Geochemical effects of rapid sedimentation in aquatic systems: minimal diagenesis and the preservation of historical metal signatures

Rapid sedimentation exerts a pronounced influence on early sedimentary diag enesis in that there is insufficient time for a sediment particle to equili brate in any one sediment layer before that layer may be displaced vertical ly by another layer. These sedimentation patterns are common in surface-wat er reservoirs whose sedimentation rates (1-10 cm yr(-1)) are several orders of magnitude greater than those for natural lakes (0.01-0.5 cm yr(-1)). Two examples of the effects of rapid sedimentation on geochemical metal sig natures are presented here. Interstitial-water data (Fe) from two sites in the Cheyenne River Embayment of Lake Oahe on the Missouri River illustrate the effects of changing sedimentation rates on dissolved species. Rapid bur ial during high-flow yrs appears to limit early sedimentary diagenesis to a erobic respiration. Solid-phase metal data (Pb) from a site in Pueblo Reser voir on the upper Arkansas River in Colorado appear to record historical re leases by flooding of abandoned mine sites upstream in Leadville, Colorado. Interstitial-water ammonia and ferrous Fe data indicate that at least one interval at depth in the sediment where solid metal concentrations peak is a zone of minimal diagenesis. The principal diagenetic reactions that occur in these sediments are aerobi c respiration and the reduction of Mn and Fe oxides. Under slower sedimenta tion conditions, there is sufficient time for particulate organic matter to decompose and create a diagenetic environment where metal oxides may not b e stable. The quasi-steady-state interstitial Fe profiles from Tidal Potoma c River sediments are an example of such a situation. This occurs primarily because the residence time of particles in the surficial sediment column i s long enough to allow benthic organisms and bacteria to perform their meta bolic functions. When faster sedimentation prevails, there is less time for these metabolic reactions to occur since the organisms do not occupy a sed iment layer for any length of time. Also, the quantity and quality of the o rganic matter input to the sediment layer is important in that reservoirs o ften receive more terrestrial organic matter than natural lakes and this te rrestrial organic matter is generally more refractory than autochthonous aq uatic organic matter.