HYDROCLIMATIC VARIABILITY AND LAND USE CHANGES INDICATED BY SEDIMENTOLOGICAL PROPERTIES OF VARVES IN TEMISKAMING LAKE, ONTARIO/QUÉBEC

Temiskaming Lake is a long (60 km) and deep (200m) water body of tectonic origin situated in the Upper Ottawa River, Ontario/Québec. The lake is currently characterised by high turbidity levels resulting from sediment delivery by the Blanche River, Wabi Creek and other minor tributaries. Density changes (X-radiography), grain size variability, major elemental changes (ITRAX core scanner) along with visual properties were recorded at high resolution (≤1 mm) on a network of short and long sediment cores (gravity cores and vibracores) retrieved in the northwest section of the lake to identify hydroclimatic controls over sedimentation. The sedimentary record contains fine and irregular laminations overlaid by an upper unit of  2-10 mm thick couplets of silt and clay. These couplets are well defined by their elemental properties (variability in Ca, Fe) recorded by the ITRAX core scanner, their micro-density structure, as well as their color and grain size properties. Independent dating methods (cesium-137 and historical palynological markers) confirm that these couplets constitute clastic varves. We hypothesise that hydrological changes associated with partial deforestation of the watershed and the development of agriculture at the end of the 19^th century are responsible for the development of these well-defined clastic varves. Individual properties of these recent varves (principally density and grain size) are controlled in part by the variability of spring river discharge. The high resolution record of elemental properties of the sediments also constitutes a unique archive of pollutant accumulation and regional mining activities. Our study shows that the use of non-destructive analytical methods (ITRAX and X-radiographs) can provide quick and accurate assessment of sedimentological properties of clastic laminated sediments.