Core Compression and Surficial Sediment Loss of Lake-Sediments of High Porosity Caused by Gravity Coring

We present Pb-210, Cs-137, and Co-60 data from three lakes of the Experimental Lakes Area, NW Ontario, with sediments of high porosity and high organic C content which indicate that gravity coring fails to capture the top 5-15 cm of sediment. Compression of gravity cores by a factor of up to 3 is unequivocal throughout at least the top 15 cm of the cores with the highest porosity but is not so significant in cores of lower porosity. Careful freeze coring, by contrast, retrieves an uncompressed core with surficial sediments intact. Porosity data suggest that the compression in the gravity cores does not result from pore-water loss and therefore must be caused by lateral thinning of the sediments as they enter the corer. Inventories of organisms or pollutants per unit of sediment area estimated from cores suffering from these artifacts would be grossly underestimated.