GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 81-29
Presentation Time: 9:00 AM-5:30 PM


SINGLETON, Adrian1, ISAACS, Chloe1, HILL, Marcus1, DIX, Monica1, GREENE, Emily Sophie2, SCHMIDT, Amanda H.1 and BIERMAN, Paul R.3, (1)Geology, Oberlin College, Geology Department, Rm. 403, 52 W. Lorain St, Oberlin, OH 44074, (2)Geology, UVM, 180 Colchester Avenue, Burlington, VT 05401, (3)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405,

Unsupported Pb-210 is a commonly used as both a tracer and geochronometer for fluvial, lacustrine, and pedogenic studies. Complications arise when Pb-210 is measured in detrital, fluvial sediments. Because Pb-210 resides both in the substrate (supported) and in grain coatings (excess or unsupported), measuring Pb-210 requires subtracting a proxy for supported Pb-210 from total Pb-210 measurements to determine unsupported Pb-210 activity. This necessarily increases the relative error in unsupported Pb-210 measurements. Typical methods use Ra-226, Pb-214 and/or Bi-214 as proxies for the activity of supported Pb-210. Samples are sealed within containers and are allowed to reach secular equilibrium before gamma counting, which unlike a natural environment, does not allow for gaseous escape and therefore leads to the overestimation of supported Pb-210.

We propose a new technique to minimize errors in unsupported Pb-210 measurements by physically separating grain coatings from substrate material in samples and counting Pb-210 activity only in the grain coatings; this approach removes the need to subtract estimates of supported Pb-210 activity. To separate grain coatings from substrate material, we leach samples for 24 hours in 6M HCl in a warm sonic bath. We then decant the leachate and dry it down. This dried leachate is counted for at least 24 hours in a low-energy, ultra-low background germanium detector.

Preliminary results on samples from western China suggest that the sum of the total Pb-210 activity in the leachate and residue for samples is within error of the total Pb-210 activity measured in the bulk sample. Furthermore, the errors in measurement of the Pb-210 activity of the leachate are significantly reduced from the errors in unsupported Pb-210 measured in bulk samples. We find that the activity of Pb-210 in the leachate is higher than the unsupported Pb-210 activity estimated from the entire sample, suggesting that traditional techniques of measuring Pb-210 overestimate supported Pb-210. We propose that this technique as a more accurate and precise way to measure activity of unsupported Pb-210.