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Connor, C.L., 1984

Late Quaternary glaciolacustrine and vegetational history of the Copper River basin, south-central Alaska

Bibliographic Reference

Connor, C.L., 1984, Late Quaternary glaciolacustrine and vegetational history of the Copper River basin, south-central Alaska: Missoula, Montana, University of Montana, Ph.D. dissertation, 115 p., illust., maps (folded).

Abstract

The late glacial (Wisconsin) history of the Copper River basin, south-central Alaska, was reconstructed from pollen and assemblages found in exposed basin sediments. Stratigraphic data and pollen samples were collected during 1979 through 1981 from 15 river bluff exposure, road cut, and borehold sites located throughout the basin. Pollen spectra recorded changes in the basinwide vegetation that occurred from approximately 65,000 to 9,400 years ago. The complex glacial history of the Copper River basin, represented by fluvial, glaciolacustrine, and full glacial sediments, includes multiple occupations by a proglacial lake as ice repeatedly advanced into the basin from surrounding source areas and dammed drainage outlets. A mid-Wisconsin intrastadial is suggested by glacial ice ablation, lowered lake levels, and the widespread deposition of fluvial sediments in the basin. This period is estimated to have occurred between 65,000 and 30,000 years ago based on radiocarbon dates and an amino acid racemization age of wood, and represents a time of climatic amelioration in the Copper River basin. Fluvial and shallow lacustrine units of probable mid-Wisconsin age at Tyone Creek and the Dadina River (Connor, 1982) yielded pollen spectra that showed high percentages of spruce, birch, alder, sedge, and sphagnum moss. These pollen assemblages were interpreted as characteristic of open, low-growing spruce forests, with intermixed treeless bogs; a vegetation type similar to the modern plant communities in the basin. Late Wisconsin glaciolacustrine deposits at Trapper's Den, Gakona, and Willow Creek contained spruce, birch, and alder pollen. These units suggested that small stands of forest persisted into the late Wisconsin glacial advance beginning about 30,000 years ago. Pollen spectra derived from peat units at Blue Lake (13,280 (+OR-) 400, W-583), Ahtell Creek (11,440 (+OR-) 400, W-429), and the Little Nelchina River (10,250 (+OR-) 250, W-767), contained rare spruce and alder. Birch, juniper, and sedge were the dominant pollen types in these units. These post-glacial pollen assemblages indicated the recolonization of the newly emerged basin surface by pioneer herb and shrub species. Spruce forests were reestablished in the Copper River basin by 9,400 years ago. Eolian silts capping river bluff sections at Gakona, the Kotsina delta, and Chitina, contained abundant spruce, poplar, birch, alder, and willow pollen. These pollen data record the return of open spruce forest with intermixed sedge bog. Presently, this plant community occupies the Copper River basin.

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