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Twentieth century climatic variability is caused by complex interactions between the ocean-atmosphere system, solar variability, volcanic processes, climatological "noise," and anthropogenic perturbations to Earth's atmosphere, or a combination of factors. Because instrumental records rarely exceed 100 yr, paleoclimate reconstructions can establish which factors are most important over multidecadal time scales, distinguish anthropogenic and natural causes, and test climate-model simulations of decadal- and centennial-scale variability. Salinity oscillations caused by multidecadal climatic variability had major impacts on the Chesapeake Bay estuarine ecosystem, most notably by altering regional precipitation, river discharge, and bay salinity. Microfossils from Chesapeake sediments dated by radiometry (14C, 137Cs, 210Pb) and pollen stratigraphy indicate that salinity in mesohaline regions of the bay oscillated 10-15 parts per thousand during periods of extreme drought (low fresh-water discharge) and wet climate (high discharge). During the past 500 yr, 14 wet-dry cycles occurred, including sixteenth and early seventeenth century "megadroughts" which exceeded twentieth century droughts in their severity. These droughts correspond to extremely dry climate also recorded in North American tree-ring records and by early colonists. Wet periods occurred every ~60-70 yr, began abruptly, lasted <20 yr, and had mean annual rainfall ~25%-30% and fresh-water discharge ~40%-50 % greater than during droughts. A shift toward wetter regional climate occurred in the early nineteenth century, lowering salinity and compounding the effects of agricultural land clearance on bay ecosystems. Further work is now underway to assess these faunal estimates of paleosalinity in terms of quantitative estimates of salinity and paleo-precipitation using oxygen isotopic records from fossil benthic foraminifera from Chesapeake Bay cores.
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