Event Title

Effects of Grain Size and Mineralogy on the Distribution of Fallout Radionuclides 7Be,10Be, 137Cs,and 210Pb in Sediment

Presenter Information

Adrian Singleton, Oberlin College

Location

Science Center A247

Start Date

10-2-2015 4:30 PM

End Date

10-2-2015 5:50 PM

Abstract

The fallout radionuclides (FRNs) 7Be, meteoric 10Be, 137Cs, and unsupported 210Pb are widely used by geomorphologists as tracers to create sediment budgets. Many studies report higher concentrations of these FRNs in finer fractions or on specific minerals in sediments and soils. Understanding how, why, and to what extent preferential distribution of FRNs occurs is essential for evaluating the assumption that FRNs are uniformly, rapidly and irreversibly adsorbed to sediment, a fundamental condition for using FRNs in erosion modeling. We evaluated the relative role 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 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.

Notes

Session III, Panel 7 - TIME: Nature & Change

Major

Geology; Trombone

Project Mentor(s)

Amanda Schmidt, Geology

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Oct 2nd, 4:30 PM Oct 2nd, 5:50 PM

Effects of Grain Size and Mineralogy on the Distribution of Fallout Radionuclides 7Be,10Be, 137Cs,and 210Pb in Sediment

Science Center A247

The fallout radionuclides (FRNs) 7Be, meteoric 10Be, 137Cs, and unsupported 210Pb are widely used by geomorphologists as tracers to create sediment budgets. Many studies report higher concentrations of these FRNs in finer fractions or on specific minerals in sediments and soils. Understanding how, why, and to what extent preferential distribution of FRNs occurs is essential for evaluating the assumption that FRNs are uniformly, rapidly and irreversibly adsorbed to sediment, a fundamental condition for using FRNs in erosion modeling. We evaluated the relative role 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 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.