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

Stratigraphic analysis of fossil chironomid head capsules was performed at North Crater Lake and Lake of the Woods, located at treeline (2250 m) in the Ashnola region of southernmost British Columbia. Prior to 10,000 yr BP, cold conditions were indicated by the lack of temperate taxa and the presence of cold-stenotherms. The abundance and diversity of warm-adapted taxa (e.g., Dicrotendipes, Microtendipes, Polypedilum and Cladopelma) increased rapidly after 9500 yr BP, whereas taxa indicative of cold conditions disappeared. Beginning prior to deposition of the Mazama ash (6730 +/- 40 yr BP), several warm-adapted taxa decreased in abundance. Mid- to late-Holocene assemblages (ca. 4500 yr BP to present) indicated continued cooling as revealed by a further reduction in diversity and abundance of warm-adapted taxa at both lakes, and the reappearance of cold-stenotherms in Lake of the Woods. Diversity changes in the cores paralleled the inferred climatic changes. Diversity was low during the late-glacial, increased in the early-Holocene, and declined after 5400 yr BP To quantitatively infer past climatic changes, a new weighted-averaging partial-least-squares (WA-PLS) model was developed and applied to the fossil midge data. The quantitative reconstructions revealed late-glacial mean July air temperatures ranging from about 8 to 10degreesC. Summer air temperatures were highest in the early Holocene (13 to 17degreesC), gradually decreasing by about 3degreesC through the mid- to late-Holocene.