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Holocene Climate Variability of Chesapeake Bay
The combined influence of natural climate variability and human-induced ecosystem degradation introduces complexities for management and restoration of coastal ecosystems of the world. Whereas many studies of coastal ecosystems have focused on anthropogenic impacts of cultural eutrophication, land-use changes, sediment influx, and hydrological changes, growing evidence suggests that estuarine ecosystems also are influenced by climate variability. With few exceptions, separating anthropogenic effects from those due to climatic causes has remained problematic due in part to the limited historical record available for most ecosystems. Sedimentary records are useful in distinguishing human impacts on ecosystems from those due to climatic variability and other "natural" factors. Sediment cores from Chesapeake Bay contain archives of climate variability throughout the Holocene, providing a baseline for comparison with changes in temperature, precipitation, and estuarine water quality over the past few centuries.
Studies of Chesapeake Bay climate variability use radiocarbon dating of fossil shells and pollen biostratigraphy to generate age models for sediment cores. Climate proxies are developed through calibration of modern indicators with specific climate parameters. These proxies include pollen (temperature and precipitation), microfaunal assemblages (salinity, dissolved oxygen levels, temperature), phytoplankton (dissolved oxygen, salinity), and shell chemistry (temperature, salinity). This Chesapeake Bay research documents natural centennial and decadal-scale fluctuations in temperature and precipitation throughout the late Holocene (past ~2,000 years). However, land-use changes of the past 250 years have altered forest composition and reduced dissolved oxygen to unprecedented low levels.
The Chesapeake Bay sedimentary record also contains evidence of periodic shifts to colder, drier conditions every ~1,400 years, based on significant decreases in pine pollen abundance. These cool events represent decreases in January temperatures of 0.2- 2° C and include temperature excursions during the Little Ice Age (~1300-1600 AD) and 8 ka cold event. The timing of these minima is correlated with a series of quasi-periodic cold intervals documented by various proxies from cores in Greenland, the North Atlantic Ocean, and Alaska and with solar minima interpreted from cosmogenic isotope records.
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