Estimating soil- saprolite residence time by coupling meteoric 10 Be with 9 Be mass balance

1. Calhoun Critical Zone Observatory Estimating soil-saprolite residence time by coupling meteoric 10Be with 9Be mass balance Allan R. Bacon and Daniel deB. Richter, NSOE, Duke University 200 Summary and Proposal 100 • We cored 30-m through soil and saprolite and 40-m into unweathered granite to estimate soil residence time (SRT) of an ancient biogeomorphically stable Ultisol in the Southern Piedmont United States. • We couple meteoric 10Be inventories with mass balance estimates of pedogenetic l9Be losses to correct the meteoric 10Be inventory and account for observed Be mobility and loss. We concluded that about half of the 9Be weathered from primary minerals has been leached from the soil and saprolitesystem. • Our minimum SRT estimates range between 1.3 to 3.1 Ma under high and low estimates of 10Be deposition (2.0 and 1.3 × 106 atoms cm–2yr–1, respectively). Established denudation rates (DR) of the physiographic region corroborate our approach. • Results indicate that interfluve soils in the region have resided at Earth’s surface for much if not all of the Quaternary, and demonstrate that retention of meteoric 10Be by acidic soilscapes cannot be safely assumed. • We propose to estimate mass balances of total and oxide-bound 9Be in other already collected soil and parent material profiles to evaluate the impact of meteoric 10Be mobility on SRT estimates. Fig. 1. Beryllium and aluminum speciation in equilibrium with Be(OH)2(s) & Al(OH)3(s)in water. LogK for hydrolysis reactions from Lindsey (1979), Renders & Anderson (1987), and Sparks (2003) Fig. 2. The Calhoun CZO is located in the largely granite-derived Southern Piedmont (upper left). The LIDAR-based digital elevation model illustrates interfluves, hill slopes, and stream bottoms (lower right), and a photo illustrates the coring site in an uncultivated hardwood forest (lower left). Fig. 3. Meteoric 10Be and oxide-bound 9Be (hhe9Be) in the Southern Piedmont Ultisol Fig. 4. Tau (τ) plots of important elements, including 9Be, in the Southern Piedmont Ultisol Fig. 5. About half of the 9Be weathered from primary minerals has been leached from the upper 18.3 of soil and saprolite. Acknowledgements This project was funded by the National Science Foundation EAR via the Critical Zone Exploration Network. For ideas and reviews, we thank Paul Bierman and the Cosmogenic Nuclide Lab Group at the University of Vermont,Dylan Rood of Lawrence Livermore National Laboratory, and Dean Hesterberg and Wayne Robarge of NC State University.We thank the Stone family of Cross Keys, SC for allowing access to their land, and the Gill Drilling Service for sample collection