Estimating U fluxes in a high-latitude, boreal post-glacial setting using U-series isotopes in soils and rivers

- U-series measurements performed upon a Scottish soil chronosequence.
- The soils have experienced both U addition and leaching, to varying degrees.
- Weathering rates are similar to other high-latitude boreal catchments.
- High-latitude rivers comprise an important U flux with high (234U/238U) to the ocean.

This study reports U-series activity ratios from river waters and six soil profiles across a soil chronosequence formed since the last glacial retreat, in Glen Feshie, Scotland. The overall aim is to examine the geochemical behaviour of the U-series nuclides in a boreal climate setting. The U-series data show that U is being both added to and leached out of the soils, to varying degrees. The U addition elevates the (234U/238U) of the bulk soils (up to 1.25), which is most pronounced in the youngest and the upper organic-rich soil horizons. The U addition appears to be linked to U adsorption, controlled by the degree of flooding by the Feshie River. The Feshie River has a high (234U/238U) ratio (~ 1.7), a feature shared with most high-latitude Northern Hemisphere rivers. For the soil profiles with no significant U addition, U-series nuclide modelling suggests U leaching rates on the order of 0.5-2 × 10− 5 y− 1, a similar range to other Northern Hemisphere high-latitude areas affected by the last glaciation, e.g. Mackenzie Basin, Canada. These two observations suggest a link between weathering rates and riverine (234U/238U) for areas that have been glaciated recently. A global compilation of major rivers shows that high-latitude Northern Hemisphere rivers comprise a significant U flux to the ocean with high (234U/238U). Thus, past changes in this Northern Hemisphere high-latitude U flux may have played a major role in oceanic (234U/238U) variation over glacial-interglacial cycles.