Importance of atmospheric inputs and Fe-oxides in controlling soil uranium budgets and behavior along a Hawaiian chronosequence

A long-term budget of uranium calculated for a chronosequence of Hawaiian soils demonstrates the importance of mineral aerosol deposition as a geochemical pathway. A series of sequential extractions on the same soils shows a strong association between Fe-oxides and uranium, especially the amorphous and poorly-crystalline Fe-oxides present in intermediate-age soils (20 and 150 ka). Despite intense leaching conditions, soil U contents were found to increase between 20-fold and 60-fold with age along the chronosequence, from 9 to 17 μg cm− 2 at 0.3 ka sites to 344–543 μg cm− 2 at the 4100 ka site. An open-system transport function comparing U to an index “least mobile” element (Nb) was used to calculate U gains or losses from the soils, relative to the input from parent material weathering. Results show small losses or small gains (± 25%) of U occurred in soils 150 ka and younger, while gains of up to 531% U occurred in soils older than 150 ka, highlighting the importance of mineral aerosol (dust) deposition at the older sites. In the 4100 ka soil, the ratio of dust-derived U inputs to parent material-derived U inputs is approximately 6. Sequential extractions show that between 0.1% and 1.2% of the total U in the soils was extracted with ammonium acetate, indicating that very little U exists as exchangeable cations. A second extraction employing citrate–dithionate and hydroxylamine hydrochloride was performed to extract U associated with Fe-oxides. Average percent occurrences for U in the Fe-oxide extraction for each profile are, 7% at 0.3 ka, 53% at 20 ka, 40% at 150 ka, and 17% at 4100 ka, illustrating an increase with age to 20 ka followed by a subsequent decline. This parallels the trend observed in amorphous Fe-oxide content, suggesting that adsorption on amorphous Fe-oxides is the dominant control on U in the intermediate-age soils.