Lead-210 Geochronology and Trace Metal Geochemistry of Sediment Cores from Lake Overstreet and Upper Lake Lafayette, Leon County, Florida

摘要

Sedimentation rates and concentrations of major, trace, and rare earth elements in sediment cores from Lake Overstreet and Upper Lake Lafayette in Leon County, Florida were investigated. The constant initial concentration method of 210Pb dating was used to derive accumulation rates of 6.94 mm/yr for the Lake Overstreet core and 5.58 mm/yr for the Upper Lake Lafayette core. Historical profiles of Upper Lake Lafayette for Pb, Zn, V, Cr, Ni, U, Th, and Sc showed two maximum enrichments during 1929 and 1997. These represent two periods: (1) 1929–1983, where enrichment decreased with time, and (2) 1983–1997, where enrichment increased with time. In the Upper Lake Lafayette sediment core, vertical trends in V, Cr, Ni, Pb, Th, and U are similar to rare earth elements, suggesting co-migration and enrichment of these elements during diagenesis. This also may be attributed to their occurrences in heavy minerals. Concentrations of these elements and concentrations of As, Sb, P, and Y are significantly enriched above crustal abundances. The probable sources of these elevated metal concentrations are heavy minerals and phosphate of the Hawthorn Group within the catchment basin. The variations in concentrations of these elements are most likely the result of the input of detrital heavy minerals during different periods. Comparing Upper Lake Lafayette with polluted lakes in Florida indicates that the degrees of enrichment for Al, Fe, P, V, Cr, Ni, Zn, Mn, Ba, and Pb are higher and for Ti, As, and Cu are lower. In contrast, Lake Overstreet is considered a pristine lake in Florida, and the results of the analysis showed that most of the elements identified in their core sediments are present in low concentrations. Wet deposition from the drainage systems of the recharge area is the probable source.