<SUP>10</SUP>Be concentrations and the long-term fate of particle-reactive nuclides in five soil profiles from California

Concentration-depth profiles of cosmic-ray-produced 10Be (t1/2 = 1.5m.y.) have been measured by accelerator-mass spectrometry in five soil profiles. These measurements were made in an effort (1) to understand the retentivity of soil surfaces for particle-reactive tracers depositing from the atmosphere on time scales of 104-106 years, and (2) to explore the application of 10Be as a chronometer of geomorphic surface age. The profiles sampled are from two wave-cut terraces located near Mendocino, California, a table mountain top and an alluvial fan, both located near Friant, California. The ages of the Mendocino terraces are inferred to be (1-5) &#215; 105 years based on amino-stratigraphic correlations and models of terrace evolution; those of the table mountain top and alluvial fan are 9.5 &#215; 106 years and 6.0 &#215; 105 years, respectively, based on K-Ar analyses. All the surfaces sampled are nearly flat and exhibit few erosional features. In addition to 10Be we measured 210Pb, 239,240Pu and 7Be to ascertain the retentivity of the soils for particle-reactive nuclides and to assess the present-day delivery rate of nuclides from the atmosphere. The 7Be inventory is 4.0 dpm/cm2 similar to those observed at nearby locations. The inventories of 210Pb and Pu isotopes conform to those predicted from model calculations and suggest that the soil surfaces sampled retain the entire burden of particle-reactive nuclides delivered to them over short time scales, ~ 100 years. The 10Be concentrations in the sample range between (0.2 and 7) &#215; 108 atoms/g soil and show strong correlations with leachable Fe and/or Al. The inventory of 10Be in the soil domain sampled is 1-2 orders of magnitude lower than that expected from the geological age of the surface and an average delivery rate of 10Be from the atmosphere, 5.2 &#215; 105 atoms/cm2 yr. The low inventory of 10Be is attributed to its loss from the soil domain sampled by solution transport. Based on a simple -model type calculation with a first-order removal process for 10Be, the residence time of 10Be in the soil domains sampled is determined to be of the order of 104 years. The low residence time of 10Be in the soil domains sampled requires that it be found either deeper in the regolith or in ground waters. In either case, the application of 10Be as a chronometer of geomorphic surface age is severely constrained. However, the study of 10Be in soils provides the only entry into the long-term (104-106 years) behavior of particle-reactive nuclides in soils and, hence, could be important for understanding the behavior of analogous nuclides introduced into soils by natural and anthropogenic processes