REGIONAL TIMING OF NEOGLACIATION IN THE MARITIME RANGES OF THE WESTERN U.S.: CONSTRAINS FROM GLACIAL AND LACUSTRINE RECORDS

Moraine and lake-sediment records indicate that cirque glaciers throughout the maritime mountains of the western U.S. began to form by ~3300 cal yr. B.P., advanced and retreated repeatedly before reaching their Holocene maxima in the last 150-200 yr during the culmination of the Little Ice Age (LIA).  The most detailed records indicate that advances lasted several hundred to perhaps a thousand years, and that the extent of glacierization increased progressively with each advance.

            Clastic sediments in lake cores from below modern glaciers in the Sierra Nevada and the North Cascades suggest that these ranges were dominated by warm, non-glacial conditions throughout the early and middle Holocene, after the Pleistocene glaciers had retreated substantially or disappeared.  Beginning ~3100-3200 ¹⁴C yr B.P. (3300-3400 cal yr. B.P.), increases in fine clastic silt document the onset of glacier growth in each of the ranges.  Glaciers thereafter reached maxima every few hundred years, achieving Holocene maxima during the late LIA (i.e., last ~200 yrs).

            Regional synchrony of both onset and maximum of Neoglaciation across the western U.S. Cordillera indicates that the primary cause was a decrease in regional temperature rather than local increases in precipitation.  Equilibrium-line altitude reconstructions suggest temperatures at the maximum were ~0.5-1.0° C cooler than present.  The synchronous onset of Neoglaciation appears to post-date late-Holocene cooling indicated by other (biologic) proxy records, and in combination, suggests that the mountains passed through a climatic threshold of cirque-glacier initiation.  Although presently retreating, the continued existence of glaciers in these areas indicates that modern alpine climates have not yet attained those characteristic of the middle Holocene prior to the 3300 yr event.