A chironomid-based paleolimnological investigation of historic and late Pleistocene-early Holocene changes in climate in the Sierra Nevada, California, U.S.A.

摘要

Modern chironomid remains (Insecta: Diptera) and subfossil chironomid remains recovered from lakes in the Sierra Nevada of California were analyzed statistically in order to reconstruct historic and late Pleistocene-early Holocene climatic changes. Chironomid remains present in the surface sediment of a suite of lakes in the eastern Sierra Nevada were identified and enumerated in order to determine the modern geographic distribution and environmental relations of chironomids in the region. Ordination analyses identified that five environmental variables—surface water temperature, elevation, lake depth, particulate organic carbon and strontium—explained independent and statistically significant amounts of variance in chironomid distribution. A number of transfer functions were developed to estimate surface water temperature from the chironomid assemblages, with the best inference model based on weighted-averaging with classical deshrinking (r2 = 0.73; RMSEP = 1.2°C; maximum bias = 0.90°C). High resolution subfossil stratigraphies were developed for two high elevation lakes, with one that supports introduced fish and one that does not, in order to identify whether the introduction of golden trout would necessarily alter chironomid community composition and compromise the application of chironomid-based water temperature inference models. The changes that occurred post-fish introduction were limited to a decrease in head capsule concentration and chironomid richness. Application of a chironomid-based inference model to the stratigraphies, revealed the existence of similar water temperature trends between AD 1830 and AD 1998 for both lakes. Fluctuations in the inferred water temperature of both lakes closely track changes in mean July air temperature in Fresno, California, suggesting that climate, specifically summer temperature, has an overriding influence on the composition of the chironomid communities in these lakes. Analysis of subfossil chironomids recovered from Greenstone Lake, a high elevation lake in the central Sierra Nevada, was undertaken to assess chironomid community development across the Pleistocene-Holocene transition. Application of a chironomid-based inference model suggests that a minimum warming of 4.7°C occurred during the Pleistocene-Holocene transition. However, warming during the late Pleistocene was not monotonic. A period of climatic deterioration occurred between 12,000 cal yr BP and 11,500 cal yr BP, which may be correlative with the Younger Dryas.