LATE HOLOCENE SEA LEVEL AND CLIMATE VARIABILITY AT POTOMAC CREEK (VIRGINIA) IN THE CHESAPEAKE BAY WATERSHED

The Chesapeake Bay contains a continuous record of sedimentation for the Holocene, yet the thickness of the sedimentary cover is highly variable between the northern and southern sides of the Potomac River. The Northern Neck region contains exceptionally thick marsh deposits which may record the combined effects of sea level rise and differential compaction. The microfossil and physical stratigraphy of sediments yield evidence for both fluvial process and compaction (autogenic) and sea level and climate (allogenic).

A 7 meter Livingston core was recovered from Potomac Creek, tributary to the Potomac River, located in the central estuarine zone near Stafford, VA. The lowermost sediments (7.85-7.45 m) comprise alternating organic-rich mud and sand, relatively intense magnetic susceptibility values, and relatively abundant Miliammina, Ammoastuta, and Thecamoebians. The central facies (7.44-3.26 m) comprises grey clay, highly variable magnetic susceptibility values, and high percentages of both Ammobaculites and Trochammina. The uppermost facies (3.25-1.01 m) comprises organic rich mud and peat and relatively high abundances of Ammoastuta and Haplophragmoides, giving way to freshwater Thecamoebians and Phragmites at the top of the core (1.00-0.00 m).

We interpret four stages of marsh evolution: Stage 1 (7.85-7.45 m) a high energy shallow marsh; Stage 2 (7.44-3.26 m) a low energy relatively deep central estuary; Stage 3 (3.25-1.01 m) a high stable marsh; and Stage 4 (1.00-0.00 m) a low salinity anthropogenically influenced marsh. The overall trend in sea level is transgressive-regressive and it is likely that climate variabilty influenced sedimentation patterns that includes the onset of enhanced peat production at the approximate time of the Medieval Warm Period.