The Stability of Metal Profiles in Freshwater and Marine Sediments

The stability of sedimentary metal profiles (defined as the absence of significant changes in profile shapes or concentrations as the accumulated metals are progressively buried), or the ability to reconstruct the original profiles following dispersion by diagenetic processes, is a key requirement for the robust interpretation of metal deposition histories from these natural archives. Diagenesis is a common problem in the study of metals in freshwater and marine sediments, but its effects are difficult to generalize because they are metal- and site-specific. There are two types of diagenetic processes, both of which may be strongly influenced by benthic biological activity: (1) physical processes, involving the mixing of surface and deeper sediments by bioturbation or wind and wave action, which may affect all metals in upper sediment layers; and (2) geochemical processes which involve chemical reactions between certain metals in solid-phases and in porewater, the vertical redistribution of dissolved metals, and their precipitation/adsorption elsewhere in the sediment column. A minority of the metals of environmental interest, such as Hg and to a lesser extent Pb, are thought to be geochemically stable in most settings. Others, such as As, Cd, Cu, Mo, Ni, Re, U and Zn, are relatively sensitive to changes in oxic-anoxic (oxidation- reduction) conditions down-core and are often redistributed during burial. This review has several aims: (1) to describe the patterns and mechanisms of physical and geochemical diagenesis; (2) to describe three tests of metal profile stability; (3) to review environmental and geochemical factors that influence the occurrence and severity of diagenesis; (4) to present case studies that illustrate working approaches for correcting diagenetically-altered profiles so that part or all of the original history of accumulation can be reconstructed; and (5) to present practical recommendations concerning study site selection, and approaches to detecting diagenesis, which may assist in minimizing or at least identifying the severity of metal redistribution.