultural areas where N-fertilizers are extensively spread, and to estimate its consequences on riverine alkalinity and uptake of CO2 by weathering. We surveyed 25 small streams in the calcareous molassic Gascogne area located in the Garonne river basin (south-western France) that drain cultivated or forested catchments for their major element compositions during different hydrologic periods. Among these catchments, the Hay and the Montousse´, two experimental catchments, were monitored on a weekly basis. Studies in the literature from other small carbonate catchments in Europe were dissected in the same way. In areas of intensive agriculture, the molar ratio (Ca + Mg)/HCO3 in surface waters is significantly higher (0.7 on average) than in areas of low anthropogenic pressure (0.5). This corresponds to a decrease in riverine alkalinity, which can reach 80% during storm events. This relative loss of alkalinity correlates well with the NO3 content in surface waters. In cultivated areas, the contribution of atmospheric/soil CO2 to the total riverine alkalinity (CO2 ATM-SOIL/HCO3) is less than 50% (expected value for carbonate basins), and it decreases when the nitrate concentration increases. This loss of alkalinity can be attributed to the substitution of carbonic acid (natural weathering pathway) by protons produced by nitrification of Nfertilizers (anthropogenic weathering pathway) occurring in soils during carbonate dissolution. As a consequence of these processes, the alkalinity over the last 30 years shows a decreasing trend in the Save river (one of the main Garonne river tributaries, draining an agricultural catchment), while the nitrate and calcium plus magnesium contents are increasing. We estimated that the contribution of atmospheric/soil CO2 to riverine alkalinity decreased by about 7–17% on average for all the studied catchments. Using these values, the deficit of CO2 uptake can be estimated as up to 0.22–0.53 and 12–29 Tg1 yr1 CO2 on a country scale (France) and a global scale, respectively. These losses represent up to 5.7–13.4% and only 1.6–3.8% of the total CO2 flux naturally consumed by carbonate dissolution, for France and on a global scale, respectively. Nevertheless, this loss of alkalinity relative to the Ca + Mg content relates to carbonate weathering by protons from N-fertilizers nitrification, which is a net source of CO2 for the atmosphere. This anthropogenic CO2 source is not negligible since it could reach 6–15% of CO2 uptake by natural silicate weathering and could consequently partly counterbalance this natural CO2 sink

Weathering microenvironments on feldspar surfaces: implications for understanding fluid-mineral reactions in soils

The mechanisms by which coatings develop on weathered grain surfaces, and their potential impact on rates of fluid-mineral interaction, have been investigated by examining feldspars from a 1.1 ky old soil in the Glen Feshie chronosequence, Scottish highlands. Using the focused ion beam technique, electron-transparent foils for characterization by transmission electron microscopy were cut from selected parts of grain surfaces. Some parts were bare whereas others had accumulations, a few micrometres thick, of weathering products, often mixed with mineral and microbial debris. Feldspar exposed at bare grain surfaces is crystalline throughout and so there is no evidence for the presence of the amorphous 'leached layers' that typically form in acid-dissolution experiments and have been described from some natural weathering contexts. The weathering products comprise sub-μm thick crystallites of an Fe-K aluminosilicate, probably smectite, that have grown within an amorphous and probably organic-rich matrix. There is also evidence for crystallization of clays having been mediated by fungal hyphae. Coatings formed within Glen Feshie soils after ∼1.1 ky are insufficiently continuous or impermeable to slow rates of fluid-feldspar reactions, but provide valuable insights into the complex weathering microenvironments on debris and microbe-covered mineral surfaces