PALYNOLOGIC DETERMINATION OF HISTORICAL PALEOECOLOGICAL VARIATION IN MARSHES ON THE ST. JONES RIVER, DELAWARE, USA

Analysis of pollen assemblages from three cores from tidal marshes along the St. Jones River in Delaware shows major shifts in vegetation over the last 1000 years. Variations in pollen assemblages reveal natural changes in the plant community and anthropogenic changes related to European settlement. This study combines pollen analysis with previous work on lithostratigraphy and local history to assess the impact of post-settlement land-use practices on sedimentation and ecological conditions. Prior to European settlement in the mid-17th century, Delaware was thickly forested with mixed conifers and hardwoods. Clear-cutting for agriculture and fuel wood supply quickly denuded the area, with 20th century recovery stalled by suburbanization. The pollen record of the cores reflects a shift from a forested setting to one with a high proportion of open areas. The lower, pre-settlement parts of the cores are dominated by Quercus, with abundant Pinus and Carya and common Graminae and Cyperaceae, indicating highly forested conditions near the marsh. The upper, post-settlement intervals are rich in Quercus, with common Pinus and Carya, but abundances of Compositae are conspicuously higher, indicating increased amounts of open spaces, and decreased forest cover. These shifts place the European agricultural horizon at depths from 2.10 m to 2.20 m in the supratidal cores and 3.45 m in an intertidal core. The agricultural horizon corresponds to the date when more than 40% of land was cleared, which was about 1680 in Delaware. Age estimates based on this horizon and radiocarbon dates from previous work indicate that sediment accumulation rates increase by approximately 4.5 times above the agricultural horizon. Increased rates were primarily an effect of decreased forest cover. The pollen record also indicates some variations in river salinity that may serve as a proxy for rainfall. The paleoenvironmental history of the St. Jones River basin over the past 1000 years is one of increasing but pulsed disturbance of natural systems with increasing human population. Combined use of pollen analysis and lithostratigraphy enabled a more detailed interpretation of environmental changes than previous studies of lithofacies alone.