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Latest Wisconsin Deglaciation and Postglacial Vegetation Development in the Turnagain Arm Area, Upper Cook Inlet, South-Central Alaska

By Thomas A. Ager and Paul E. Carrara

Figure 1. Chugach Mountains bordering the north shores of Turnagain Arm, a marine estuary and former glacial fiord in upper Cook Inlet.

Radiocarbon-dated pollen records from peat deposits in the Turnagain Arm and Anchorage lowland areas of northern Cook Inlet, south-central Alaska, provide a basis for reconstructing the history of vegetation development following the retreat of the final late Wisconsin glacial advance (Elmendorf stade). Basal radiocarbon ages from silty deposits under peat collected from the Girdwood area and Hope area in Turnagain Arm indicate that the retreat of Elmendorf glaciers occurred more than 13,800 calendar years before present (cal yr B.P.).

Figure 2. Present-day climate zones of Alaska. Turnagain Arm in upper Cook Inlet trends east-west (Figure 3) and has a strong maritime influence towards its eastern end, and a drier, transitional climate in the western end.

Pollen assemblages dated to 13,800-11,500 cal yr B.P. indicate that shrub-herb tundra was the pioneering vegetation in Turnagain Arm and the Anchorage lowland. Beginning about 10,350 cal yr B.P. alder shrubs rapidly colonized northern Cook Inlet and became the dominant vegetation type for thousands of years, especially in eastern Turnagain Arm. In the Anchorage lowland, which has a drier, transitional climate between maritime and continental climates, boreal forest vegetation was able to colonize by about 9150 cal yr B.P. This vegetation spread quickly southward into the northern Kenai lowland, which shares a drier, colder climate with the Anchorage lowland. The spread of boreal forest species into Turnagain Arm east of the Anchorage and Kenai lowlands was much slower, probably because the wetter climate in Turnagain Arm was less favorable for many boreal species. Some boreal forest tree species such as white spruce reached the Hope area by 7100 cal yr B.P. Eastern Turnagain Arm (east of Bird Creek) remained dominated by alder thickets, and only outlier populations of some boreal forest tree species became established there during the Holocene (e.g., black spruce, balsam poplar, paper birch). Alders were finally displaced over much of eastern Turnagain Arm after 3200 cal yr B.P., when tree species from coastal forests in Prince William Sound spread westward. After 3200 cal yr B.P., Sitka spruce and mountain hemlock became the dominant trees in eastern Turnagain Arm. Between about Bird Creek and the Anchorage lowland, the vegetation that developed during the late Holocene was a transitional forest of mixed coastal and boreal species. The most common conifer in that transition zone is Lutz spruce, a hybrid between white spruce and Sitka spruce.

The distribution of boreal forest, coastal forest, and transition zone vegetation in Turnagain Arm and Anchorage lowland appears to be controlled mostly by precipitation. High precipitation in eastern Turnagain Arm creates favorable conditions for coastal forest species, and the low precipitation in the Anchorage-Kenai lowlands favors boreal forest species. The ecotone between these forest types is occupied by a mixture of coastal and boreal taxa that occupy areas of intermediate precipitation. The substantial variations in precipitation within the Cook Inlet- western Prince William Sound region reflect the relative influences of maritime climates on the southern and eastern coasts of Kenai Peninsula and eastern Turnagain Arm, versus the much drier, more continental climates of Kenai and Anchorage lowlands, formed in part by the precipitation shadow of the Chugach and Kenai Mountains.

Figure 3. Map of upper Cook Inlet and adjacent areas showing Turnagain Arm, the Anchorage lowland (A.L.), the Kenai lowland, Prince William Sound (east of Turnagain Arm), the Kenai Mountains, the Chugach Mountains, the Alaska Range and Talkeetna Mountains. Numbered sites refer to the following:

1. Pollen records from the Anchorage lowland (Ager and Brubaker, 1985 and this abstract, Fig. 9)
2. A pollen record from near Hope (this web page)
3. Bird Creek, the approximate boundary between coastal forests to the east and transitional and boreal forest to the west
4. Pollen records from the Girdwood area, within predominantly coastal forest vegetation (this abstract; Fig. 9)
5. A pollen record from Golden in northwestern Prince William Sound (coastal forest and alder vegetation today; Fig. 9)
6. A pollen record from the Kenai Mountains, in a region of mixed boreal and coastal forest species (Ager, 2001).

