AN ASSESSMENT OF SEDIMENT LOSS AND DISTORTION AT THE TOP OF SHORT GRAVITY CORES

Examination of laminated bottom sediment cores from two glacier-fed lakes in the Canadian Rocky Mountains allows an assessment of the amount of material lost or badly distorted by open-barrel gravity corer impact. Analysis of sediment 137Cs content indicates that the laminae are of annual periodicity and allows precise dating of individual laminae. Examination of 8 cores from Hector Lake indicates that a mean of 4.6 mm of sediment was missing from the top of each, using sedimentation rates in the recovered sections of core and lake wide sedimentation rate variations to approximate sedimentation rates in the missing sections of the core. An additional mean thickness of 3.6 mm was strongly distorted. Analysis of 14 cores from Bow Lake indicates that on average 6.4 mm of sediment was lost at core tops and an additional 11.4 mm or less suffered severe distortion. These data suggest that, at least in the lakes studied, high impact velocity gravity coring has less of an effect on core-top sediment than has previously been reported. © 1990.