SENSITIVE RESPONSES OF CALCITE STABLE ISOTOPES AT LAKE GRINNELL (NJ) TO CHANGES IN INSOLATION AND MOISTURE CONDITIONS DURING THE HOLOCENE

Lake sediments have been widely used in documenting patterns and understanding mechanisms of climate changes in the past. However, there are few isotopic studies with adequate resolution from lakes in northeastern North America. Here, we present a ~35-year resolution isotopic record of endogenic calcite from a sediment core at Lake Grinnell in northern New Jersey. Lake Grinnell is a hard water lake recharged dominantly by shallow groundwater. Hydrological measurements indicate that this lake has a short residence time of ~80 days so the lake water isotopes could rapidly track change in precipitation isotope values. Loss-on-ignition analysis shows that the core has high and uniform carbonate content of 90-95%. Chronology of the core was controlled by 7 calibrated AMS-14C dates on terrestrial plant macrofossils. Between 12 ka and 11 ka, a negative excursion of about 2‰ in ä18O corresponds to the Younger Dryas cooling event, which is associated with an increase of about 3‰ in ä13C. During the Holocene, a long-term decreasing trend of about 1‰ in ä18O closely follows the summer insolation curve at 40°N latitude, suggesting a dominant control of summer insolation and temperature on overall meteoric water ä18O trend. A decrease of 0.5‰ in ä18O values from a relatively stable value of -7.4‰ at 11-5 ka to -7.9‰ at 4.3-1.5 ka might have been caused by a shift in atmospheric circulation that was responsible for low lake level and dry climate at the time, as has been documented in other studies in northeastern North America. Carbon isotopes show an increase about 2.5‰ at 11-9 ka and then remain at about -5.5‰ during the last 9 ka. Superimposed on this long-term carbon isotope trend is large-amplitude millennial- and centennial-scale oscillations, with periods of ~3000 and 1500 years. The low ä13C values at 11-9ka and 5.2-3.4ka are corresponding to dry periods in this region, possibly because the carbonate with high ä13C value (~0‰) from limestone bedrock had been decreased when groundwater supply was lower at dry condition. Our new results suggest that temperature and moisture associations might have shifted during the last 12,000 years, and that orbital-scale insolation change not only controls the general Holocene climate pattern but its gradual decrease trend in the mid-Holocene might have triggered an abrupt shift in atmospheric circulation around 5 ka.