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

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