Neoglacial fluctuations and sedimentation of an iceberg-calving glacier resolved with tree rings (kenai Fjords national park, Alaska)

Abstract

The McCarty Fjord iceberg-calving glacier and its land-terminating tributaries have fluctuated with some asynchroneity during the past two millennia. During advance, McCarty Glacier shed outwash along the fjord, and into ice-dammed tributaries inundating forests. A radioacarbon framework has revealed at least two major late Holocene glacial advances that occurred following a poorly documented expansion about 3600 BP. These two later advances are resolved within this fjord with tree-ring dating of in situ and transported tree trunks. The first event was an early medieval expansion of the McCarty trunk glacier beyond midfjord about 596 A.D.; this followed an interval of tree growth of at least 206 years. Tributary glaciers probably also advanced at this time. However, continuous tree-growth occurred in the distal (southern) tributary valleys during this advance, while northern tributaries were being dammed by the advancing trunk glacier. The down-valley extent of this expansion is thus constrained to a position within 12 km of the present contracted ice margin. The tree-ring refined chronology shows that a second McCarty ice expansion began in the 9th century, and reached midfjord by 900 A.D., and continued to advance through the Little Ice Age. In contrast, expansions of land-terminating glaciers here began after 1300 A.D. in concert with mountain glaciers worldwide. A log from a diamict cross-dated with a living tree-ring chronology shows that McCarty Glacier was advancing within a kilometer of its Little Ice Age maximum at 1790 A.D. Since about 1905 A.D., dramatic ice retreat has uncovered more than 20 km of McCarty Fjord.