What was done
The authors "examine[d] the late Holocene (2300 yr BP to present) record of Chesapeake Bay and the adjacent terrestrial ecosystem in its watershed through the study of fossil dinoflagellate cysts and pollen from sediment cores."

What was learned
The authors report that "several dry periods ranging from decades to centuries in duration are evident in Chesapeake Bay records."  The first of these periods of lower-than-average precipitation, which spanned the period 200 BC-AD 300, occurred during the latter part of the Roman Warm Period, as delineated by McDermott et al. (2001) on the basis of a high-resolution speleothem ð¹⁸O record from southwest Ireland.  The next such period (~AD 800-1200), in the words of the authors, "corresponds to the 'Medieval Warm Period', which has been documented as drier than average by tree-ring (Stahle and Cleaveland, 1994) and pollen (Willard et al., 2001) records from the southeastern USA."  Other periods consisting of several decadal-scale dry intervals span the years AD 1320-1400 and AD 1525-1650.

The authors say that "mid-Atlantic dry periods generally correspond to central and southwestern USA 'megadroughts', described by Woodhouse and Overpeck (1998) as major droughts of decadal or more duration that probably exceeded twentieth-century droughts in severity."  They further indicate that "droughts in the late sixteenth century that lasted several decades, and those in the 'Medieval Warm Period' and between ~AD 50 and AD 350 spanning a century or more have been indicated by Great Plains tree-ring (Stahle et al., 1985; Stahle and Cleaveland, 1994), lacustrine diatom and ostracode (Fritz et al., 2000; Laird et al., 1996a, 1996b) and detrital clastic records (Dean, 1997)."

On another note, the authors find that "European colonization had severe impacts on the watershed and estuary."  Specifically, they note that "after European colonization in the early seventeenth century, forest clearance for agriculture, timber and urbanization altered estuarine water quality, with dinoflagellate assemblages indicating reduced DO [dissolved oxygen] and increased turbidity."  In addition, they report that "following peak limber harvesting between 1880 and 1910, sedimentation rates increased two- to four-fold (Brush, 1984; Colman et al., 2002; Cronin et al., 1999)" and that "several dinocyst taxa nearly disappeared."  Another such degradation of the coastal environment occurred after 1950, when "dinocyst assemblage diversity decreased, reflecting water-quality changes associated with increased urbanization, greater hypoxia (Karlsen et al., 2000) and increased agricultural nutrient input (Jaworski et al., 1997)."

What it means
This study does three important things.  First, it demonstrates the reality of the millennial-scale hydrologic cycle that accompanies the millennial-scale temperature cycle that is responsible for producing alternating warm and cold intervals such as the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age and Modern Warm Period.  Second, it demonstrates that the global warming of the 20th century has not produced unusually strong wet and dry periods, contradicting climate-alarmist claims that warming will exacerbate extreme climate anomalies.  Third, it demonstrates that coastal waters of the United States began to experience significant quality degradation from the very first appearance of European settlers, providing addition evidence for the hypothesis we describe in our Editorials of 12 March and 26 March 2003, i.e., that the historical and ongoing worldwide increase in sediment-induced stress, which includes the debilitating effects of various nutrients and toxins of anthropogenic origin that are intimately associated with and carried by sediments, is what is predisposing today's corals to bleach more readily than they did in the past in response to periodic increases in water temperature.

Reference
Brush, G.S.  1984.  Patterns of recent sediment accumulation in Chesapeake Bay (VA, MD, U.S.A.) tributaries.  Chemical Geology 44: 227-242.

Colman, S.M., Baucom, P.C., Bratton, J.F., Cronin, T.M., McGeehin, J.P., Willard, D.A., Zimmerman, A.R. and Vogt, P.R.  2002.  Radiocarbon dating, chronologic framework, and changes in accumulation rates of Holocene estuarine sediments from Chesapeake Bay.  Quaternary Research 57: 58-70.

Cronin, T., Colman, S., Willard, D., Kerhin, R., Holmes, C., Karlsen, A. Ishman, S. and Bratton, J.  1999.  Interdisciplinary environmental project probes Chesapeake Bay down to the core.  EOS, Transactions of the American Geophysical Union 80: 237, 240-241.

Dean, W.E.  1997.  Rates, timing, and cyclicity of Holocene eolian activity in north-central United States: evidence from varved lake sediments.  Geology 25: 331-334.

Fritz, S.C., Ito, E., Yu, Z., Laird, K.R. and Engstrom, D.R.  2000.  Hydrologic variation in the northern Great Plains during the last two millennia.  Quaternary Research 53: 175-184.

Jaworski, N.A., Howarth, R.W. and Hetling, L.J.  1997.  Atmospheric deposition of nitrogen oxides onto the landscape contributes to coastal eutrophicaton in the northeast United States.  Environmental Science and Technology 31: 1995-2004.

Karlsen, A.W., Cronin, T.M., Ishman, S.E., Willard, D.A., Kerhin, R., Holmes, C.W. and Marot, M.  2000.  Historical trends in Chesapeake Bay dissolved oxygen based on benthic foraminifera from sediment cores.  Estuaries 23: 488-508.

Laird, K.R., Fritz, S.C., Grimm, E.C. and Mueller, P.G.  1996a.  Century-scale paleoclimatic reconstruction from Moon Lake, a closed-basin lake in the northern Great Plains.  Limnology and Oceanography 41: 890-902.

Laird, K.R., Fritz, S.C., Maasch, K.A. and Cumming, B.F.  1996b.  Greater drought intensity and frequency before AD 1200 in the Northern Great Plains, USA.  Nature 384: 552-554.

McDermott, F., Mattey, D.P. and Hawkesworth, C.  2001.  Centennial-scale Holocene climate variability revealed by a high-resolution speleothem ð¹⁸O record from SW Ireland.  Science 294: 1328-1331.

Stahle, D.W. and Cleaveland, M.K.  1994.  Tree-ring reconstructed rainfall over the southeastern U.S.A. during the Medieval Warm Period and Little Ice Age.  Climatic Change 26: 199-212.

Stahle, D.W., Cleaveland, M.K. and Hehr, J.G.  1985.  A 450-year drought reconstruction for Arkansas, United States.  Nature 316: 530-532.

Willard, D.A., Weimer, L.M. and Holmes, C.W.  2001.  The Florida Everglades ecosystem, climatic and anthropogenic impacts over the last two millennia. In: Wardlaw, B.R. (Ed.). Paleoecology of South Florida.  Bulletins of American Paleontology 361: 41-55.

Woodhouse, C.A. and Overpeck, J.T.  1998.  2000 years of drought variability in the Central United States.  Bulletin of the American Meteorological Society 79: 2693-2714.