EFFECTS OF GRAIN SIZE AND MINERALOGY ON THE DISTRIBUTION OF FALLOUT RADIONUCLIDES 7BE, 10BE, 137CS, AND 210PB IN SEDIMENT
We evaluated the relative roles of composition and grain size in determining FRN distribution in sediments via a natural delivery experiment and by measuring concentrations in different grain size fractions of detrital river sediments. For the natural delivery experiment, we monitored the retention of FRNs 210Pb and 7Be in five grain sizes (from <63μm to 850μm) of quartz and river sediment as well as five sheet silicates. Samples were exposed outside to fallout in filter-bottomed beakers for over a year. In addition, we measured the concentration of 10Be, 137Cs, and 210Pb in five grain sizes of detrital river sediment from the Mekong watershed in Yunnan, China, and the Vermilion River in Ohio, USA. We constrained sediment composition via microscopy, XRD, and XRF.
Grain size dependency of FRN concentration was significant in the detrital samples, but not in the experimental setup. This outcome may display the importance of time-dependent geochemical, pedogenic, and sedimentary processes in the development of preferential FRN distribution. Mineralogy controlled FRN concentration in the detrital samples even when grain size was accounted for. Likewise, composition had a large effect on FRN retention in the experimental setup. For example, we observed 7Be retention in all samples except for kaolinite. Our findings show that FRN distribution in sediment is greatly affected by mineralogy and suggest that sieving sediments to a single grain size may not fully control for preferential distribution of FRNs.