The Transport of Heavy Metals by the Mississippi River and Their Fate in the Gulf of Mexico
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The Transport of Heavy Metals by the Mississippi River and their Fate in the Gulf of Mexico (May 1977) John Harold Trefry III, A.B., Syracuse University; M.S., Texas A&M University Chairman of Advisory Committee: Dr. B. J. Presley Significant differences in total and leachable concentrations of some metals were observed between Mississippi River suspended matter and Gulf of Mexico sediments. These variations are related tc diversity in Gulf sediment redox conditions, accumulation rates and grain-size distribution. Suspended particulates transport >90% of the Mississippi River metal load, hydrous oxides and aluminosilicate lattices are the primary phases of particulate metal transport. In general, total and leachable metal concentrations in river suspended matter are seasonally and spatially uniform. During low river flow, an eight-fold increase in the organic content of the suspended matter brought about a. decrease in particulate Fe and A1 values and a marked increase in Mn, Zn, Cu and Cd concentrations. Average Si02, AI2O3 and K2O concentrations in Mississippi River particulates are similar to crustal abundances. Particulate Na20, CaO and MgO values are 60-80% below crustal levels; however a high river dissolved load of these elements offsets the low particulate levels. No significant change in the total or leachable heavy metal concentrations of river suspended matter was found across the freshwater-seawater interface. Mississippi Delta sediments have 20-^0% lower Mn, Cu, Co, Ni and Zn concentrations and metal/Al ratios than river particulates which is the direct result of a decrease in oxide-phase metal content. Iron, Cr, V and A1 concentrations and grain-size distribution in suspended matter and sediments are similar. Lead and Cd concentrations in delta sediments are 70 and 200% respectively above natural levels. The onset of this Pb and Cd pollutant input is traced to the late 1800's. Reducing conditions in delta sediments induce a loss of oxide- phase Mn, Fe and perhaps other metals via reduction-diffusion. Interstitial water Mn gradients,, in particular, support such a mechanism. Calculated fluxes of dissolved Mn from the sediments to the overlying seawater range from 200 to >1000 yg Mn cm ^ y ^. This process occurs over at least 1000 km of the delta where sedimentation rates are >0.5 g cm ^ y ^ as determined by Pb-210 dating (SMOKES, 1976). Gulf of Mexico outer shelf and slope sediments which accumulate at <0.5 g cm ^ y ^ are characterized by surficial Mn-rich zones (2000-8000 yg Mn/g) and complementary interstitial Mn profiles. Manganese, Cu, Co and Ni concentrations are high in deep Gulf sediments relative to river particulates. Observed metal enrichment is most striking in the strictly pelagic Sigsbee Knoll sediment where Cu, Co and Ni concentrations (ratioed to Al) are '^50% above river particulate levels and Mn concentrations are almost triple river values. Little change in average Fe, Zn, Pb, Cr, and V concentrations occurs between nearshore and abyssal environments. Evidence of post-depositional remobilization of Mn, and to a lesser degree Ni, Fe, Cu and Co, is found throughout the deep Gulf. For example, surface layers of 1.5% Mn show up in slope cores. In abyssal areas, metal maxima are found at depth as a relict of past environmental conditions.
Trefry, John Harold (1977). The Transport of Heavy Metals by the Mississippi River and Their Fate in the Gulf of Mexico. Doctoral dissertation, Available electronically from