Postglacial midge community change and Holocene palaeotemperaturereconstructions near treeline, southern British Columbia (Canada)

Stratigraphic analysis of fossil chironomid head capsules wasperformed at North Crater Lake and Lake of the Woods, located at treeline (2250m) in the Ashnola region of southernmost British Columbia. Priorto 10,000 yr BP, cold conditions were indicated by the lack oftemperate taxa and the presence of cold-stenotherms. The abundance anddiversity of warm-adapted taxa (e.g., Dicrotendipes,Microtendipes, Polypedilum and Cladopelma)increased rapidly after 9500 yr BP, whereas taxa indicative ofcold conditions disappeared. Beginning prior to deposition of the Mazama ash(6730 ± 40 yr BP), several warm-adapted taxa decreasedin abundance. Mid- to late-Holocene assemblages (ca. 4500yr BP to present) indicated continued cooling as revealed by afurther reduction in diversity and abundance of warm-adapted taxa atboth lakes, and the reappearance of cold-stenotherms in Lake of theWoods. Diversity changes in the cores paralleled the inferred climatic changes.Diversity was low during the late-glacial, increased in theearly-Holocene, and declined after 5400 yr BP.To quantitatively infer past climatic changes, a newweighted yphen;averaging partial-least-squares (WA-PLS)model was developed and applied to the fossil midge data. The quantitativereconstructions revealed late-glacial mean July air temperatures rangingfrom about 8 to 10°C. Summer air temperatures were highest inthe early Holocene (13 to 17°C), gradually decreasing by about3°C through the mid- to late-Holocene.