The approximate maximum positions of Elmendorf stade glacial ice in the Anchorage lowland and Turnagain Arm are shown by arcuate hatchured lines. Ice retreated from those terminal positions some time before 13,800 in Turnagain Arm and before 14,500 cal yr B.P. in the Anchorage lowland area (Reger and others, 1995; Schmoll and others, 1999). The dashed line is a fault which forms the boundary between the Anchorage-Kenai lowlands and the Kenai-Chugach Mountains.

Figure 4. Map of major vegetation types of Alaska. While fairly accurate, it fails to show the broad transition zones between boreal forest and coastal forest in the Cook Inlet region.

Figure 5. Climate data from upper Cook Inlet and northwestern Prince William Sound, showing the dramatic range in precipitation along the west-to-east transect of Turnagain Arm.

Figure 6. Present-day coastal forest (from eastern Kenai Peninsula) dominated by Sitka spruce and mountain hemlock. In some areas black cottonwood and western hemlock are also present. Western hemlock is rare or absent in Turnagain Arm.

Figure 7. Present-day mixture of boreal forest and coastal forest species in western Turnagain Arm (Hope area, Figure 3). Also shown are bog vegetation in foreground, and alder thickets and shrub tundra on mountain slopes in background.

Figure 8. Present-day boreal forest vegetation in south-central Alaska, composed of white spruce, black spruce, paper birch, balsam poplar, aspen, alder, and willow.

Conclusions

1. The final late Wisconsin deglaciation of Turnagain Arm in northern Cook Inlet occurred sometime before 13,800 cal yr B.P.
2. Shrub-herb tundra was the first vegetation type to colonize Turnagain Arm following deglaciation.
3. Alder colonization of Turnagain Arm occurred rapidly after about 10,350 cal yr B.P. Alders remained the dominant vegetation type in much of Turnagain Arm for thousands of years.
4. Boreal forest vegetation spread from interior Alaska to the Anchorage lowland in northern Cook Inlet by about 9150 cal yr B.P., then spread quickly south into the Kenai lowland. The spread of boreal forest eastward into Turnagain Arm was inhibited by wetter climates.
5. Coastal forest vegetation colonized eastern Turnagain Arm beginning about 3200 cal yr B.P. During the late Holocene a broad ecotone between boreal forest and coastal forest developed in western Turnagain Arm. This ecotone consists of a complex mixture of boreal forest and coastal forest species.
6. It appears likely that the environmental factor that most effectively controls the distribution of coastal forest and boreal forest vegetation in south-central Alaska is precipitation. Most boreal trees and shrub species are intolerant of wet coastal climates, while coastal forest species have limited tolerance for the dry climates of the interior and for dry areas in the precipitation shadows of coastal mountains.

References

Ager, T.A., 2001, Holocene vegetational history of the northern Kenai Mountains, south-central Alaska: U.S. Geological Survey Professional Paper 1633, p. 91-107.

Ager, T.A., and Brubaker, L.B., 1985, Quaternary palynology and vegetational history of Alaska: in Bryant, V., and Holloway, R., Pollen Records of Late-Quaternary North American Sediments. American Association of Stratigraphic Palynologists Foundation, Dallas, p. 353-384.

Ager, T.A., and Carrara, P.E., 2006, Latest Wisconsin deglaciation and postglacial vegetation development in the Turnagain Arm area, upper Cook Inlet, south-central Alaska: 36th Arctic Workshop Program and Abstracts.

Heusser, C.J., 1983, Holocene vegetational history of the Prince William Sound region, south-central Alaska: Quaternary Research 19, p. 337-355.

Reger, R.D., Combellick, R.A., Brigham-Grette, J., 1995, Late Wisconsin events in upper Cook Inlet, south-central Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 117, p. 33-55.

Schmoll, H.R., Yehle, L.A., and Updike, R.G., 1999, Summary of Quaternary geology of the Municipality of Anchorage, Alaska: Quaternary International 60, p. 3-36.

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