Environmental history and tephrostratigraphy at Carp Lake, southwestern Columbia Basin, Washington, USA

Sediment cores from Carp Lake provide a pollen record of the last ca. 125,0 00 years that helps disclose vegetational and climatic conditions from the present day to the previous interglaciation (120-133 ka). The core also con tained 15 tephra layers, which were characterised by electron-microprobe an alysis of volcanic glass shards. Identified tephra include Mount St. Helens Ye, 3.69 ka; Mazama ash bed, 7.54 ka; Mount St, Helens layer C, 35-50 ka; an unnamed Mount St. Helens tephra, 75-150 ka; the tephra equivalent of lay er E at Pringle Falls, Oregon, <218 ka; and an andesitic tephra layer simil ar to that at Tulelake, California, 174 ka, Ten calibrated radiocarbon ages and the ages of Mount St. Helens Ye, Mazama ash, and the unnamed Mount St, Helens tephra were used to develop an age-depth model. This model was refi ned by also incorporating the age of marine oxygen isotope stage (IS) bound ary 4/5 (73.9 ka) and the age of IS-Se (125 ka). The justification for this age-model is based on an analysis of the pollen record and lithologic data . The pollen record is divided into 11 assemblage zones that describe alter nations between periods of montane conifer forest, pine forest, and steppe. The previous interglacial period (IS-5e) supported temperate xerothermic f orests of pine and oak and a northward and westward expansion of steppe and juniper woodland, compared to their present occurrence. The period from 83 to 117 ka contains intervals of pine forest and parkland alternating with pine-spruce forest, suggesting shifts from cold humid to cool temperate con ditions. Between 73 and 83 ka, a forest of oak, hemlock, Douglas-fir, and f ir was present that has no modem analogue. It suggests warm wet summers and cool wet winters. Cool humid conditions during the mid-Wisconsin interval supported mixed conifer forest with Douglas-fir and spruce. The glacial int erval featured cold dry steppe, with an expansion of spruce in the late-gla cial. Xerothermic communities prevailed in the early Holocene, when tempera te steppe was widespread and the lake dried intermittently. The middle Holo cene was characterised by ponderosa pine forest, and the modern vegetation was established in the last 3900 yr, when ponderosa pine, Douglas-fir, fir, and oak were part of the local vegetation. (C) 2000 Elsevier Science B.V. All rights reserved.