The negative effect of environmental geological conditions of some geo-archaeological sites of North Coast and Alexandria

Three geo-archaeological sites at the North Coast and Alexandria, namely, the Alexandria wall (El Shalalat Park site), Abu Soir temple, and Marina excavations, were investigated to determine the negative impact of a salty environmental condition. The monuments suffer from rock decay of different rates. The geo-archaeological sites were built mainly from oolitic limestone blocks (i.e., the Alexandria wall at the El Shalalat Park site and Abu Soir temple) or excavated on them (i.e., Marina excavations). Field inspection and a lab analysis were carried out to understand the weathering mechanism. Salt weathering criteria such as disintegration, pitting, scaling, exfoliation, and honeycomb are observed on the Alexandria wall and upper parts of the Abu Soir temple, while dangerous cracks are detected on the Marina excavation tombs. The petrographic study of the oolitic limestone samples shows that they consist mainly of oolities and drusy sparite as a cement (oolitic grainstone). Some oolities have quartz grains as nuclei. Hydrochemical analysis shows that the total dissolved salts of extracted solutions of the North Coast quarry samples range from 539 to 686 ppm and dramatically increase (i.e., ten times) for extracted solutions from monument samples, ranging from 5395 to 6880 ppm. The dominant cation is sodium while the dominant anion is chlorine. Acid insoluble residue analysis shows that the carbonate content ranges from 89.2% to 96.4% for fresh samples from quarries and from 9.2% to 94.8% for weathered monument samples. The weight loss of the quarry oolitic limestone samples range from 30.7% to 32.7% and its physical and mechanical properties become worse after being subjected to 15 cycles of a durability simulation soundness test (using a sodium sulphate solution). Our main recommendations are to use suitable grouting for binding the cracks, high durability reconstruction rocks, and suitable cleaning methods to remove salts from the monuments.     

L'érosion chimique et l'érosion mécanique à long terme du substrat granitique (Vendée,France)

We calculate the chemical depletion fraction of the granitic bedrock by analysing the rock-soil enrichment of zirconium. In Vendée (France), chemical weathering rates account for 26% of the denudation rates. Such a chemical depletion fraction characterizes temperate regimes. It is three times lower than that of humid tropical regimes. To cite this article: J.-C. Maurin et al., C. R. Geoscience 337 (2005).     

Sulfuric acid as a weathering agent of carbonate weathering constrained by δ^13C： Examples from Southwest China

Rock weathering by carbonic acid is one of the important atmosphere CO2 sequestration. Actually, it depends on whether carbonic acid or other acids as weathering agents, which is important to understand the model of global carbon cycle. For example, sulfuric acid derived from oxidation of pyrite takes part in the rock weathering, which might counteract one part of CO2 drawdown by silicate weathering. In this study, chemicals and carbon isotopic composition of waters were determined in the Beipan River, Guizhou. The δ^13C values of dissolved inorganic carbon in the province, Southwest China. The values of the samples range from -13.1‰ to -2.4‰, which show a good negative correlation with the equivalent ratio of [HCO3]/（[Ca^2＋＋Mg^2＋]） and a good positive correlation with the equivalent ratio of [SO4^2-]/（[Ca^2＋＋Mg^2＋]） and [SO4^2-]/[HCO3^-]. The relationships suggest that sulfuric acid took part in carbonate weathering. Acid rain is thus a significant source of sulfuric acid to the karstic rivers of Guizhou Province.     

An index for describing the core-stone shape in weathered columnar joints

The shape of core-stones remained in the weathering basalt column is an index to classify its weathered degree. In this paper, a roundness index (RI) like the box dimension using in fractal theory, was employed to quantitatively measure the shape of core-stone in various weathering grades. The rounded core-stone that is weathered seriously has a smaller RI value. According to the value of RI, the five classes shown in the Russell–Taylor–Pettijohn chart for visually classifying grain shape is available to approximately determine the weathered grades of core-stones remained in the basalt column in Taiwan. A slump test is preliminarily tested to show the difference in repose angle of a stability slope assembled by the grains in a similar shape. The collection of angular core-stones those are less weathered (with a higher RI value) indeed reveals a greater repose angle.     

