Factors affecting landslides occurrence in agro-ecological zones in the Merek catchment, Iran

Part of the mountainous semi-arid areas of Iran is geologically and topographically susceptible to landslides. A study was conducted at the Merek catchment, Iran which is geologically characterized by limestone, sandstone, shale and marls so as to determine the factors responsible for the occurrence of landslides. In this study, geomorphological units within the catchment were mapped by overlapping geological, topographical, land use and erosion data, using satellite image and GIS. Rocks and soils were sampled within each unit to determine their physico-chemical and mineralogical properties. It was found that landslides have affected about 15% of the Merek catchment and that their occurrence were promoted by unstable soils, being eroded at the rate of 18.57 t ha⁻¹ yr⁻¹. These landslides were initiated by crack formation in areas having smectite, a mineral which can swell and contract, depending on the availability of water. Smectite is the dominant mineral in about 80% of the area affected by landslides. Low aggregate stability in the topsoil further promotes the occurrence of landslides. Field verification showed that landslides occur mainly in the forest where the canopy cover is less than 15% with slope of 10-40%. Deforestation, overgrazing and improper tillage practices were found to accelerate landslides in this catchment.     

Chemical Weathering of New Pyroclastic Deposits from Mt. Merapi （Java）, Indonesia

The Java Island, Indonesia with abundant amount of pyroclastic deposits is located in the very active and dynamic Pacific Ring of Fires. Studying the geochemical weathering indices of these pyroclastic deposits is important to get a clear picture about weathering profiles on deposits resulting from the eruption of Mt. Merapi. Immediately after the first phase of the eruption （March to June 2006）, moist and leached pyroclastic deposits were collected. These pyroclastic deposits were found to be composed of volcanic glass, plagioclase feldspar in various propoitions, orthopyroxene, clinopyroxene, olivine, amphibole and titanomagnetite. The total elemental composition of the bulk samples （including trace elements and heavy metals） was determined by wet chemical methods and X-ray fluorescence （XRF） analyses. Weathering of the pyroelastic deposits was studied using various weathering indices. The Ruxton ratio, weathering index of Parker, Vought resudual index and chemical index of weathering of moist pyroclastic deposits were lower than those of the leached samples, but the alteration indices （chemical and plagioclase） were slightly higher in the moist compared to the leached pyroelastic deposits.     

Preservation of organic matter in soils of a climo-biosequence in the Main Range of Peninsular Malaysia

Limited information is available about factors of soil organic carbon(SOC) preservation in soils along a climo-biosequence. The objective of this study was to evaluate the role of soil texture and mineralogy on preservation of SOC in the topsoil and subsoil along a climo-biosequence in the Main Range of Peninsular Malaysia. Soil samples from the A and B-horizons of four representative soil profiles were subjected to particle-size fractionation and mineralogical analyses including X-ray diffraction and selective dissolution. The proportion of SOC in the 250-2000 μm fraction(SOC associated with coarse sand) decreased while the proportion of SOC in the 53 μm fraction(SOC associated with clay and silt)increased with depth. This reflected the importance of the fine mineral fractions of the soil matrix for SOC storage in the subsoil. Close relationships between the content of SOC in the 53 μm fraction and the content of poorly crystalline Fe oxides [oxalate-extractable Fe(Fe_o) – pyrophosphate-extractable Fe(Fe_p)] and poorly crystalline inorganic forms of Al [oxalateextractable Al(Al_o) – pyrophosphate-extractable Al(Al_p)] in the B-horizon indicated the importance of poorly crystalline Fe oxides and poorly crystalline aluminosilicates for the preservation of SOC in the Bhorizon. The increasing trend of Fe_o-Fe_p and Al_o-Al_p over elevation suggest that the importance of poorly crystalline Fe oxides and poorly crystalline aluminosilicates for the preservation of SOC in the Bhorizon increased with increasing elevation. This study demonstrates that regardless of differences in climate and vegetation along the studied climobiosequence, preservation of SOC in the subsoil depends on clay mineralogy.