Holocene climate trend, variability, and shift documented by lacustrine stable-isotope record in the northeastern United States

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.