TREE-RING ANALYSIS OF A MEDIEVAL OLD-GROWTH FOREST AT NELLIE JUAN GLACIER, SOUTHERN ALASKA

Nellie Juan Glacier is a small iceberg-calving tongue situated on the western side of Prince William Sound, southern Alaska. Ice descends steeply in a narrow valley from the northern Sargent Icefield to terminate in a tidewater lagoon that has opened during glacial retreat over the past 100 years. Scattered along the shores of this lagoon and exposed in stream gullies are many subfossil tree stumps and logs; tree-ring records from these logs record climate in this area when the trees were alive and enable the Little Ice Age advance of Nellie Juan Glacier to be reconstructed in considerable detail. Fieldwork in July 2000 complimented and expanded on previous sampling efforts in June 1992. A total of 37 new stumps and logs were sampled to add to the 16 earlier samples, and some of the 1992 logs were relocated and their positions fixed with GPS to provide a more detailed spatial picture of the subfossil forest. Tree-ring samples have been cross-dated with living trees at the lagoon entrance and show that the subfossil forest was alive from at least AD 1034 to 1608. Six trees were over 400 years old when they died and the preservation of many of the samples is excellent, making this one of the best subfossil tree-ring sites in southern Alaska. Preliminary analysis of the ring-width data shows strong decadal signals, and comparison with local climate records suggest that trees at this site are primarily responding to summer temperatures. Consideration of the exact location of each subfossil tree and the year when it died allows the most recent advance of Nellie Juan Glacier to be reconstructed with great detail. Between 1539 and 1605 the terminus advanced at between 25 and 35 m/yr. This is similar to advance rates recorded at modern glaciers, and reflects control of the terminus position by the rate at which a stabilizing terminal moraine shoal can be reworked and advanced down-valley.