The effect of weathering regime on uranium decay series and osmium in two soil profiles

Abstract

Two soil profiles from the United States with radically different emplacement and climatic histories were analyzed for U, Th and members of the ²³⁸U decay series (²³⁴U, ²³⁰Th, ²²⁶Ra, ²¹⁰Pb), ¹³⁷Cs and osmium isotopes. The arid New Mexico profile is developed on an approximately 250,000 years old colluvium while the temperate New Hampshire profile is formed on till after the last glaciation at about 10,000 years ago. Both the profiles show significant ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and ²²⁶Ra/²³⁰Th disequilibria, however, in the New Hampshire profile, the disequilibria are far more pronounced in mid-depths (20-50 cm). High Os concentration with highly radiogenic ¹⁸⁷Os/₁₈₈Os is another characteristic of the mid-depths of the New Hampshire profile. This layer, particularly at about 30-40 cm depth has the characteristics of a soil developed on black shale, as evidenced from both the high U and Os concentrations and the large excess of ²³⁰Th over ²³⁸U. This profile clearly shows that the regolith on which the contemporary soil is developing was not homogeneous. The presence of measurable excess ²²⁶Ra activity over ²³⁰Th activity in both profiles suggests the need for a source of ²²⁶Ra external to the regolith in both cases. Atmospheric deposition of ²²⁶Ra is a possible source for this ²²⁶Ra excess and brings to light the important role of atmospheric deposiion of nuclides and their transport in the soil profile in pedogenic processes. It also shows that regolith developed by glacial processes need not be homogeneous, thereby confounding the understanding of vertically modified soil profiles.