engleski

Clay mineralogy of soils developed on Miocene marl sections of Mt Medvednica, NW Croatia: Origin and transformation in temperate humid climate

Clay mineralogy of marl deposits and overlaying soil horizons (rendzina and brown soils after Škorić, 1977 and Bogunović et al., 1996) was investigated in the southeastern foothills of Mt Medvednica, Croatia. The marl was deposited during Miocene ages in various depositional environments (saline, brackish and fresh water). The overlaying soil profiles are poorly developed with A and C horizons usually present. C horizon is developed as a transitional horizon towards the underlying rock and often contains rock debris. Altogether nine marl-soil sections were investigated for discrete changes in clay mineralogy among and within sections. The mineral composition of the bulk samples was determined using X-ray diffraction (XRD). Carbonates, organic matter and Fe-Mn oxyhydroxides were chemically removed to separate clay mineral fraction. The residual was centrifuged and clay fraction was extracted for preparation of oriented mounts. The oriented mounts were air-dried, glycolated and heated at 400 and 550°C for half an hour, and in each step analyzed by XRD. As indicated by the position and shape of 10 Å peak, illite is present in all the samples. The varying constituents are smectite and interstratified illite/smectite. A regular pattern of occurrence was observed: pure smectite never occurs in the topsoil and is present in the transitional horizon and underlying rock. Interstratified illite/smectite is always present in the topsoil, and sometimes also in the underlying horizons. Smectite and illite/smectite were mutually distinguished largely by their behavior after glycolation Illite/smectite is randomly interstratified with illite component reaching maximally 30%, mainly in A horizon. Distribution of these two minerals seems to be mostly governed by stratigraphic position, topographic outline, and soil profile development. Chlorite and kaolinite are present as minor constituents in majority of the samples with exemption of the sections related to Lower Miocene marl deposits. Kaolinite is generally not present in the marl horizon of any section. Cation exchange capacity was determined using copper(II)-ethylenediamine complex. The results show a significant change of values upon removal of non-clay components from marl and especially soil. Two trends can be generally observed: a) in the bulk samples CEC values decrease with depth, and b) in the separated clay fraction CEC values increase with depth. For the bulk samples the CEC values are the highest in horizon A which is the only horizon with decreasing CEC values after removal of non-clay components. This indicates a strong contribution of organic matter to the bulk CEC values in top-soil horizon (SILVEIRA et al., 2002 ; VEGA et al., 2007). The increase of the CEC values with depth in clay fraction implies higher quantity of clay minerals with higher adsorption capacity ; in this case it can be related to the increase of smectite phase or smectite component in illite/smectite. Additionally, deblocking of adsorption sites due to the removal of Fe-Mn-Al oxyhydroxides also could play a role (WU et al., 1999). The measured CEC values correspond well to the observed clay mineral composition of the samples. The bedrock marls contribute as a source of expandable clays in overlying soil horizons. However, a portion of them could have been formed by transformation of other inherited clay minerals like illite or by precipitation from soil solution. The neoformation could be supported by slightly alkaline pH due to the presence of carbonates, an increased Mg activity provided by dissolution of carbonates, and poor drainage of the marl terrains (REID-SOUKUP & ULERY, 2002).

clay mineralogy, miocene marls

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