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

Laterites and paleoclimates. Weathering processes and anthropogenic impact

The European Union of Geosciences held its 9th biannual meeting in Strasbourg, March 23–27, 1997. During this meeting, Symposium No. 63, Weathering Processes: Mineral deposits and soil formation in tropical enÍironments, was merged with Symposium No. 78, Anthropogenic Impact on Weathering processesco-sponsored by IGCP Project 405. The resulting symposium No. 63, Weathering processes and Anthropogenic Impact, was held under the sponsorship of EUROLAT1 and attracted 36 oral and poster presentations and about 100 participants, reflecting the interest of the Earth Sciences community in weathering processes and anthropogenic impact

A normalised seawater strontium isotope curve: possible implications for Neoproterozoic-Cambrian weathering rates and the further oxygenation of the Earth

The strontium isotope composition of seawater is strongly influenced on geological time scales by changes in the rates of continental weathering relative to ocean crust alteration. However, the potential of the seawater 87Sr/86Sr curve to trace globally integrated chemical weathering rates has not been fully realised because ocean 87Sr/86Sr is also influenced by the isotopic evolution of Sr sources to the ocean. A preliminary attempt is made here to normalise the seawater 87Sr/86Sr curve to plausible trends in the 87Sr/86Sr ratios of the three major Sr sources: carbonate dissolution, silicate weathering and submarine hydrothermal exchange. The normalised curve highlights the Neoproterozoic-Phanerozoic transition as a period of exceptionally high continental influence, indicating that this interval was characterised by a transient increase in global weathering rates and/or by the weathering of unusually radiogenic crustal rocks. Close correlation between the normalised 87Sr/86Sr curve, a published seawater δ34S curve and atmospheric pCO2 models is used here to argue that elevated chemical weathering rates were a major contributing factor to the steep rise in seawater 87Sr/86Sr from 650 Ma to 500 Ma. Elevated weathering rates during the Neoproterozoic-Cambrian interval led to increased nutrient availability, organic burial and to the further oxygenation of Earth's surface environment. Use of normalised seawater 87Sr/86Sr curves will, it is hoped, help to improve future geochemical models of Earth System dynamics

Investigating Variation in the Prevalence of Weathering in Faunal Assemblages in the UK: A Multivariate Statistical Approach

This article presents an exploratory multivariate statistical approach to gaining a more comprehensive understanding of variation in subaerial bone weathering in a British context. Weathering is among the most common taphonomic modifications and provides a crucial line of evidence for reconstructing the taphonomic trajectories of faunal assemblages and archaeological deposits. It provides clear evidence for prolonged subaerial exposure either before deposition in a context or because of later disturbance. In combination with other taphonomic indices such as gnawing, trampling, abrasion and fracture patterns, weathering can be used to reconstruct depositional histories and to investigate the structured treatment of different body parts or taxa in deposition. However, a broad range of factors affect the prevalence and severity of weathering, and therefore patterns can rarely be interpreted at face value. Many variables such as predepositional microenvironment cannot be traced archaeologically. Other contributory factors pertaining to the structural properties of elements and taxa can be discerned and must be taken into account in interpreting weathering signatures. However, disagreement exists regarding which variables are most important in mediating weathering. In addition for zooarchaeologists to interpret modification patterns, it is necessary for elements and taxa that are most likely to be affected by weathering to be defined. This is the case as deposits that are dominated by those classes of remains are likely to exhibit greater modification than those that are not, even if depositional histories were similar. Through a combination of classification tree and ordinal regression analysis, this article identifies which archaeologically recoverable variables explain the greatest variance in weathering and which anatomical elements and taxa are most likely to be affected in archaeological deposits in the UK

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

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

Reinforcing quantitative skills with applied research on tombstone-weathering rates.

This Journal of Geoscience Education article describes a tombstone weathering exercise that reinforces quantitative skills with applied research. The article describes an exercise designed and carried out by students in a surficial processes course. Students measured the rates of tombstone weathering, compiled and analyzed data. The complexity of the data involved in this exercise helps students demonstrate for themselves that mathematical analysis can give meaning to data. Educational levels: Graduate or professional, Undergraduate lower division

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

I. Collisional evolution and reddening of asteroid surfaces: The problem of conflicting timescales and the role of size-dependent effects

Space weathering is the generic term used for processes that modify the optical properties of surfaces of atmosphereless rocky bodies under exposure to the space environment. The general agreement about the relevance of the effects of space weathering on the spectral properties of S-complex asteroids fails when some basic quantitative estimates are attempted. In particular, there is severe disagreement regarding the typical timescales for significant spectral reddening to occur, ranging from 1 Myr to 1 Gyr. Generally speaking, the spectral reddening of an individual object can be considered as the sum of three terms, one (which is relevant for statistical analyses) depending on the exposure of the object to space weathering during its lifetime, a second one due to the original surface composition, and a third one (a "noise" term) due to the combination of poorly constrained effects (e.g., structure and texture of the surface). The surface of an asteroid is usually covered by regolith, and its presence and properties presumably play a critical role in the weathering processes. In this paper we discuss the role played by collisional evolution in affecting the spectral properties of asteroids and refreshing the surfaces due to the formation of ejecta, and the necessity of a simultaneous modeling of collisions and weathering processes. We introduce a new idea, based on the possibility of a sort of saturation of the refreshing process whenever a massive reaccumulation of the impact ejecta takes place. In this case, a dependence of the overall reddening on the asteroid size should naturally come out. We show that this conclusion is indeed supported by available main belt asteroid spectroscopic data.Comment: Accepted by MNRA