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In many states of central Mexico, hardened volcanic materials called tepetate (hardened layers from tuff) cover large proportions of the land surface, and are therefore of economic and social importance. Under natural conditions, tepetates are unproductive due to their hardness and low porosity, but they can be used agriculturally after the hardened layer is broken up and fertilizer and organic matter are added. Just broken up tepetate consists of angular clods of the original consolidated material of various sizes and they are devoided of aggregates. Because of the scarce presence of organic matter, these fragmented tepetates present low availability of N and P, limited water infiltration, and small water holding capacity. Additions of manure and cultivation of Leguminosae aim to improve the physical properties of tepetates and particularly to promote the formation of aggregates. Such ameliorated tepetates alleviate the demand for agricultural land. The aim of this study was to evaluate the formation of aggregates from initially broken up and fragmented tepetates, their dry and wet stability, and the distribution of organic carbon in aggregates and fragments of different sizes in tepetates cultivated between 1 and 100 years. The tepetates were arbitrarily classified as (I) recently broken up; (II) affected by severe erosion; (III) cultivated mostly under monoculture; (IV) cultivated with Leguminosae for at least 20 years and receiving approximately 2 Mg·ha-1 of farm yard manure (FYM) per year; (V) cultivated under no-tillage and few external inputs; (VI) cultivated with cereals and Leguminosae and receiving more than Mg·ha-1 per year of FYM; (VII) Used as greenhouse bed; (VIII) cultivared with no-tillage and approximately 5 to 10 Mg·ha-1 of FYM; (IX) used as greenhouse bed and addition of approximately 2 Mg·ha-1 of composted wood residues per year. In addition to the above mentioned tepetates, two agricultural soils: one managed with traditional tillage (X) and a second with no tillage (IX) were included as checks. We collected 97 samples of tepetates and four soils in the Basin of the Río Texcoco and neighboring communities. The aggregate stability test overestimated stable aggregates due to the presence of remaining hard fragments of tepetates of different size; the hardness of the latter was one to three times greater than that of the aggregates. The relative amount of stable aggregates increased with the time of cultivation, approaching 80% after 100 years. The recently broken up tepetates contained only little of C, whereas stable aggregates showed one to three times larger concentration. The accumulation of C with time in tepetate fragments was small, in contrast to the aggregates formed in the same period. The accumulation of organic C was larger in smaller aggregate particles and followed logarithmic or potential models. The results of this study show that ameliorated tepetates have the potential to store carbon. However, the accumulated amount of C depends on the aggregate formation, which in turn is related with the agricultural management. Classes VII, VIII y IX produced more stable aggregates and thus accumulated more carbon. 

 This paper summarizes mineralogical results from temperate, subtropical and tropical soil chrono- and climosequences and aims to understand the role of the factor time for the formation of kaolinites. A mid-Pleistocene welded F6-paleosol at Stari Slankamen, Serbia, developed discontinuously in a time span of 140 ka, shows much greater pedochemical weathering and clay mineral formation than Holocene loess soils: more than 40% of the feldspars and almost 80% of the micas are decomposed; mainly smectites dominate the fine clay fraction followed by illites, but no kaolinites have formed. In the Atlantic coastal region of Morocco during the first 100 ka only weathering of calcarenites to Rendzinas (Typic Calcixerolls) has taken place with little formation of 2:1 clay minerals. In a time span of several 100 ka, however, the direction of weathering goes towards strong kaolinite formation. Six “red soils” (Typic Dystrudepts to a Typic Hapludalf) in hyperhermic SW Nepal with 1,500–1,750 mm annual rainfall (five humid months), and a “black soil” (Vertic Haplustoll) near Borada, Gujarat, India with 930 mm annual rainfall were studied. Two of the “Red Soils” have thermoluminescence (TL) ages between 10 and 30 ka, the “Black Soil” has an age of about 10 ka. Surprisingly, little pedogenic clay mineral formation could be identified. The illites and kaolinites are mostly of detrital (inherited) origin. Only in the Vertic Haplustoll, a small increase mainly of smectites but no kaolinites were found, although the content of weatherable minerals is large. In South India, nine soils in a climatic sequence derived from saprolite of weathered granitic gneiss were examined for recent and relict features. Above a threshold of about 2,000 mm in an Udic Rhodustalf deep weathering is a recent process leading to the formation of kaolinites; above 2,500 mm in a Typic Rhodudult it leads also to gibbsite. In Aridic Rhodustalfs (three to one humid months), besides the formation of 2:1 clay minerals, still strong formation of kaolinites has taken place. However, this process has now almost ceased; instead, secondary carbonate is accumulating in the lower part of the profile proving that these Alfisols are relict soils, formed in an earlier period of much moister climate. However, according to our recent results from Morocco and Nepal besides a climatic change, the soil forming factor time is very important: strong pedogenic formation of kaolinites also in the seasonal tropics needs a longer time, probably some 100 ka.

