A paleoecological study of a one-hundred year sedimentary record of Galveston Bay, Texas
Yuill, Richard Matthew
Fisher, Frank M.
Doctor of Philosophy thesis
Paleoecological and geochemical methods were used to study changes in Galveston Bay for the period 1850-1988. The purpose of the study was to: (1) examine foraminiferal species distribution in space and time, (2) measure changes in sediment geochemistry, and (3) determine if human actions have measurably affected the distribution of foraminifera. During 1987-1988, gravity cores were collected, examined by x-radiography for evidence of disturbance, and subsampled at various depths. Sediment accumulation rates were determined by lead-210 analysis. Subsamples were analyzed for particle size, TOC, foraminifera, and selected chemical elements including Al, Ba, Ca, Cd, Cr, Cu, Fe, Mg, Na, Ni, Sn, Sr, V, and Zn. Foraminifera were identified to species and enumerated as percent relative abundance. Barium concentration was depth-dependent for a number of cores. A regression of year before present against Log10 of barium was used to establish the chronology of additional cores. There were two significant geochemical trends: (1) a high correlation of the metals, including Al, Cr, Cu, Fe, Mg, Na, and V, to percent silt and clay, and (2) high intercorrelations between metals, especially to Al and Fe. Barium was poorly correlated to percent silt and clay and to metals. Thus, adsorption onto silt and clay particles and co-precipitation of metals with iron and aluminum hydrous oxides are apparently two of the dominant processes controlling the fate of metals in Galveston Bay. Dominant foraminifera in Galveston Bay were Miliammina fusca, Ammotium salsum, Ammonia parkinsoniana, and Elphidium spp. The species composition of individual subsamples generally fit into one of two biofacies described for Gulf of Mexico estuaries: (1) the Miliammina-Ammotium biofacies, generally confined to low salinity and (2) the Ammonia-Elphidium biofacies, dominant in the middle and lower estuary. Results showed a temporal shift from Miliammina-Ammotium to the Ammonia-Elphidium. These shifts occurred in the late 1800s in lower and middle Galveston Bay and as recently as the 1970s in Trinity Bay. This stepwise progression of a high-salinity biofacies further into the estuary coincides with dredging in the Houston Ship Channel. The pattern and timing of the species shift supports the hypothesis that dredging of the Houston Ship Channel has been a major contributing factor to salinity intrusion in Galveston Bay.