William Alexander Webster

Master of Science

March 21, 1997

(B. S., University of South Alabama, 1991)

Directed by Robert A. Gastaldo

The Mobile-Tensaw River Delta has formed as the result of sea-level rise into a Pleistocene river valley. The delta is comprised of swamp, marsh, distributary-channel, and estuarine environments which are integrally related to each other. Distributary- channel bifurcation sets the stage for interdistributary-bay development. Infilling of bays by overbank-flood and/or crevasse-splay deposits result in colonization by aquatic and semi-aquatic vegetation. Sediment baffling by these plants allows for establishment of marshes, and ultimately, swamps. This transition is represented by bioturbated muddy sands (open bayfill); organic-rich sandy muds (submerged aquatic vegetation communities); and heavily rooted muds (marshes and swamps). These environments were sampled using a vibracoring program in an infilled estuarine bay, Mudhole Creek and Little Bateau Bay, to identify metal-concentration trends in the subsurface of this environmentally sensitive area.

Samples were taken at regular intervals throughout the cores and analyzed by ICP (inductively coupled plasma) for 32 major and trace metals, and by LECO combustion techniques for percent total organic carbon (TOC). Samples were also taken for sediment grain-size analysis. Selected metals were then plotted using core to depth versus concentration in parts per million. Elemental concentrations across the study area were compared to determine if concentrations correlated with specific sediment facies, with each other, or with changes in percent TOC. Results show a series of increasing and decreasing trends in the metal concentrations upwards through the bayfill sequence. When metal trends within a single core are compared, each element follows a similar trend. When cores across the study area are compared, the metal concentrations and trends are similar. However, the overall trend for the metals is one of increasing concentration regardless of sediment facies, and is not dependent upon changes in percent TOC or decreasing mean grain-size of the sediment.

This study showed that metal increases in the subsurface occurred before local anthropogenic activity began near the study area. Based on the calculated age range of the increases, it is speculated that increasing metal concentrations upsection in the cores are the result of anthropogenic activity in the form of aerosol deposition from smelting operations in Europe rather than fluctuations in percent TOC or decreasing sediment grain size.

See Felton and Slone for related M.Sc. Theses.