Influence of changing atmospheric circulation on precipitation delta O-18-temperature relations in Canada during the Holocene

Postglacial precipitation delta(18)O history has been reconstructed for two regions of Canada. Long-term shifts in the oxygen-isotope composition of annual precipitation (delta(18)O(p)) in southern Ontario appear to have occurred with a consistent isotope-temperature relation throughout the past 11,500 C-14 yr. The modern isotope-temperature relation in central Canada near present boreal tree-line evidently became established between 5000 and 4000 years ago, although the relation during the last glacial maximum and deglaciation may also have been similar to present. In the early Holocene, however, unusually high delta(18)O(p) apparently persisted, in spite of low temperature locally, probably associated with high zonal index. A rudimentary sensitivity analysis suggests that a small reduction in distillation of moisture in Pacific air masses traversing the western Cordillera, perhaps accompanied by a higher summer:winter precipitation ratio, could have been responsible for the observed effect. Equivalent isotope-temperature ''anomalies'' apparently occurred elsewhere in western North America in response to changing early-Holocene atmospheric circulation patterns, suggesting that a time-slice map of delta(18)O(p) for North America during this period might provide a useful target for testing and validation of atmospheric general circulation model simulations using isotopic water tracers. (C) 1996 University of Washington