BENTHIC FORAMINIFERAL RECORD OF HOLOCENE CLIMATIC AND ANTHROPOGENIC IMPACTS ON CHESAPEAKE BAY, USA

Holocene benthic foraminiferal records from the Chesapeake Bay provide evidence that regional climatic and oceanographic fluctuations and human land-use activities have significantly impacted the bay’s ecosystems. Quantitative analyses of foraminiferal assemblages were carried out on sediment core MD99-2209, recovered from the northernmost axial basin of the Chesapeake Bay (38.886°N, 76.395°W, 26 m water depth) in June 1999 during the IMAGES V cruise of the R/V Marion-Dufresne. Calibrated radiocarbon dates on bivalves reveal two main periods of deposition, from ~7570 to 5800 yr BP and from 2200 to present.

Early and late Holocene benthic foraminiferal assemblages differ considerably from one another, suggesting the Bay was on average warmer and slightly more saline during the early Holocene than the late Holocene. The early Holocene was also characterized by high-frequency (~100 years) fluctuations in foraminiferal assemblages. Assemblages dominated by Ammonia parkinsoniana tepida and Bolivina spp. alternate with those dominated by Elphidium spp. signifying marine salinities/low oxygen and normal marine conditions, respectively. Although the causes of these oscillations are uncertain, they may signify early Holocene centennial scale climate variability related to thermohaline circulation changes in the North Atlantic region.

Two distinct periods, 2200 to 200 yr BP and 200 yr BP to present, corresponding to periods prior to and following major 19th century land clearance in the Bay’s watershed characterize the late Holocene. Prior to clearance assemblages are dominated by Elphidium spp. with small percentages of A. parkinsoniana typica. Since land clearance there has been a significant and permanent decrease in abundance of Elphidium spp. and an increase in A. parkinsoniana typica and Ammobaculites sp. This faunal shift, documented at other sites and in ostracode assemblages in Chesapeake Bay, represents the permanent and unprecedented changes in the Bay’s water quality (greater hypoxia, increased turbidity) and biota due to anomalously high nutrient and sediment influx from the drainage basin.