Paleoclimatic fluctuations from 50,000 yr BP to the present are recorded in pollen assemblages from buried Pleistocene peat and in postglacial bogs in the Puget Lowland of Washington.

Two peat beds in Possession Drift, radiocarbon-dated at 47,600 + 3.300/− 1,800 and 34,900 + 3,000/− 2,000 yr B.P., contain high percentages of pine with minor spruce, fir, and western red cedar. Significant representation of total NAP suggests an open landscape dominated by herbs, with intermittent patches of lodgepole pine, characteristic of a cool climate and unstable physiographic conditions.

The nonglacial interval immediately preceding the last glacial advance was originally defined as the Olympia Interglaciation, but new radiocarbon and palynological evidence now suggest that it should be considered to be a nonglacial interval of less than interglacial rank. Olympia peat beds yielded radiocarbon dates of 22,700 ± 550, 22,700 ± 600, 24,800 ± 600, 26,850 ± 1,700, 27,200 ± 1,000, and 27,600 ± 1,000 yr B.P. Pine maintains a dominant role throughout the units, although spruce, mountain hemlock, and total NAP increase in the younger units, suggesting a trend toward a cooler climate.

Pollen from sediments of the Everson Interstade of the Fraser Glaciation is dominated by lodgepole pine, suggesting that the environment was characterized by extreme edaphic disturbance and severe climatic conditions.

Postglacial bogs show a lower pine–Douglas fir zone and an upper western red cedar–western hemlock zone, separated by an ash younger than 7,140 ± 600 yr B.P. The Hypsithermal is marked by high pollen values for Douglas fir about 7,000 yr ago, followed by increased western red cedar and western hemlock, implying a moister, cooler climate.

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