Radiometric sand-mud characterisation in the Rhine-Meuse estuary part A. Fingerprinting

Abstract The composition of the underwater sediment bed is often defined in terms of the mud ( 63 and 40 K content of the sediment. Differences in age or composition, in terms of mineralogy, organic matter or carbonate content, only slightly affected the radiometric characteristics based on 232 Th and 238 U. On the other hand, no reliable correlation between 40 K and the mud content was obtained; therefore, 40 K is excluded from the fingerprint. The fingerprints based on the 232 Th and 238 U contents characterise mud ( 63 μm) as: 46.2±1.9 Bq/kg 238 U and 9.3±0.9 Bq/kg 238 U and 45.6±1.9 Bq/kg 232 Th and 9.7±0.9 Bq/kg 232 Th, respectively. The assessed radiometric fingerprint allows a quantitative interpretation of the mud and sand content through the total 238 U+ 232 Th activity. This interpretation is confirmed by the high correlation ( R 2 =0.96) for the mud percentages obtained from radiometric analysis and those by laser diffraction in the laboratory. It can be concluded that radiometric sedimentology provides a tool for reliable sediment characterisation. New perspectives are opened when radiometric fingerprints are combined with in situ radiometric characterisation of sediments, which will be presented in part B of this paper (this issue).