T82 Developing Proxies for Human Impact on Soil and Sediment Mass Transfer throughout the Holocene

Monday, 2 November 2015
8:00 AM - 12:00 PM
Room 318

EFFECTS OF GRAIN SIZE AND MINERALOGY ON THE DISTRIBUTION OF FALLOUT RADIONUCLIDES 7BE, 10BE, 137CS, AND 210PB IN SEDIMENT

Paper number: T82-6
Program: Technical Sessions
Time: 9:20 AM – 9:35 AM

Authors

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 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.