Palynologic determination of historical sediment accumulation rates and paleoecological variation in marshes on the St. Jones River, Delaware, USA
Date
2005
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Publisher
University of Delaware
Abstract
The analysis of pollen assemblage zones in three cores from salt marshes on the St. Jones River in central Delaware shows major shifts in vegetation over the last 1000 years. Variations in pollen assemblages reveal both natural changes in the plant community and anthropogenic changes related to European settlement. This interdisciplinary study combines pollen analytic biostratigraphy with lithostratigraphy and well-documented local history to develop a more comprehensive understanding of the recent history of the study area. Age estimates derived from changes in the plant community combined with a well-documented physical stratigraphy of the study area allow assessment of the impact of post-European-settlement land-use practices on sedimentation and ecological conditions in the St. Jones estuarine system. The area surrounding the St. Jones River has been continuously inhabited since 1650, when the English established the settlement of St. Jones Neck, while other portions of Delaware were settled as early as 1631. Prior to the advent of European agricultural practices, Delaware was thickly forested with mixed conifers and hardwoods. Clear-cutting for agriculture and fuel wood supply, beginning in the late 1600’s, quickly denuded the area, which did not begin to recover until late in the twentieth century. Analysis of the pollen record of three cores reflects a shift from this forested setting to one with a significant 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. The upper, post-settlement intervals are rich in Quercus, with common Pinus and Carya, but the abundances of Compositae are conspicuously higher, indicating increased amounts of open spaces, and decreased forest cover. These shifts place the European agricultural horizon at a depth between 2.10 m and 2.20 m in the supratidal cores and 3.45 m in an intertidal core. Age estimates based on this palynologically derived horizon and independent radiocarbon dates indicate that sediment accumulation rates increased by approximately 4.5 times following European settlement. The paleoenvironmental history of the St. Jones River basin over the past 1000 years as seen in the pollen record is one of increasing disturbance of natural systems with increasing population growth. Additional results of pollen analysis indicate that the pollen record can provide general patterns of river salinity as well as the degree of forest clearance, leading to the possibility of more detailed studies comparing pollen analysis to other paleosalinity techniques. Micropaleontological analysis of pollen assemblages yields considerably greater temporal and paleoenvironmental resolution than lithostratigraphy alone.