Earlier studies indicated that the general pattern of the Holocene climate in the northeastern United States changed from cool and dry (11.6-8.2ka; 1ka=1000 cal yr BP) to warm and wet (8.2-5.4ka) to warm and dry (5.4-3ka) to cool and wet (after 3ka). A new ∼35-year resolution stable isotope record of endogenic calcite from a sediment core for Lake Grinnell in northern New Jersey provided a chance to examine the Holocene climate variations of the region in a finer detail. After the Younger Dryas cold climate reversal, the δ18O fluctuated around a constant value of -7.4‰ until 5.8ka, thereafter shifted to a steadily decreasing trend to the most recent value of -8.2‰ Responding to this shift, the widely observed hemlock decline in the northeastern USA occurred about ∼350-500 (±143.5) years later. Detrended δ18O and δ13C records show a clear covariance at 910-year periodicity. The amplitudes of centennial-scale δ18O variations became much smaller after 4.7ka. At the same time, the dominant frequency of these variations changed from 330 to 500 years. We suggest that a non-linear response of atmospheric circulation to the gradual decrease in insolation is responsible for the shift in the climate trend at 5.8ka as indicated by the deceasing δ18O values. A dominant frequency shift in solar forcing and the decreased seasonal contrast of insolation might have caused the change in climate variability at 4.7ka through modulating ocean and atmosphere circulations.
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We thank Robert Moeller, Yongxiang Li, Long Li and EES 357 Spring 2005 class at Lehigh University for field coring assistance, John Naby and his neighbors around Lake Grinnell for water sampling assistance, and two anonymous reviewers for helpful comments that improved the manuscript. This project was supported by American Chemical Society – Petroleum Research Fund (#42418-AC2) and US NSF (EAR 0518774) to Z.C. Yu. This is Contribution 09-22 of the Limnological Research Center of the University of Minnesota.