『Abstract
Numerous studies of weathering fluxes have been carried out on
major world rivers during the last decade, to estimate CO2 consumption rates, landscape evolution and global
erosion rates. For obvious logistical reasons, most of these studies
were based on large scale investigations carried out on short
timescales. By comparison, much less effort has been devoted to
long term monitoring, as a means to verify the temporal variability
of the average characteristics, their trends, and the representativeness
of short-term investigations. Here we report the results of a
three-year survey (November 2000 to December 2003) of the major
and trace element composition of dissolved and suspended matter
in the lower Rhone River (France), the largest river of the Mediterranean
area. Subsurface water samples were collected in Arles, about
48 km upstream of the estuary, twice a month routinely, and at
higher frequency during flood events.
During each flood event, the suspended particulate matter (SPM)
show the usual trend of clockwise hysteresis with higher SPM concentrations
on the rising limb of the flood than at the same discharge on
the falling limb. We show that the annual average SPM flux of
the Rhone River to the Mediterranean Sea (7.3±0.6×106
tons yr-1) was largely controlled by the flood events
(83％ of the solid discharge occurred in less than 12％ of the time),
and that the precision on the total output flux depends strongly
on the precise monitoring of SPM variations during the floods.
The chemical composition of water and SPM are characterized by
the predominance of Ca2+ due to the abundance of carbonate
rocks in the Rhone watershed. Chemical budgets have been calculated
to derive the contributions of atmospheric deposition, carbonate,
silicate and evaporite weathering, and anthropogenic inputs. The
chemical weathering rate of carbonates is estimated to be 89±5
t km-2 yr-1 compared to 14.4±3 t km-2
yr-1 from silicates. By contrast, the physical erosion
rate of silicates is about 51 t km-2 yr-1
against 19 t km-2 yr-1 for carbonates.
The steady-state model of Gaillardet et al.(1995) has been applied
to the chemical composition of dissolved and solid products. The
results show that the Rhone River currently exports much less
material than produced at steady-state by weathering in its watershed.
The sediment flux inferred from the steady-state calculation (21-56×106
tons yr-1) is on the same order as that estimated in
literature for the 19th and the beginning of the 20th centuries.
This imbalance may suggest that the Rhone is under a transient
erosion regime following climate change (i.e. significant decrease
of the flooding frequency since the beginning of the 19th century).
On the other hand, the imbalance may also be due to the trapping
of alluvion by the numerous dams on the river and its tributaries.
Our data corroborate with previous studies that suggest a strong
coupling between chemical and physical erosion fluxes, during
the hydrological seasonal cycle of the Rhone River. The correlation
between physical and chemical transport rates is, however, clearly
different from that reported for global annual averages in large
world rivers.』
1. Introduction
2. Study area
3. Materials and methods
4. Results
4.1. Suspended sediments concentration and flux
4.2. Chemical composition of the dissolved and particulate phases
4.2.1. Dissolved composition
4.2.2. Chloride concentration and correction of the atmospheric
input
4.2.3. Particulate composition
5. Discussion
5.1. Physical erosion
5.2. Chemical erosion: major sources of the dissolved load
5.3. Carbonate versus silicate denudation rates and CO2
consumption
5.4. Physical versus chemical erosion in the Rhone River
5.5. Steady-state prediction of the suspended sediment discharge
6. Conclusions
Acknowledgments
Appendix A
References