Very long hillslope transport timescales determined from uranium-series isotopes in river sediments from a large, tectonically stable catchment

Abstract

The uranium-series isotopic compositions of soils and sediments evolve in response to time and weathering conditions. Therefore, these isotopes can be used to constrain the timescales of river sediment transport. Catchment evolution depends on the sediment dynamic timescales, on which erosion imparts a major control. Erosion rates in tectonically stable catchments are expected to be lower than those in tectonically active catchments, implying longer sediment residence times in tectonically stable catchments. Mineralogical, elemental and isotopic data are presented for modern channel sediments, alluvial and colluvial deposits from the Murrumbidgee River, a large catchment in the passive margin highlands of south-eastern Australia and three of its tributaries from the headwaters to the alluvial plain. Low variability in Si-based Weathering Index indicates that there is little chemical weathering occurring in the Murrumbidgee River during sediment transport. However, quartz content increases and plagioclase content decreases downstream, indicating progressive mineralogical sorting and/or physical comminution with increasing transport distance. U-series isotopic ratios in the Murrumbidgee River trunk stream sediments show no systematic downstream variation. The weathering ages of sediments within the catchment were determined using a loss–gain model of U-series isotopes. Modern sediments from a headwater tributary, the Bredbo River at Frogs Hollow, have a weathering age of 76±30kyr but all other modern channel sediments from the length of the Murrumbidgee River and its main tributaries have weathering ages ∼400±180kyr. The two headwater colluvial deposits have weathering ages of 57±13 and 47±11kyr, respectively. All the alluvial deposits have weathering ages similar to those of modern sediments. No downstream trend in weathering age is observed. Together with the soil residence time of up to 30kyr for ridge-top soils at Frogs Hollow in the upper catchment area of the Murrumbidgee River (Suresh et al., 2013), the current results indicate, for the first time, that sediments in the Murrumbidgee catchment are stored in hill slope for long time (∼200kyr) before carried by the river. The long residence times of sediments indicate a low erosion rate from the catchment. The sediment transport timescales estimated are up to two orders of magnitude higher than those reported for tectonically active catchments in Iceland (Vigier et al., 2006) and in the Himalayas (Granet et al., 2007), indicating the influence of tectonism on catchment erosion.