EVIDENCE FOR CIRQUE GLACIER CHRONOLOGIES AND RAPID ALPINE DEGLACIATION IN THE NORTH CASCADES DURING THE HOLOCENE/PLEISTOCENE TRANSITION

Moraines of two ages occur in north aspect cirques at the headwaters of Swift and Bagley Creeks in the North Cascade Range, WA. The upper moraines were formed ~AD 1900 and correlate to many other LIA moraines in the range. Lower moraines in south Swift Creek cirque (SSCC) have a minimum age of 9560+/-50 14C yrs BP on charcoal from the base of a bog just outside the lowermost moraine. Several sediment cores associated with small recessional moraines indicate rapid deglaciation (or at least rapid colonization by vegetation). A core from a small pond just above the uppermost of these moraines indicates that gyttja and tree macrofossils were being deposited by 9350+/-180 14C yrs BP, demonstrating that subalpine vegetation similar to today was established at this elevation (1360 m) by that time. No record of glacier advance is evident between the uppermost recessional moraine and outermost LIA moraine.

Other cirque moraines in the Mt. Baker area with similar geomorphic position to the SSCC moraines are likely similar in age. Other possible correlations may be the McNeely II moraines near Mt. Rainier and the Brisingamen moraines in the southern North Cascades.

ELAs are calculated for modern and paleo-glaciers using a 0.6 AAR. The delta-ELA for the SSCC LIA moraines is 75 m, from a local ELA of the glacier at the head of the cirque. The LIA paleo-glacier was subject to the same localized conditions that preserve the modern glacier. The delta-ELA for the late-Pleistocene/early Holocene moraines in Swift and Bagley Creeks is ~ 490 m and is calculated from a regional ELA from modern north facing glaciers on nearby Mt. Baker and Mt. Shuksan. This delta-ELA is similar to the McNeely II moraines delta-ELA of 400 to 500 m from the modern ELA near Mt. Rainier.

Additional work is needed to augment paleo-climatic/environmental records in alpine and subalpine areas. Numerous sites exist which may yield new data on climatic and environmental change in this unique and dynamic region.