Comparing X-ray densitometry and BSE-image analysis of thin section in varved sediments

A 10-cm long section from varved sequence of Lake Nautajärvi, Finland, has been analysed using both X-ray densitometry and image analysis of Backscattered Scanning Electron microscope Images (BSEI) of thin sections. Lake Nautajärvi's clastic-organi c varves appear in X-ray images as a succession of dark and pale stripes, pale ones being the spring detrital minerogenic layers and dark ones organic matter laid down during the time of summer, autumn and winter ice cover. Owing to the sharp contrast of the minerogenic spring layer, a semiautomatic tree-ring width and density measurement DendroScan system was applied to obtain seasonal/annual data of relative X-ray density. Mean density of BSEI was also acquired. Then BSEI were processed to produce black and white images, where white pixels represent the clay-rich sedimentary matrix and black pixels the particles in the matrix: silt- or sand-sized, terrigenous grains, authigenic particles and diatoms. Measurements of the size of the detrital particles forming the varves were obtained. For each varve, mean grey-level values in BSEI correlated well with the mean grey-level values of X-ray images, but grain size obtained on BSEI did not correlate with any parameters computed from X-ray radiography.