Iron in a post-glacial lake sediment core; a Mössbauer effect study

The distribution of iron in a 6-m core of post-glacial sediment from an oligotrophic lake (Connistonwater, England) was determined, principally by Mossbauer spectroscopy on dried samples. The immediate post-glacial deposits contain 4.8 wt. % of iron, with a Fe²⁺; Fe³⁺ ratio ～- 4. The iron there is predominantly in the form of chlorite, but there are small amounts in hematite and illite. The distribution of iron is different, and very variable in the recent sediments (~ < 13,000 BP), which contain 25–35 per cent organic matter and 5.2 wt. % of iron relative to the inorganic fraction. Typically half the iron is present there as chlorite, and the rest is ferric, mostly in the form of an amorphous gel which is also present in undried samples. To explain the observed ferrous:ferric profile, it is proposed that the latter includes iron which was once mobile, having been leached from the Fe²⁺-bearing clays under reducing conditions in the soils of the drainage basin, or in the sediment itself. It was subsequently precipitated as ferric hydroxide on contact with the oxic lake water. In contrast, the ferrous iron in the sediments is immobile iron, which remained locked in the chlorite phase of the clay particles as they were carried from soil to sediment intact.All the sediments are rather inhomogeneous. Chlorite, and especially hematite, are mechanically concentrated in pink varves in the immediate post-glacial deposits. In the partly inorganic sediments, the concentration of ferrous iron (chlorite) is approximately uniform, but the ferric content may differ by a factor of five between regions only a few millimeters apart.