The emergence of tepetates (hardened volcanic ash layers) on the surface after erosion of the overlying horizon is widespread all over the Trans-Mexican Volcanic Belt, and in particular in the State of Tlaxcala where it covers 15% of the state surface. The rehabilitation of tepetates can be a solution to the lack of arable land and to environmental problems related to surface runoff and soil erosion. The purpose of this investigation was to evaluate the effect of organic farming on erosion rates and to determine the relationship between runoff, soil loss, rain erosivity (EI30), soil organic carbon (SOC), vegetation cover (V) and aggregate stability (AS), by multiple regression analysis performed for individual erosive events (36) and for cumulative annual values.Erosion and runoff rates were measured from 2003 to 2005 in five farmer’s plots (580 to 2,020 m2) of 3% slope inclination. Two plots were reclaimed and cultivated since 2002 and three since 1989. Three different management techniques were applied in plots reclaimed in 1989: “conventional”, “improved” and “organic”, corresponding mainly to increasing incorporation of organic matter. Soil loss in plots reclaimed in 2002 ranged from 8.6 to 19.1 ton×ha-1×year-1 under conventional management and from 5.5 to 14.1 ton×ha-1×year-1 under organic farming. In plots reclaimed in 1989 the erosion rates are up to three times smaller than in recently reclaimed tepetates, with soil losses ranging from 1.1 to 5.6 ton×ha-1×year-1 and no significant (P<0.05) differences between management techniques. For individual events, runoff and soil loss are significantly dependant on EI30, SOC and V (r2=0.73 and 0.54 respectively). For annual values, the model accounted for an even larger proportion of the variance (r2=0.94 and 0.87, respectively). Even moderate incorporation of fresh organic matter significantly increased aggregate stability in both recently and older reclaimed tepetates. However, the percolation stability index was not correlated to SOC nor to erosion rates as we expected. In recently reclaimed tepetates, organic management enhanced carbon accumulation and vegetation cover, reducing runoff and soil loss in relation to conventional management.

There is a good consensus about the age of the most primitive materials in the Solar System represented by carbonaceous chondrites. From these, a canonical age of the Solar System of 4567 Ma has been defined. Nevertheless, the age of the subsequent events that modified the accreted material that formed meteorites has been only partially constrained by geochronological methods. In this work, we report K-Ar and Pb-Pb ages for individual chondrules from eight Mexican chondrites: three H5 (Cosina, Nuevo Mercurio and Aldama), one LL5 (Tuxtuac), one L4 (Zapotitlán Salinas), one L5 (El Pozo), one L6 (Pácula), and one carbonaceous CV3 (Allende). Analysis were performed by using combined isotope dilution thermal ionization mass spectrometry and noble gas mass spectrometry. The purpose was to obtain some understanding of the thermal history of these meteorites and their parent bodies. The analyzed chondrules and matrix show a comprehensive K-Ar age range from the Solar System formation to 442 Ma, demonstrating that ordinary chondrites preserve an extensive record of disruption and accretion of their parent bodies. The large time span recorded in the analyzed meteorites suggests that some chondrules were able to maintain a closed isotopic system, whereas the whole rock and other chondrules did not. To explain these ages, we suggest that partial heating from impact events, presumably from protracted bombardment of the parent body surface that also affected the surfaces of the Moon and Mars, is responsible for resetting the K-Ar chronometer. It is remarkable that some of the studied chondrules show pre-solar ages similar to those reported for the Allende meteorite by the Ar-Ar method, implying that primordial bodies incorporated some kind of pre-solar materials, as suggested previously.

