RECONSTRUCTION OF HOLOCENE HYDROLOGIC VARIABILITY, USING LAMINATED LAKE SEDIMENTS FROM CLELAND LAKE, SOUTH EASTERN BRITISH COLUMBIA, CANADA

Visible derivative spectroscopy (VDS) is a non-destructive method of quantitatively analyzing the organic matter content (including plant pigments), clay minerals, carbonates and iron oxides in sediments. Diffuse spectral reflectance (DSR) was measured by VDS at 0.5 cm resolution on three sediment cores collected from Cleland Lake located in southern British Columbia. We applied a varimax-rotated Principal Component Analysis to reduce the complexity of the data set and identify underlying patterns and trends. Four principal components (PC) were identified that explained 95% of the variance in the data. Three of the PCs correlated with the standard reflectance curves of diatom, algae and blue-green algae pigments and therefore can be used as paleo-productivity (LP) proxies. Surface sediments ranging in age from 1829 to 2007 A.D. were compared with historical climate records in the area. A significant correlation (r=0.82, α= 0.05, n=13) was observed between PC 3 (diatom pigments and inverse spectrum of phycocyanin) and total annual rainfall. This suggests that low rainfall is likely correlated with greater warmth and lake stratification and favor the growth of blue-green (BG) algae, while higher rainfall leads to transition from a B-G algae community to a diatom community when runoff and nutrient flux increases. The highest recorded BG algae productivity and low diatom productivity was observed between 11,500 and 9.500 years B.P., coincident with the northern hemisphere summer insolation maximum. Reconstruction of paleo-rainfall based on the LP proxy suggested a rapid decline in the rainfall and low lake levels during the early Holocene around 11,500 to 9,200 years B.P. The local climate returned to higher rainfall and lower BG around 8,500 years B.P. The climatic (rainfall) variability observed during the early Holocene was an order of magnitude greater than the present climate variability in the region. These results are consistent with the regional pollen records, which suggest a transition to drier and warmer conditions around 10,300 years B.P at the start of the postglacial period (xerothermic interval; Hallet and Hills, 2006; Mathews and Heusser, 1980).