Similarities and discrepancies between chironomid- and diatom-inferred temperature reconstructions through the Holocene at Lake 850, northern Sweden

摘要

A quantitative temperature reconstruction using chironomids and diatoms has been attempted from a high elevation lake in northern Sweden (Lake 850). Since 7000 cal. years BP, both chironomids and diatoms recorded similar temperatures (in the range of present-day estimates) but the correspondence between chironomid and diatom-inferred temperatures was highest in the recent Holocene (2500 cal. years BP to the present). Between ca. 9000 and 7000 cal. years BP, inferred temperatures from chironomids were warmer than today (ca. 1-2℃), in accord with other climate reconstruction using pollen, plant macrofossils and oxygen isotope analysis in lakes of northern Scandinavia. In contrast, diatom analysis did not infer warmer temperatures during this period. The insensitivity of diatoms to temperature in Lake 850 between 9000 and 7000 cal. years BP could be attributed to other environmental factors affecting the diatom assemblages through time, especially lake-water pH. Diatom-inferred pH showed a gradual decrease (0.5 pH units) between 9000 and 7000 cal. years BP while it remained more or less constant since 7000 cal. years BP. Changes in lake-water pH acting on diatoms seem to mask the effect of climate, leading to temperature reconstructions that are inaccurate. Ways of disentangling climate and other environmental factors when attempting climate reconstruction should be further investigated.