Climatic variability in the eastern United States over the past millenniumfrom Chesapeake Bay sediments

Salinity oscillations caused by multidecadal climatic variability had major impacts on the Chesapeake Bay estuarine ecosystem during the past 1000 yr. Microfossils from sediments dated by radiometry (C-14, Cs-137, Pb-210) and pollen stratigraphy indicate that salinity in mesohaline regions oscillate d 10-15 ppt during periods of extreme drought (low fresh-water discharge) a nd wet climate (high discharge). During the past 500 yr, 14 wet-dry cycles occurred, including sixteenth and early seventeenth century megadroughts th at exceeded twentieth century droughts in their severity, These droughts co rrespond to extremely dry climate also recorded in North American tree-ring records and by early colonists. Wet periods occurred every similar to 60-7 0 yr,began abruptly, lasted <20 yr, and had mean annual rainfall similar to 25%-30% and fresh-water discharge similar to 40%-50% greater than during d roughts. A shift toward wetter regional climate occurred in the early ninet eenth century, lowering salinity and compounding the effects of agricultura l land clearance on bay ecosystems.