wAbstract
@The uranium-series isotope signatures of the suspended and dissolved
load of rivers have emerged as an important tool for understanding
the processes of erosion and chemical weathering at the scale
of a watershed. These signatures are a function of both time and
weathering-induced fractionation between the different nuclides.
Provided appropriate models can be developed, they can be used
to constrain the residence time of river sediment. This chronometer
is triggered as the bedrock starts weathering and the inferred
timescale encompasses the residence time in the weathering profile,
storage in temporary sediment deposits (e.g. floodplain) and transport
in the river. This approach has been applied to various catchments
over the past five years showing that river sediments can reside
in a watershed for timescales ranging from a few hundreds of years
(Iceland) to several hundreds of thousands of years (lowlands
of the Amazon). Various factors control how long sediment resides
in the watershed: the longest residence times are observed on
stable cratons unaffected by glacial cycles (or more generally,
climate variability) and human disturbance. Shorter residence
times are observed in active orogens (Andes) or fast-eroding,
recently glaciated catchments (Iceland). In several cases, the
residence time of suspended sediments also corresponds to the
time since the last major climate change. The U-series isotope
composition of rivers can also be used to predict the river sediment
yield assuming steady-state erosion is reached. By comparing this
estimate with modern sediment yield obtained by multi-year sediment
gauging, it is clear that steady-state is seldom reached. This
can be explained by climate variability and/or human disturbance.
Steady-state is reached in those catchments where sediment transport
is rapid (Iceland) or where the region has been unaffected by
climate change and/or human disturbance. U-series are thus becoming
an important tool to study the dynamics of erosion.
Keywords: erosion; timescale; U-series; sediment transportx
1. Introduction
@1.1. Uranium-series isotopes
2. Analytical techniques
3. Behavior of radionuclides during weathering - theory and observations
@3.1. Theory
@3.2. Sampling issues
@3.3. Observations
4. The residence time of sediments in a catchment
@4.1. Models of chemical weathering
@4.2. Timescales and rates of chemical weathering
@4.3. Timescale of sediment transport
5. Dynamics of erosion inferred from U-series isotopes
6. Future directions
Acknowledgements
References