Diatom, Pollen, and Chemical Evidence of Postglacial Climatic Change at Big Lake, South-Central British Columbia, Canada

Abstract

Postglacial climatic conditions were inferred from cores taken from Big Lake in southern British Columbia. Low concentrations of nonarboreal pollen and pigments near the base of the core suggest that initial conditions were cool. Increases in both aquatic and terrestrial production suggest warmer and moister conditions until ∼8500 cal yr B.P. Hyposaline diatom assemblages, increases in nonarboreal pollen, and increased concentrations of pigments suggest the onset of arid conditions from ∼8500 to ∼7500 cal yr B.P. Slightly less arid conditions are inferred from ∼7500 until ∼6660 cal yr B.P. based on the diatoms, small increases and greater variability in biogenic silica and pigments, and higher percentages of arboreal pollen. At ∼6600 cal yr B.P., changes in diatoms, pigments, biogenic silica, and organic matter suggest that Big Lake became fresh, deep, and eutrophic until ∼3600 cal yr B.P., when water levels and nutrients decreased slightly. Our paleoclimatic inferences are similar to pollen-based studies until ∼6600 cal yr B.P. However, unlike these studies, our multiple lines of evidence from Big Lake imply large changes in effective moisture since 6000 cal yr B.P.

Climatic control of ultraviolet radiation effects on lakes

“…Our analysis focused on Big Lake, a site characteristic of the region but with prior paleoecological study of climate change and lake response during the Holocene era (Bennett et al 2001). Big Lake is located within a mixed coniferous forest zone and presently has DOM levels (Ͼ20 mg dissolved organic carbon [DOC] L Ϫ1 ) sufficient to limit UVR penetration to a few centimeters.…”

“…Low-elevation lakes were cored using a standard combination of Livingstone piston and Glew gravity corers (Bennett et al 2001), whereas Crowfoot Lake sediments were collected using a light-weight percussion corer (Reasoner et al 1994). Full postglacial sequences were recovered in all instances (ϳ12,000 14 C yr).…”

“…Full postglacial sequences were recovered in all instances (ϳ12,000 14 C yr). Sediment ages were determined using accelerator mass spectrometric (AMS) analyses of 14 C activities in Crowfoot and Big lakes (10-18 dates core Ϫ1 ; Reasoner et al 1994;Bennett et al 2001), volcanic ash layers (tephra), including the Bridge River (2,360 14 C yr BP) and Mazama eruptions (6,730 14 C yr BP) in all lakes (e.g., Reasoner et al 1994), and AMS-dated lithostratigraphic transitions at the base of each core (Heinrichs et al 1999;Reasoner and Huber 1999).…”

“…UVR penetration was elevated and algal abundance was at a historical minimum before ϳ10,700 14 C yr BP. Details of accelerator mass-spectrometric 14 C (AMS; black squares), volcanic ash (open triangles), and clay interface (black triangles) dates are as in Bennett et al (2001) and Heinrichs et al (1999). Replicate analyses are indicated with multiple symbols.…”

“…Replicate analyses are indicated with multiple symbols. Nonarboreal pollen (NAP) abundance (as % fossil pollen sum; panel b) was modified from Bennett et al (2001). (Leavitt et al 1997) to identify the potential mechanisms that cause long-term ecosystem change.…”