New pollen data from a 770-cm core of a mire at Humptulips on the southwestern Olympic Peninsula (47 degrees 17'00"N, 123 degrees 54'40"W) and from a nearby deposit regarded as interglacial in age on the West Fork of the Humptulips River provide evidence for a reinterpretation of previous accounts of the Quaternary vegetation and climate. Using a new age model, the highly variable record at Humptulips is here correlated with marine oxygen-isotope stages (OIS) 1-5a. Vegetation during OIS 5a was pine-dominated, mixed woodland and tundra. In OIS 4, 3, and 2, oscillations in pollen assemblages imply repeated intervals of mountain hemlock parkland and tundra. Pine, as an indicator of openness, is also abundant in these stages. Interstadial fluctuations in OIS 3 reflect episodes of lowland western hemlock communities. The earliest of the episodes is of infinite radiocarbon age and the latest dates to approximately 30,000 C-14 yr B.P Parkland in OIS 2, occurring after 24,600 until at least 18,440 C-14 yr B.P., was replaced by transitional pine-alder in OIS 1 (Holocene), which, unlike any of the earlier stages, conveys the development of modem lowland forest dominated by western hemlock. Pollen assemblages over the length of record imply that temperature and humidity at no time were as high as at present; only in OIS 3 do conditions approach those occurring in OIS 1. During OIS 2, 3, and 4, when tree line apparently stood at the location of the site, climate was colder and drier. Temperatures were depressed an estimated greater than or equal to 5 degrees C with precipitation close to 1000 mm compared with 2000-3000 mm at present. Atmospheric circulation during OIS 2 appears to have been much controlled by the location of the Laurentide ice sheet in the continental interior, whereas under the current climatic regime beginning in the early Holocene, westerly air flow has dominated, regulated by interplay between the North Pacific high in summer and the wintertime Aleutian low. (C) 1999 Elsevier Science B.V. All rights reserved.
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