SMEARING OF GRAVITY CORE PROFILES IN SOFT SEDIMENTS

Gravity coring of soft sediments can result in smearing of sediment between adjacent vertical sections in a core, We have quantified this artifact in a laboratory experiment with soft, highly porous gyttja using radiotracers to label the surface of the sediments. After coring, extrusion, and sectioning, the radiotracers were measured down to 5 cm in inner core sections and to 10 cm in outer core sections. We have estimated the impact of smearing on the interpretation of concentration-depth profiles, and gradients calculated from smeared profiles can be less than those occurring in situ in the sediments. This effect seems most pronounced near the mud-water interface. We have simulated radionuclide profiles in sediments from Chernobyl fallout (e.g. Cs-134) and natural emissions (e.g. Pb-210) to examine this problem. Concentration gradients of these radionuclides and of other contaminants used to establish geochronologies can be altered Significantly depending on sediment type. A tracer layered on the surface of the sediments can be used to quantitate smearing effects, and corrections can then be made to concentration-depth profiles.