Sedimentology and geochemistry of fine-grained sediments in the Solent estuarine system

Abstract

Grain size distributions, clay mineralogy and trace metal distributions associated with fine-grained sediments are used to define the prevailing sedimentological conditions and to trace sediment transport pathways in the Solent Estuarine System. Grain size distributions are determined to define the variations in energy conditions throughout the System. The clay mineralogy of the samples analyzed (by XRD) indicates the presence mainly of illite, smectite, and kaolinite, with trace amounts of chlorite. Clay mineral ratios are calculated from their peak heights. The relative abundance of the clay minerals provides evidence of two sources of supply of sediments into the area: one is high in smectite, whilst the other is high in illite. It is likely, therefore, that the distribution pattern of the clay mineral ratios is consistent with the mixing of fine-grained sediments from two sources (a smectite-rich riverine source and a smectite-poor marine source) throughout Southampton Water and the East Solent. The effect of the mixing of the different sources is detected by decreases in the S/I and S/(K+ C) ratios.

Trace metal analyses are undertaken on total (bulk) samples and for the different grain size fractions. An association is identified between the concentrations of Co, Cr, Ni and Mn, and the fine-grained size fraction; they increase as the proportion of the clay-sized fraction increases. The concentrations of Cu, Pb and Zn are shown to be independent of grain size. Al is used to normalize the trace metal concentrations, in order to compensate for the natural variability of trace metals in the sediments and to undertake an intercomparison of sediments from the different localities. Normalization indicates that Cu, Pb and Zn are enriched anthropogenically in the upstream sections of the Rivers Itchen, Test and Hamble and in the bay-head locations of the harbours. The distribution pattern of total trace metal concentrations suggests, once again, physical mixing of two sources (riverine, with higher metal contents, with the lower marine contents), decreasing from landward to seaward. Irregular variations in the normalized metal/Al ratios are attributable to changes in the mixing ratio of marine and fluvial-derived sediments.

Finally, a conceptual model is proposed for the supply and mixing of fine-grained sediments within the Solent Estuarine System.

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