Radiometric fingerprinting of fluvial sediments in the Rhine-Meuse delta, the Netherlands - A feasibility test

Abstract

The deposits of the Rhine and the Meuse in the Netherlands alternate in their delta in a complex way. This paper discusses a method to distinguish the deposits of the Rhine and the Meuse based on the differences in natural radioactivity of 40K, 238U and 232Th, and the effect of the age of the deposits on the radiometric signal. In total, six channel belts of the Rhine and the Meuse were selected for sampling with an approximate age of about 2000, 4000 and 6000 14C years B.P. Of each channel belt 5 samples of different lithology were taken: clay (C), clay loam (CL), sandy clay loam (sCL), sandy loam (sL) and sand (S). All samples were analysed on organic matter content, grain size, geochemistry and radioactivity of the radionuclides 40K, 238U and 232Th. The radioactivity of the sample is mainly influenced by the grain size of the sample. Therefore, this signal is divided in partial radioactivities for three grain size fractions - clay (<16 μm), silt (16 · 63 μm) and sand (>63 μm) - to make the radiometric fingerprint, which is independent of the grain size of the sample. These fingerprints show a difference between the Rhine and the Meuse. Additionally, the radiometric signal strongly depends on the age of the deposits. Remarkably, this trend with age is opposite in the deposits of the Rhine and the Meuse and opposite in the clay and silt fraction. Because the radiometric differences between the samples seem more distinct than the geochemical differences, the radiometric fingerprints are more suitable to distinguish the deposits of the Rhine and the Meuse. A method is presented to derive the contribution of the Rhine and the Meuse in a deposit of unknown origin, assuming that the radiometric fingerprints found are consistent and valid for the Rhine-Meuse delta. To distinguish the deposits of the Rhine and the Meuse, both the grain size composition and the age of the deposits have to be known.