2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 82-5
Presentation Time: 9:20 AM


SINGLETON, Adrian1, HENCK SCHMIDT, Amanda2, SOSA-GONZALEZ, Veronica3, QIU, Yue4, NEILSON, Thomas3, GREENE, Emily Sophie5, BIERMAN, Paul3, ROOD, Dylan6, CAMPBELL, Mary K.7 and WOODMANSEE, Sylvia Jane8, (1)Geology, Oberlin College, 52 W. Lorain St., Carnegie Building, Rm. 403, Oberlin, OH 44074, (2)Geology, Oberlin College, 52 West Lorain Street, Oberlin, OH 44074-1044, (3)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave., Burlington, VT 05405, (4)Geology, Oberlin College, 52 West Lorain Street, Oberlin, OH 44074, (5)Geology, UVM, 180 Colchester Avenue, Burlington, VT 05401, (6)Earth Research Institute, University of California, 6832 Ellison Hall, University of California, Santa Barbara, CA 93106-3060, (7)Geology, Oberlin College, 173 W Lorain St., Oberlin, OH 44074, (8)135 West Lorain St, OCMR 2883, Oberlin, OH 44074, asinglet@oberlin.edu

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.