Ice-Core Evidence of Millennial and Slower Variability in North Atlantic Ocean Heat Transport

R.B. Alley, (Earth System Science Center and Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA; ph. 814-863-1700; fax 814-865-3191; Internet: ralley@essc.psu.edu); A.-M. Agustsdottir (same; Internet: annamari@essc.psu.edu); P.J. Fawcett (Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA; ph. 505-277-3867; fax 505-277-8843; Internet: fawcett@unm.edu)

Large, abrupt, widespread, millennial climate changes are documented clearly in the Greenland ice cores and in many other records. The magnitudes, rapidity, and spatial patterns of these changes are explained well by the effects of reorganizations of ocean circulation, with an important role for north Atlantic processes. The several-millennial trend of late-Holocene climate recorded in the GISP2 ice core from central Greenland, and from some other records, mimics in many ways the faster cooling trends of millennial oscillations. Both are plausibly explained at least in part by decreasing ocean heat transport into the north Atlantic.

The most direct ice-core evidence of late-Holocene reduction in oceanic heat transport to the north Atlantic is a mean-annual cooling greater than summertime cooling inferred from comparison of melt-layer frequency to borehole-temperature-calibrated ice-isotopic ratios. The difference between the magnitude of the ice-isotopic calibration to temperature and that expected from modern spatial gradients also is explainable based on changes through the Holocene in precipitation seasonality driven by changes in oceanic heat flux. Changes in oceanic heat flux also can explain observed changes in climatic variability. Some differences exist between the faster and slower cooling trends, which may reflect influences unrelated to oceanic heat flux, or may be related to spatial differences in the changes in the oceanic heat flux between the faster and slower trends.