Runoff is one of the most important hydrological variables used in most of the water resources applications. Reliable prediction of runoff from land surface into streams and rivers is difficult and time consuming to obtain for ungauged basins. However, Remote Sensing (RS) and Geographic Information System (GIS) technologies can augment to a great extent the conventional methods used in rainfall-runoff studies. These techniques can be used to estimate the spatial variation of the hydrological parameters, which are useful as input to the rainfall-runoff models. The main objective of this study was to model the rainfall-runoff process in a selected ungauged basin for the purpose of groundwater artificial recharge. This model simulation was carried out using an hydrological modeling system assisted by GIS. Two model runs were carried out using precipitation data of the Intensity-Duration-Frequency (IDF) curves at Zarqa rainfall station for 10 years and 50 years return periods. With the first model run, the total direct runoff volume and the peak discharge for the 10 years return period were estimated to be 151,000 m3 and 5.43m3/s, respectively. For the 50 years return period, the total direct runoff volume and the peak discharge were estimated to be 280,000 m3 and 12.77m3/s, respectively. The model was optimized against observed runoff data, measured during a storm event that occurred between the 2nd and the 4th of April, 2006. The flow comparison graph indicates that the calibrated model fits well with the observed runoff data, with a peak-weighted root mean square error (RMS) of less than 2%. This calibration was performed by applying different curve numbers in the simulated model. It was possible to obtain a reasonable match between the simulated and the observed hydrographs.

The study was conducted in a subtropical mountainous area of the Sierra Sur de Oaxaca, southern Mexico. This region has been reported to have a high diversity of soils due to erosion and deposition by slope processes triggered by seismic activity and rainstorms. In this study, lithologic properties indicate that 18 of 24 soil profiles received sediments as result of slope processes (landslides, colluviation, accumulation of the material washed out by sheet and inter-rill erosion). The profiles showing sediment deposition on the surface were formed on all landforms, except steep backslopes, where the loss of material is more intensive. The effect of newly accumulated sediment was previously underestimated, and varies from slight modification of soil properties to complete burial of a soil that forces pedogenesis to begin from “time zero”. Sediment accumulation due to slope processes should be included into a general pedogenic model for the study area.

Soils are natural bodies. Their characteristics result from the interaction of meteorologi- cal conditions (climate) on outcropping lithospheric materials over long periods of time. Plant cover produces organic residues which are incorporated into the soil by numerous and diverse soil organisms. Landscape position determines if the new formed mineral and organic (humic) materials stay in place, or if they are transported down slopes and eventually accumulate on foothills, valleys or plains, or end as sediments in rivers, lakes or oceans. On stable landscape positions, soil formation progresses and the soil differentiates into several horizons with distinct properties that reflect the dominant soil forming processes. Soil forming processes include weathering of primary minerals and the neoformation of clay minerals and oxides, humification of organic matter and aggregation of primary soil particles, along with translocation of clay (clay illuviation) or of metal-organic complexes (podsolization), dissolution and precipitation of secondary carbonates, among several other processes... SEE FULL ARTICLE

  Preface Over the last three decades, studies of the Solar System have become a well-established research tradition at the Universidad Nacional Autónoma de México (UNAM). Investigation has been conducted in the fields of solar physics, asteroids and meteorites, but recently, lines of investigation have diversified to include the study of protoplanetary objects, as well as the evolution of the Earth and its biotic environment. In order to promote the interaction between different specialized groups working directly or indirectly on topics related to the study of the Solar System and the Earth, the Instituto de Astronomía, Instituto de Geofísica and Instituto de Geología, UNAM, organized the symposium “Origins and Structure of the Solar System”, which provided a forum for researchers and students to present and discuss the results of their investigations on planetary geology and geophysics, the nature of the Earth and... DOWNLOAD FULL PAPER.

