Humptulips revisited: a revised interpretation of Quaternary vegetation and climate of western Washington, USA

New pollen data from a 770-cm core of a mire at Humptulips on the southwest ern 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 Hump tulips River provide evidence for a reinterpretation of previous accounts o f 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 wood land and tundra. In OIS 4, 3, and 2, oscillations in pollen assemblages imp ly repeated intervals of mountain hemlock parkland and tundra. Pine, as an indicator of openness, is also abundant in these stages. Interstadial fluct uations in OIS 3 reflect episodes of lowland western hemlock communities. T he earliest of the episodes is of infinite radiocarbon age and the latest d ates 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 pi ne-alder in OIS 1 (Holocene), which, unlike any of the earlier stages, conv eys the development of modem lowland forest dominated by western hemlock. P ollen assemblages over the length of record imply that temperature and humi dity at no time were as high as at present; only in OIS 3 do conditions app roach those occurring in OIS 1. During OIS 2, 3, and 4, when tree line appa rently stood at the location of the site, climate was colder and drier. Tem peratures 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 b y the location of the Laurentide ice sheet in the continental interior, whe reas under the current climatic regime beginning in the early Holocene, wes terly air flow has dominated, regulated by interplay between the North Paci fic high in summer and the wintertime Aleutian low. (C) 1999 Elsevier Scien ce B.V. All rights reserved.