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

Paper No. 204-9
Presentation Time: 9:00 AM-6:30 PM


GREENE, Emily Sophie, Geology, UVM, 180 Colchester Avenue, Burlington, VT 05401, BIERMAN, Paul, Burlington, VT 05405 and PERDRIAL, Nicolas, Burlington, VT 05404, esgreene@uvm.edu

By measuring 10Be produced in rock and soil (in situ 10Be - 10Beis), one can obtain average denudation rates over millennia. However, 10Beis analysis is methodologically limited to locations where quartz-bearing rocks crop out and to systems where one can isolate sand-size quartz grains. In other systems, meteoric 10Be (10Bemet) could provide a similar tool, but pedogenic processes, soil chemistry, and grain size control concentrations. Because 9Be weathered from crustal rocks has the same reactivity as 10Bemet, others have hypothesized that 10Bemet/9Be ratios of grain coatings normalize the 10Bemet inventory to remobilization and grain size dependency. To test this hypothesis, we determined the relationship between 10Bemet, 10Beis and 9Be by extracting 9Be from three sets of fluvial sand samples previously measured for 10Bemet and 10Beis.

We leached 9Be from 206 samples by reacting powdered sands in 6M HCl for 24 hours while reacting in a heated ultrasonic bath – a method that did not leach 9Be in mineral grains. This is significant because 9Be in grain coatings is subject to the same post-depositional influences as 10Bemet, while 9Be incorporated into mineral matrixes is uninfluenced by post-depositional conditions, similar to 10Beis. The leachate was analyzed by ICP-OES.

Samples show different 10Bemet /9Be correlations with 10Beis across sampling locations. Samples from the Mekong River watershed in China have a positive correlation between 10Beis and 10Bemet (R2=0.46, n=119) and a stronger positive correlation between 10Beis and 10Bemet/9Be (R2=0.69). Samples from the Barron River Watershed in Queensland Australia have no correlation between 10Beis and 10Bemet (R2=0.05, n=16) but a positive correlation between 10Beis and 10Bemet/9Be (R2=0.56). Samples from the Potomac River Watershed in east-central North America show no correlation between 10Beis and 10Bemet (R2=0.06, n=71) or 10Beis and 10Bemet/9Be (R2=0.00). Considering all data, correlations between 10Bemet/9Be ratios and 10Beis (R2=0.70, n=206) are only slightly stronger than correlations between 10Beis and 10Bemet (R2=0.66). These results suggest that while measuring 9Be can improve correlations between 10Bemet and 10Beis, further study is needed to understand the variables that influence Be dynamics across a diversity of watersheds.