Radiometric fingerprinting of fluvial sediments in theRhine-Meuse delta, the Netherlands – a feasibility test

Abstract

The deposits of the Rhine and the Meuse in the Netherlands alternate intheir delta in a complex way. This paper discusses a method to distinguishthe deposits of the Rhine and the Meuse based on the differences in naturalradioactivity of 40K, 238U and 232Th, andthe effect of the age of the deposits on the radiometrie signal. In total,six channel belts of the Rhine and the Meuse were selected for sampling withan approximate age of about 2000, 4000 and 6000 14C years B.P. Ofeach channel belt 5 samples of different lithology were taken: clay (C),clay leads (CL), sandy clay loam (sCL), sandy loam (sL) and sand (S). Allsamples were analysed on organic matter content, grain size, geochemistryand radioactivity of the radionuclides 40K, 238U and 232Th. The radioactivity of the sample is mainly influenced bythe grain size of the sample. Therefore, this signal is divided in partialradioactivities for three grain size fractions – clay (<16 μm), silt (16– 63 μm) and sand (>63 μm) – to make the radiometric fingerprint, whichis independent of the grain size of the sample. These fingerprints show adifference between the Rhine and the Meuse. Additionally, the radiometricsignal strongly depends on the age of the deposits. Remarkably, this trendwith age is opposite in the deposits of the Rhine and the Meuse and oppositein the clay and silt fraction. Because the radiometrie differences betweenthe samples seem more distinct than the geochemical differences, theradiometric fingerprints are more suitable to distinguish the deposits ofthe Rhine and the Meuse. A method is presented to derive the contribution ofthe Rhine and the Meuse in a deposit of unknown origin, assuming that theradiometric fingerprints found are consistent and valid for the Rhine-Meusedelta. To distinguish the deposits of the Rhine and the Meuse, both thegrain size composition and the age of the deposits have to be known.