The northwestern U.S. during deglaciation; Vegetational history and paleoclimatic implications

Pollen records from northern Grand Teton National Park, the Pinyon Peak Highlands, and southern Yellowstone National Park were examined to study the pattern of reforestation and climatic change following late-Pinedale Glaciation. The vegetational reconstruction was aided by analyses of associated plant macrofossils and the modern pollen rain of the region. Radiocarbon-age determinations and tephrochronology provided a chronologic framework to help correlate pollen records among sites. The fossil records indicate that alpine meadow communities, with Betula and Juniperus, were present between - 14 000 and - 1 1 500 yr BP. This early assemblage implies a lowering of modern upper treeline by at least 600 m and a climate that was 5-60C colder than present. Between 11 500 and 10 500 yr BP, increased temperature and winter precipitation allowed first Picea, and then Abies and Pinus cf. albicaulis to spread into areas underlain by andesite and nonvolcanic bedrock. By 10 500 yr BP, the fossil record in these areas resembled modern spectra from subalpine forest. In contrast, the Central Plateau of Yel- lowstone, which is underlain by infertile rhyolitic soils, was treeless prior to - 10 000 yr BP. The absence of late-glacial subalpine parkland in this area is attributed to the same edaphic factors that limit Picea, Abies, and Pinus albicaulis from the rhyolite plateau today. Between 10 000 and 9500 yr BP, Pinus contorta forest developed throughout the region in response to further warming. Pseudotsuga and Populus were present between 9500 and 5000 yr BP, suggesting a warmer, drier climate than today and more frequent fires. In the last 5000 yr BP mixed forests of Picea, Pinus, and Abies have developed on andesitic and nonvolcanic terrain, and closed forests of Pinus contorta have persisted on rhyolitic sub- strates. The vegetational patterns are attributed to a combination of climatic and nonclimatic controls operating on different spatial and temporal scales. Climatic changes brought about by the retreat of Laurentide ice and the amplification of the seasonal cycle of radiation explain the broad patterns of vegetational change on millennial time scales. On shorter time scales and smaller spatial scales, substrate differences and migration history shaped the vegetational variability.

Postglacial Vegetation and Climate of Grand Teton and Southern Yellowstone National Parks

Publisher Summary This chapter discusses the advances in both global and regional understanding of Quaternary history, deposits, and geomorphic processes that have brought new information and new techniques for characterizing the growth, decay, and products of the Cordilleran Ice Sheet during the Pleistocene. The Cordilleran Ice Sheet, the smaller of two great continental ice sheets that covered North America during Quaternary glacial periods, extends from the mountains of coastal south and southeast Alaska, along the Coast Mountains of British Columbia and into northern Washington and northwestern Montana. Ice has advanced south into western Washington at least six times, but the marine-isotope record suggests that these are but a fraction of the total that entered the region in the past 2.5 million years. Reconstruction of the Puget lobe of the Cordilleran Ice Sheet during the last glacial maximum requires basal sliding at the rates of several hundred meters per year, with pore-water pressures nearly that of the ice overburden. Landforms produced during glaciation include an extensive low-gradient outwash plain in front of the advancing ice sheet, a prominent system of subparallel troughs deeply incised into that plain and carved mainly by subglacial meltwater, and widespread streamlined landforms.

https://faculty.washington.edu/dbooth/Ch_02_INQUA_volume.pdf

The Cordilleran Ice Sheet

Rapid climate oscillations in the Northeast Pacific during the last deglaciation reflect northern and southern hemisphere sources

The late Quaternary climate of the North Pacific, where according to modelling the solar radiation in the early Holocene at the time of the summer solstice is high and in the late Holocene is relatively low, is investigated. Quantitative temperature and precipitation estimates from southern Alaska are compared with estimates from western Washington and British Columbia. Data extending over more than 10,000 years show a broadly consistent pattern of climatic change in general agreement with predicted variations in solar radiation and their effect on atmospheric circulation and seasonal duration of pressure systems over the North Pacific Ocean. In the early Holocene, the subtropical North Pacific anticyclone annually regulated climate for a longer period at higher latitudes than at present, so that warmth and dryness increased in southern Alaska. The Aleutian low-pressure center intensified during the late Holocene, resulting in colder and more humid coastal climate and increased frequency of glacier growth in the cordillera.

Late-Quaternary climatic change on the American North Pacific Coast

Palynologists utilize present-day pollen rain to interpret the climatic setting of pollen records from Quaternary deposits. Analogues are sought which relate the present with the past. Because climatic conditions at mid-latitudes during the Quaternary were diverse, often ranging from a tundra type at one extreme to a closed forest type at the other, a modern data set should cover the extremes of vegetation and climate expected during this time. For interpreting climatic parameters from Quaternary pollen in land and marine cores, we calculated a pair of regression equations relating modern pollen rain from the Pacific coastal forest and tundra to mean July temperature and mean annual precipitation at a series of sites from the Aleutian Islands to northern California. We describe here how application of these equations to Quaternary pollen profiles from western Washington enabled us to quantify temperature and precipitation over the past ∼80,000 yr.

S. S. Streeter

Papers

Quaternary temperatures and precipitation for the north-west coast of North America