The distribution of (234U/238U) activity ratios in river sediments

Type Article
Date 2020-12
Language English
Author(s) Thollon Maude1, 2, 3, Bayon Germain1, Toucanne SamuelORCID1, Trinquier AnneORCID1, Germain Yoan1, Dosseto Anthony2
Affiliation(s) 1 : IFREMER, Marine Geosciences Research Unit, 29280 Plouzané, France
2 : Wollongong Isotope Geochronology Laboratory, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia
3 : Université de Bretagne Occidentale, 29200 Brest, France
Source Geochimica Et Cosmochimica Acta (0016-7037) (Elsevier BV), 2020-12 , Vol. 290 , P. 216-234
DOI 10.1016/j.gca.2020.09.007
WOS© Times Cited 1
Keyword(s) World rivers, Uranium-series isotopes, Chemical weathering, Erosion, Sediment transport, Comminution age, Clays, Silts

Uranium (U) isotopes can be used to estimate the comminution age of sediments, i.e. the time elapsed from sediment production on continents, via weathering and physical erosion, to deposition in the sedimentary record. The calculation of this comminution age is based on measured (234U/238U) activity ratios in river sediments, and inferred time-dependent recoil effect, which leads to the preferential release of 234U from mineral lattices during erosion processes. In this study, we report on a large-scale (234U/238U) investigation of modern river sediments worldwide, with the aim to determine the extent to which parameters such as grain size, lithology, weathering, climate and geomorphology may influence the distribution of U isotopes in fine-grained sediments. Our extensive dataset (N=64) includes U isotopic measurements for many of the world’s largest rivers, but also rivers draining particular climatic and geological settings. Our results indicate that sediments collected from river basins draining mostly igneous, metamorphic or volcanic rocks often display (234U/238U) ratios >1, with clay-size fractions (<4 µm) being less depleted in 234U (higher 234U/238U) than corresponding silt-size fractions (4-63 µm). In contrast, sediments derived from multi-lithological basins or draining sedimentary rocks are typically characterized by (234U/238U) ratios <1, with clays generally exhibiting more depleted 234U signatures than silts. Taken together, these observations suggest that the formation of secondary clay minerals in soils from basins draining mostly igneous, metamorphic is accompanied by partial incorporation by recoil injection of 234U initially released during weathering processes, possibly from U-rich minerals, such as sphene or apatite. Instead, in multi-lithological catchments draining sedimentary rocks, we propose that the erosion of recycled sediments having experienced several cycles of weathering, possibly over glacial-interglacial timescales, could explain the much lower (234U/238U) ratios observed in clay-size fractions. While no direct relationships can be identified between sediment (234U/238U) ratios and lithology, weathering intensity, climatic or geomorphic parameters in corresponding river basins, we show that the catchment size probably plays an important role in controlling the distribution of (234U/238U) in river sediments, through its direct influence on the sediment residence time. Finally, a multiple regression analysis of our data, combining various environmental parameters for the lithology, climate and geomorphology of studied river basins, indicates predicted (234U/238U) values that are very similar to measured values (with R2 ∼ 0.8). This finding provides further support for the usefulness of (234U/238U) ratios in the sedimentary record for reconstructing past landscape changes and their effect on sediment transport and residence time in river basins.

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