Two-Dimensional and Fine Structure in the Concentrations of Iron and Manganese in Sediment Pore-Waters

Abstract

The technique of diffusive equilibration in thin films (DET) was used to measure sediment pore-water concentrations of dissolved iron and manganese at high-spatial resolution (1 mm). Measurements in a laboratory microcosm of homogeneously mixed sediment gave reproducible concentration-depth profiles with no fine scale features. By contrast, measurements in undisturbed surface sediments showed fine structure that was only occasionally reproduced in profiles from several replicate field deployments. Although redox-related features in the profiles, indicative of processes traditionally considered to occur with lateral uniformity, were generally present, the details observed in the replicate deployments were different. DET was modified to allow horizontal as well as vertical measurement of pore-water solutes. The resulting 2D images of the distribution of iron and manganese concentrations in pore-waters showed systematic vertical and horizontal structure. The individual component vertical profiles of the 2D measurement were only 3 mm apart and yet they were sometimes quite different from one another. These findings suggest that (i) microniches play a major role in metal remobilization, (ii) results from single vertical profiles may not be representative, and (iii) caution should be exercised when estimating exchange fluxes from vertical concentration gradients.