In Situ Collection of Diagenetic and Induced Oxyhydroxide Precipitates from Riverine and Estuarine Sediments

Abstract

The use of plastic sheets as collectors of diagenetically-formed oxyhydroxides from riverine and estuarine sediments was evaluated. In a range of materials tested in the laboratory, Teflon and polycarbonate were found to be the most efficient collector materials. Laboratory optimisation experiments indicated that the deposition of oxyhydroxides on both materials occurred quickly (<2 weeks). In the field, oxyhydroxides were collected on Teflon sheets as depositional bands just below the sediment-water interface. These bands were much wider and more variable (1–5 cm) than those observed in laboratory experiments (typically 0.5 cm). This was attributed to the frequently changing redox conditions at the field sites caused by a combination of sediment movement and tidal processes. The deposits collected were predominantly iron with lesser amounts of manganese. At several sites strong correlations were observed between the concentrations of manganese and zinc deposited on the Teflon collector surfaces. In both the laboratory and field experiments, biologically-mediated iron oxyhydroxide deposition in anoxic sediments was frequently observed (mainly caused by the activities of burrowing organisms). The mechanism of iron deposition on polycarbonate was investigated in detail. This plastic contains high concentrations of oxygen which is released over sustained periods, thereby inducing the deposition of iron oxyhydroxide even in anoxic sediments. It is proposed that polycarbonate may be used for investigating the impacts of oxygen introduction to anoxic sediments by processes such as bioturbation and oxygen release from aquatic plant roots.

Keywords:

• Iron oxyhydroxide
• estuaries
• polycarbonate
• Teflon
• sediment