Erosion timescales derived from U-decay series measurements in rivers

The relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades. The principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates. In this study, (234U/238U), (230Th/238U), and (226Ra/238U) were measured by TIMS and by MC¯ICP¯MS in the dissolved and suspended loads of rivers from the Mackenzie Basin (Northwest Territories, Canada). The data show a complementary nature between (234U/238U), (230Th/238U) in the dissolved and suspended loads while 226Ra is characterized by a more complex behavior. Modeling of fractionation of U-series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed (9¯28±10 ka), as well as the rates of release of U-series nuclides. The results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the Mackenzie River.