Barium in Deccan Basalt Rivers: Its Abundance, Relative Mobility and Flux

The concentration of dissolved Ba in a number of rivers having their drainage almost entirely in Deccan Trap basalts has been measured. These results along with available data on the abundances of major elements in these waters, and on Ba and major elements in bed sediments of these rivers provide a measure of (i) the relative mobility of Ba during chemical weathering and erosion of basalts, particularly with respect to alkaline earths, Mg, Ca and Sr, and (ii) the flux of Ba out of the Deccan and its global significance. The concentration of dissolved Ba ranges from 8 to 105 nM. The average Ba/Mg*, Ba/Ca* and Ba/Sr (* is concentration corrected for atmospheric contribution) in waters is lower than the corresponding mean ratios in Deccan basalts, though they overlap within errors. Majority of the water samples, however, have ratios less than that in basalts. These findings can be interpreted as a cumulative effect of limited release/mobility of Ba during chemical weathering and erosion of basalts and its reactive behaviour in waters which promote its association with clays and oxy-hydroxides of Fe. These results also indicate that during chemical erosion of Deccan basalts, Ba is the least mobile among the alkaline earth elements. The abundance of Ba in sediments and their Ba/Al ratios relative to basalts are consistent with the above conclusion. Ba/Mg and Ba/Ca ratios in water and in sediments from the same location are strongly correlated; however, the mean ratios in waters are far less than those in sediments. This is a result of limited Ba mobility, effectively 5–6 times lower than that of Mg. The annual flux of dissolved Ba out of the Deccan Traps is ~1 × 10⁷ moles, ~ 0.2% of its global riverine transport to oceans. The contribution of dissolved Ba from Deccan Traps, seem lower than its aerial coverage, ~ 0.5% of the global drainage area; the potential causes for this could be the lower abundance of Ba in basalts relative to “average continental crust”, and its behaviour during chemical weathering and erosion.     

Carbonate weathering rates in the Jura Mountains, France-The influence of vegetation

In order to better understand what controls carbonate weathering rates, we report results from the Jura Mountains （East France）, an area exclusively composed of carbonate rocks. This region presents an altitude gradient increasing from 250 m to 1300 m. Over the basin, this gradient generates climatic contrasts of 5℃ and a runoff three times higher in altitude. This place offers a good opportunity for understanding the controlling factors of carbonate weathering. Contrary to thermodynamic calculations that predict the highest concentrations at low temperature, we observe that carbonate dissolution is two times higher at low elevation than in the mountains. This observation can only be explained by a variation in soil pCO2. In order to better constrain the observed variation in cation contents （principally Ca and Mg） of rivers, we used the ASPECTS ecological model （Rasse et al., 2001）. Based on both hourly climatic data, vegetation type and soil type, ASPECTS may reconstruct pCO2 in soil.     

Effect of Aspergillus niger on weathering of phosphorite rock

Weathering mechanism of phosphorite rock by Aspergillus niger has been studied. Ten strains of phosphate-solubilizing fungi were isolated from rhizosphere soils by using dilute plate method in solid inorganic phosphorus culture. According to the size of soluble phosphate circle in the medium, a strain of stronger phosphate-solubilizing fungus was acquired, which was identified as, Aspergillus niger using identification methods of morphology and ITS sequences comparison in Genbank. In liquid culture, incubation in static state, the weathering of apatite rock with the fungus by contrast method： putting apatite powder （200 mesh） in liquid culture medium of flask, inoculation fungus after autoclave （setting sterile control） for studying direct weathering process; putting dialysis bag with apatite powder （200 mesh） in liquid culture medium of flask, inoculation fungus after autoclave （setting sterile control） for studying indirect weathering process.     

Coeval biochemical and biophysical weathering processes on Quaternary sandstone terraces south of Rabat (Temara), northwest Morocco

