The geochemistry of iron in Recent tidal-flat sediments of the Wash area, England: a mineralogical, Mössbauer, and magnetic study

Abstract

Undisturbed core samples of Recent sediments from the Wash tidal flats, East Anglia, England, obtained using a Delft corer, were studied with special reference to the diagenesis and geochemical behaviour of iron. The Mössbauer effect in 57Fe was used to monitor the distribution of Fe between different phases as a function of depth, together with the magnetic mineralogy and palaeomagnetic properties. The cores consist of, successively downwards: 0.36 m brown clay; 1.5 m finely laminated silts and fine sands, and 7.14 m homogeneous fine sands. The dominant minerals are quartz, feldspar, calcite and clay minerals, and chemical analysis for Al, Si, Mg, Mn, Ca, Fe, Na, K showed variations closely linked to lithological changes. Illite is the most abundant clay mineral (mean 48%), followed by mixed layer illite-montmorillonite and montmorillonite, kaolinite and chlorite. Chlorite is the major iron-bearing clay mineral and represents 4 to 10% of the <2 μm fraction throughout the core. Sulphide minerals are present throughout the core, including framboidal pyrite. Computer fit analysis of the Mössbauer spectra of best quality showed contributions from Fe 2+ and Fe 3+ in clay minerals (essentially chlorite), low-spin Fe 2+ in pyrite, and magnetically ordered iron in greigite (Fe 3S 4). Systematic variations, as a function of sample depth, indicate a relative increase in the amount of Fe in pyrite at the expense of the clay minerals. Magnetite and titanium-bearing magnetite are the carriers of natural magnetic remanence in these sediments. The direction and intensity of natural remanence in the samples compare well with the known secular variation of the Earth's magnetic field derived from the historic-archaeomagnetic record and this enables the samples to be dated and sedimentation rates to be determined (1.5 mm yr −1 for the upper 2 m and ~7.7 mm yr −1 for the lower 7 m).