The inactive tailings of the Santa Bárbara mining unit in northern Mexico were studied in order to evaluate the environmental impact on surrounding soils and sediments. Total concentrations as well as the concentrations of water-soluble, potentially toxic elements were analyzed to assess the potential for leaching and contamination. The near-neutral pH values of the oxidized tailings throughout the impoundments suggest that the dissolution of minerals with neutralizing potential was sufficient to consume the acidity released by metal sulfide oxidation. The tailings contain high total concentrations of Zn (1,762-21,059 mg·kg-1), Pb (972-16,881 mg·kg-1), As (410-3,281 mg·kg-1) and Cu (472-2,415 mg·kg-1). The total concentrations of the other elements studied were relatively low (Ba: 62-586 mg·kg-1; Cd: 10-239 mg·kg-1; and Se: 1-10 mg·kg-1) and even not detected by the analytical techniques in the case of the Cr, Ni, Hg and Ag. However, the concentrations of water-soluble, potentially toxic elements were low (Zn ≤ 2.0 mg·l-1, Pb < 0.07 mg·l-1, As ≤0.05 mg·l-1, Cu < 0.02 mg·l-1, Cd ≤ 0.2 mg·l-1 and Fe < 0.25 mg·l-1). Considering the near-neutral pH values and the low concentrations of potentially toxic elements available for leaching, we concluded that the potential for net acid generation was very low. The total concentrations of Zn, Pb, As, Cu and Cd in samples of soils and sediments located around the inactive tailing were greater than the natural background values determined for this study (background values: 974 mg·kg-1 Zn; 947 mg·kg-1 Pb; 190 mg·kg-1 Cu; 342 mg·kg-1 As; and 28 mg·kg-1 Cd ). These results indicate that the soils and sediments have been contaminated by dispersed tailings. However, the concentrations of water-soluble, potentially toxic elements in all samples of soils and sediments were low: Cu ≤ 0.16 mg·l-1 and Cd ≤ 0.005 mg·l-1. For the case of Pb, in 39 samples of a total of 47 (included superficial and subsuperficial samples), concentrations below the detection limit (0.07 mg·l-1) were registered, and only eight samples had water soluble Pb concentrations between 0.1 and 0.89 mg·l-1. The concentrations of water soluble As varied between 0.0005 and 0.65 mg·l-1, and those of Zn varied between 0.02 and 2.85 mg·l-1. The low potential for leaching of potentially toxic elements from the contaminated soils and sedi- ments indicate that they do not represent a serious environmental risk under present conditions. This is corroborated by the low concentration of potentially toxic elements found in shallow groundwater, values that did not exceeded the limits for drinking water.

In this work, the processes and products involved in the generation of acid rock drainage – metal leaching (ARD-ML) from mine waste material (tailings) derived from the exploitation of an ore type Pb- Zn-Ag skarn were characterized. Laboratory tests (static and kinetic) of historic and recent tailings were conducted along with the mineralogical characterization of solids, and chemical analyses of solids and leachates. Pyrite (FeS2) is the most abundant sulfide phase, and one of the main minerals promoting ARD- ML generation, followed by pyrrhotite (Fe1-xS) and arsenopyrite (FeAsS). Carbonates are present as calcite (CaCO3) and lesser amounts of ankerite [Ca(Fe,Mg)(CO3)2]. Also, smaller concentrations of quartz and feldspar were identified. Results of the static tests (acid base accounting: ABA) indicate that both, recent and historic tailings, have a likely ARD-ML potential (neutralization potential, NP <1). Kinetic tests were performed in humidity cells to simulate the natural oxidation of primary mineral samples; the obtained leachates had circum-neutral pH values (4.5–7.9) and relatively low heavy metals contents. Nevertheless, oxidation-neutralization curves suggest that oxidation capacity in both sample types is higher than their neutralizing capacity, and that sulfide oxidation is more important in historic than in recent tailings. The obtained information, along with statistical tests (F ratio and t-student), demonstrated that the tailings have a strong capacity to generate ARD-ML, and that the particular characteristics of the historic and recent mine waste materials created different conditions for the generation of ARD-ML.