ANTHROPOGENIC SEDIMENT LOADING & CENTENNIAL CLIMATE CHANGE IN THE VIRGINIA TIDEWATERS OF CHESAPEAKE BAY

Microfossil paleoecological trends and sedimentation rates observed in cores collected from the Rappahannock and Potomac estuaries record evidence for dramatic changes in sediment loading and the salt-wedge structure in the tidal reaches of the Chesapeake Bay pre and post industrial revolution. Sedimentary cores were collected from the proximal and central estuarine zones with known seasonal salinity stratification. Sedimentation rates established on the basis of isotopic (¹³⁷Cs) and radiometric (AMS ¹⁴C) dating indicate an increase from approximately 1.0 mm/yr to 7 mm/yr up core. Microfossil associations comprise alternating populations of marine and brackish ostracodes (bivalved crustaceans) that are particularly prominent in the basal, clay-rich mud. The uppermost organic-rich, silty mud contains brackish microfossil associations that give way to a freshwater thecamoebians (unicellular protists) and ostracodes. The synthesis of the combined paleontologic and sedimentologic data indicate a recent trend towards increased sediment loading and gradual deterioration of the salinity gradient in both the Rappahannock and Potomac Estuaries with a marked shift during the mid-to late 19^th century. Sedimentation rates since the time of the Industrial Revolution have increased considerably in response to aggressive agricultural practices in the Chesapeake Bay Watershed. We attribute the changes in paleosalinity to multidecadal climate changes that might include the last vestiges of the late 18^th century Little Ice Age and prolonged droughts during the late 19th and early-to middle 20th Centuries.