Estuarine sediments from the boreal region: an indication of weathering

Ore estuary (Northern Sweden) was chosen as a model for other antropogenically unpolluted estuaries in boreal region. Sediments were studied in details to characterize weathering products of silicate rocks. The primary rocks are composed mostly of granite and gneiss. Chemical analysis of total sediments was performed by XRF method. In all samples SiO2 predominates (71.1 - 59.2%) decreasing seaward. Al2O3 (11.58 - 12.89%) and Fe2O3 (3.71 - 6.92%) increase seaward. Fourteen elements in fine silt + clay fraction (f < 32 ľm) were analyzed by ICP-AES method. From microelements the most abundant is Ti, then Mn, Zn, Ba, Cr, Sr, V, while Cu, Co and Ni are the least abundant. Organic matter was characterized as total carbon, hydrogen and nitrogen. Mineralogical composition was studied by XRD method. The minerals found were quartz, different minerals from feldspar group (albite, microcline, plagioclase, sanidine), biotite, chlorite and hornblende. In fine silt + clay fraction (f < 32 mm) some weathering products were detected, such as montmorillonite-15A and Al(OH)3. Amorphous iron hydroxide could not be detected by XRD method. Most of trace elements determined by ICP-AES (Co, Mg, Cr, Ni, Cu, Mn, Zn) are in very good correlation with iron. 57Fe Mossbauer spectroscopy was used for further characterization. Measured at room temperature, Mossbauer 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 splitting parameters. The only exception was spectrum of sample 1b fitted with five doublets and fraction f < 32 ľm of the same sample fitted with six doublets. Relative amounts of Fe(II) and of Fe(III) were determined in each sample and the ratio Fe(III)/Fe(II) was calculated. 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 might be retained in the silicate sites or precipitated as amorphous iron oxide. Mossbauer 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 confirm the hypothesis that sediments can be used for studying weathering. Some of the reactions, according to the literature, are connected with consumption of CO2. The examples of such reactions are weathering of K-feldspar, albite and hornblende.