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

Abstract

Concentration-depth profiles of cosmic-ray-produced ¹⁰Be (t_1/2 = 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) × 10⁵ years based on amino-stratigraphic correlations and models of terrace evolution; those of the table mountain top and alluvial fan are 9.5 × 10⁶ years and 6.0 × 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) × 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 × 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.