Despite numerous investigations on substrate‐inhabiting microflora, especially lichens, very little is known about the colonization of coastal escarpments by lithobiontic micro‐organisms, inland of a retreating coastline in Africa. Reported herein are the results of a combined field observation and microscopy study focusing on the connection between microrelief of the substrate, colonies of lithobiontic micro‐organisms (in particular the lichen Xanthoria parietina) and microstructures of putative bacterial origin. The occurrence of weathering pits in which the early stages of the biotic development occurs, and the subsequent disintegration of the rock indicate that lichens, mosses and fungi act synergistically by alternating chemical and mechanical weathering. Penetration of grains by expansion and contraction of the hyphae depletes the rock matrix and contributes to the mechanical breakdown of the rock. Calcite rhombs on the weathered surfaces of the calcite‐cemented sandstones are severely etched with well‐developed rhomb‐shaped etch pits (‘spiky calcite’), holes, or has one or more of the faces removed, and their cores exposed and leached. Nanofilaments (c. 100–700 nm) and ‘nanomicrobial’ fruiting bodies (c. 250 nm) emanating from micropores appear to be common on affected crystalline structures. Weddellite present immediately below the thallus is a strong indicator of biomineralization. Quartz responds differently to chemical weathering by producing peeling structures and microbrecciation features. The dissolution of these crystals appears to be a surface reaction‐controlled process mediated by microbial microfilaments and nanofilaments. A model is proposed, firstly indicating early‐stage biochemical weathering, followed by biophysical weathering. Disintegration of the rock outcrops in due to a complex interplay of several events, probably beginning at the nanoscale with penetration of sites on crystal faces. Copyright © 2006 John Wiley & Sons, Ltd.     

Using Rock‐Eval 6 pyrolysis for tracking fossil organic carbon in modern environments: implications for the roles of erosion and weathering

This work relates to the debate on the fossil organic carbon (FOC) input in modern environments and its possible implication for the carbon cycle, and suggests the use of Rock‐Eval 6 pyrolysis as a relevant tool for tracking FOC in such environments. Considering that such a delivery is mainly due to supergene processes affecting the continental surface, we studied organic matter in different reservoirs such as bedrocks, alterites, soils and rivers in two experimental catchments at Draix (Alpes de Haute Provence, France). Samples were subjected to geochemical (Rock‐Eval 6 pyrolysis) investigations and artificial bacterial degradations. After comparing the geochemical fingerprint of samples, geochemical markers of FOC were defined and tracked in the different reservoirs. Our results confirm the contribution of FOC in modern soils and rivers and display the various influences of weathering and erosional processes on the fate of FOC during its exchange between these pools. In addition, the contrasting behaviour of these markers upon the supergene processes has also highlighted the refractory or labile characters of the fossil organic matter (FOM). Bedrock to river fluxes, controlled by gully erosion, are characterized by a qualitative and quantitative preservation of FOM. Bedrock to alterite fluxes, governed by chemical weathering, are characterized by FOC mineralization without qualitative changes in deeper alterites. Alterite to soils fluxes, controlled by (bio)chemical weathering, are characterized by strong FOC mineralization and qualitative changes of FOM. Thus weathering and erosional processes induce different FOM evolution and affect the fate of FOC towards the global carbon cycle. In this study, gully erosion would involve maintenance of an ancient sink for the global carbon cycle, while (bio)chemical processes provide a source of CO₂. Finally, this study suggests that Rock‐Eval 6 pyrolysis can be considered as a relevant tool for tracking FOC in modern environments. Copyright © 2006 John Wiley & Sons, Ltd.     

黄土高原南缘午城黄土底部火山碎屑的来源和成因

一种与多类撞击记录共存的灰黑色碎屑，发现于黄土高原南缘段家坡黄土地层剖面午城黄土底部Ｇａ／Ｍ边界（２．４８ＭａＢ．Ｐ．）附近，通过对此碎屑的矿物学和地球化学特征研究，表明它们最初是源自青藏高原北部及其邻近地区碱性玄武质火山喷发的碎屑物，后来又在表生条件下与黄土层一起经历了包括水解、成土化和氧化等作用在内的风化改造。此外，本文更为强调的是，在火山碎屑形成过程中存在具有富Ｎｉ和Ｃｅ，高Ｓｍ／Ｎｄ比以及低Ｅｕ／Ｓｍ比等特点的地外物质的参与，据以论证这种火山活动确系Ｎ／Ｑ界面撞击事件引发。在黄土地层中，深入研究各种类型的火山碎屑，无疑对全面探讨第四纪环境变迁有着十分重要的意义。关键词午城黄土底部松山－高斯边界火山碎屑风化作用地外物质新近的研究表明，第四纪气候和环境变迁往往与起因于撞击和火山事件的大量尘埃物质有关［１］，而地外物体的撞击又常能引发火山活动［２、３］，因此第四纪沉积物中的火山喷发空落堆积物研究，无疑应该引起更大的关注。火山学发展至今，对各种火山碎屑物质，已有十分详尽的研究［４、５］。从国内第四纪火山碎屑的研究现状来看，虽然丁国瑜等早在１９６４年就曾在河北平原及太行山东麓的黄土地层中，首次发现了火山碎屑堆。