THE USE OF NUCLEAR MICROPROBE TECHNIQUES TO STUDY THE CHEMISTRY OF LACUSTRINE SEDIMENTS AND PARTICLES

The Oxford SPM has been used in two novel studies of take chemistry: (a) The distribution of dissolved iron in sediment pore waters close to the sediment/water interface has been measured using the novel technique of diffusive equilibration in a thin film (DET). In this technique, which has a spatial resolution of < 1 mm, much less than that of competing techniques (I cm), a thin layer of polyacrylamide gel is inserted into the sediment and after the rapid equilibration with the pore water, the gel is dried and fixed. The distribution of trace elements can then be measured using microbeam PIXE. Preliminary results have shown for the first time a subsurface maximum of Fe consistent with current theories of Fe dynamics. This paper presents some results obtained using the technique and discusses the limits on resolution and sensitivity. (b) Individual suspended lake particles (predominantly iron oxides and sulphides) have been analysed using point microbeam RBS and PIXE. Of particular interest in this study is the oxidation state of iron rich particles, so RBS with a 1 mum beam was used to determine the Fe:0 stoichiometry of single particles. The particles were filtered from a depth of 14 m in Esthwaite Water in the English Lake District and handled in anoxic conditions until evacuation in the SPM sample chamber. Two distinct compositions of iron oxide were determined in clusters of about 5 mum diameter. Analysis by PIXE revealed that FeS was uniformly distributed in the particulate material and that it also contained elevated levels of Cu and Zn. This study was the first to demonstrate directly that discrete clusters of iron oxides are present in black particulate material which is commonly considered to comprise iron sulphides.