Explicit treatment of inheritance in dating depositional surfaces using in situ (super 10) Be and (super 26) Al

We describe a new strategy for dating depositional landscape surfaces using in situ - produced cosmogenic radionuclides (CRNs) that removes the complication of nuclide inheritance by clasts prior to deposition. Two amalgamated samples, each consisting of 30 clasts, one from the surface and one from a fixed depth in the subsurface, constrain this CRN inheritance and date the surface. The inheritance may be used to estimate minimum exhumation rates and maximum transport times within the geomorphic system. We test the technique using (super 10) Be and (super 26) Al to date the third (FR3) of five terraces along the Fremont River, Utah, and the third (WR3) of 15 along the Wind River, Wyoming. Whereas effective ages based solely upon the surface samples yield 118 - 138 ka (FR3) and 93 ka (WR3), the subsurface samples reveal that inheritance accounts for 22% (WR3) to 43% (FR3) of the total CRN concentration. Taking this into account yields terrace ages of approximately 61 - 81 ka for FR3 and approximately 67 - 76 ka for WR3. We explore the dependence of age estimates on the accumulation history of the terrace silt caps.