In situ collection of diagenetic and induced oxyhydroxide precipitates from riverine and estuarine sediments

The use of plastic sheets as collectors of diagenetically-formed oxyhydroxi des from riverine and estuarine sediments was evaluated. In a range of mate rials tested in the laboratory, Teflon and polycarbonate were found to be t he most efficient collector materials. Laboratory optimisation experiments indicated that the deposition of oxyhydroxides on both materials occurred q uickly (<2 weeks). In the field, oxyhydroxides were collected on Teflon she ets as depositional bands just below the sediment-water interface. These ba nds were much wider and more variable (1-5 cm) than those observed in labor atory experiments (typically 0.5 an). This was attributed to the frequently changing redox conditions at the field sites caused by a combination of se diment movement and tidal processes. The deposits collected were predominan tly iron with lesser amounts of manganese. At several sites strong correlat ions were observed between the concentrations of manganese and zinc deposit ed on the Teflon collector surfaces. In both the laboratory and field exper iments, biologically-mediated iron oxyhydroxide deposition in anoxic sedime nts was frequently observed (mainly caused by the activities of burrowing o rganisms). The mechanism of iron deposition on polycarbonate was investigat ed in detail. This plastic contains high concentrations of oxygen which is released over sustained periods, thereby inducing the deposition of iron ox yhydroxide even in anode sediments. It is proposed that polycarbonate may b e used for investigating the impacts of oxygen introduction to anoxic sedim ents by processes such as bioturbation and oxygen release from aquatic plan t roots.