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 (11.6–8.2 ka; 1 ka = 1000 cal yr BP) to warm and wet (8.2–5.4 ka) to warm and dry (5.4–3 ka) to cool and wet (after 3 ka). 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 5.8 ka, 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 4.7 ka. 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 5.8 ka 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 4.7 ka through modulating ocean and atmosphere circulations.