Biogeochemical Control on Metal Distribution and Accumulation in Louisiana Sediments

Abstract

AbstractMetal accumulation rates were determined from sedimentation rates and the distribution of 137Cs and total metal content in cores collected from freshwater, brackish, and saltwater bay bottoms in Barataria Basin, LA. Vertical accretion rates ranged from 0.65 cm/yr in the northern freshwater part of the basin up to 1.1 cm/yr in the lower saline estuary. Metal sedimentation and accumulation rate estimates along this salinity gradient indicated that metals were concentrated by a factor of two to four in the saline end‐member. Manganese, however, exhibited an estimated depletion of 1.95 g/(m2 yr) in the lower part of the basin, due to its postdepositional mobility. A principal component and regression analysis of total metal distribution showed that the metal content could be predicted by the C content of the substrate (R = 0.89**; significant at the 0.01 probability level). This suggested the sedimentation of metal‐rich organic matter, most likely originating from adjacent marshes. Clay and silt content were insignificant as predictor variables. In the subsurface layer (0–4 cm), 18% of the variability in Fe and 27% in the variability in Mn content were attributed to redox processes, and were adequately predicted by the redox potential of the sediment (R = 0.60*; significant at the 0.05 probability level). Partial metal partitioning in soluble, exchangeable, and reducible fractions indicated that Fe, Al, Cu, Zn, Pb, and Cd held in the water soluble and exchangeable phase represented a minor part of the total metal content. However, water soluble plus exchangeable Mn accounted for 19,10, and 1% of the total Mn in freshwater, brackish, and saltwater environments, respectively. Iron and manganese oxides were found to scavenge metals to 50% of the total heavy metal content.