engleski

Estuarine sediments from boreal region - an indication of weathering

Sediments from Öre estuary (North Sweden were studied in details to characterize weathering products of silicate rocks. This estuary was chosen as a model for other antropogenically unpolluted estuaries in boreal region, in which both physical and chemical weathering products could be observed. The primary rocks are composed mostly of granite and granodiorite. Chemical analysis was performed by XRF method. Organic matter was characterized as total carbon, hydrogen and nitrogen. IR spectra were not particularly useful, due to the presence of different minerals, but have confirmed the presence of organic matter. Thermal behavior was studied using TG and DTA techniques. Mineralogical composition was studied by XRD method. The minerals found were quartz, alkaline feldspars, plagioclases, biotite, chlorite and hornblenda. In silt and clay fraction < 32 ?m some weathering products were detected, such as montmorillonite-15A, hematite and Al(OH)3 in trace quantities. Since iron is substantially present in sediments and most of trace elements are in very good correlation with iron, additional characterization of iron compounds was performed by 57Fe Mössbauer spectroscopy. Measured at room temperature, Mössbauer spectra do not display magnetic ordering. In only one sample, closest to the river mouth, better fit was obtained adding one sextet pattern, but it was of minor significance. Spectra were fitted with three to four doublets, showing different isomer shifts (?) and quadrupole parameters (?). Relative amounts of Fe(II) and of Fe(III) was determined in each sample. However, it was not possible to distinguish Fe(II) from biotite and chlorite, as they have similar parameters. Fe(II) was not observed in M1 site. It was observed that the amount of Fe(III) increases with the distance from the river mouth. It can be retained in the silicate sites or precipitated as amorphous iron oxide, what could not be distinguished. Mössbauer spectroscopy indicates that studied sediments are mostly formed from metamorphic rocks on the basis of the presence of altered biotite and chlorite. The results of this work prove that weathering can be studied in sediments and that some of the reactions are connected with consumption of CO2 from atmosphere.

boreal region; estuarine sediments; silicate rocks; weathering

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