『Abstract
This study investigates dissolved, suspended loads and sands
of major Icelandic rivers and determines chemical and mechanical
erosion rates as well as rates of CO2 consumption
by the chemical weathering. A steady state model of erosion is
used to locally calculate the river suspended load fluxes needed
to balance chemical weathering.
The total dissolved solid concentrations range from 20 to 179
mg/kg. The highest concentrations are for spring fed rivers draining
young rocks in the vicinity of active volcanoes, and the lowest
for direct runoff rivers draining old Tertiary rocks. Total dissolved
loads, “corrected” for atmospheric, geothermal, and magmatic inputs
are used together with mean annual discharges to estimate low-temperature
chemical erosion rates of 16 to 111 t/km2/yr. These
rates increase with runoff but decrease with the age of the rocks.
Icelandic chemical erosion rates are higher than the world average
for silicate rocks, reflecting both high reactivity of the basalt
and high runoff, but lower than those for other basalt-draining
rivers (in Reunion(eの頭に´), Java, Azores or
Deccan). CO2 consumption rates associated
to chemical denudation range between 0.18 and 2.12 106
mol/km2/yr with an average with value of 0.74 106
mol/km2/yr, higher than the world average for rivers
draining silicate rocks.
Chemical compositions of suspended sediments and sands are similar,
showing a very low weathering stage. The elements most soluble
during the weathering show slightly lower concentrations in the
suspended sediments. River sediment chemical compositions are
assumed to reflect a mixture between 3 initially pristine rock
end-members: high Mg-basalt, tholeiite and rhyolite. The most
insoluble elements (REE and Th) are used to re-define the mean
chemical composition of the initially unaltered rocks of each
drainage basin.
A mass budget between the unaltered rock of the catchments and
the river dissolved and suspended loads (steady state model of
erosion) is used to calculate the average annual solid load of
the rivers, which range from 650 to 4300 mg/l. For some rivers
there is a good agreement between calculated and measured suspended
loads but for others the calculated load is much higher than the
measured one. The difference stems from groundwater inputs, man-made
dams and other sedimentary traps. If the relevancy of the steady
state model of erosion can be questioned, the accuracy of sediment
load measurements is also questionable. Pros and cons of both
methods are argued. The calculated solid loads lead to very high
mechanical erosion rates, ranging from 940 to 10200 t/km2/yr.
Those increase with the glacier cover but decrease with the age
of the catchment rocks. Icelandic mechanical erosion rates rank
among the maximum reported rats, underscoring the importance of
glaciers, tectonics, glassy basaltic rocks and high runoff. In
association with low chemical weathering rates, these place Icelandic
rivers as an end-member in the observed anti-correlation between
mechanical to chemical erosion ratios and temperature for volcanic
islands.』
Introduction
General settings of Iceland
Sampling and analysis
Dissolved load
Solid load
River dissolved loads
Chemical compositions
Major elements and nutrients
Trace elements
Comparison with other data, temporal variability of the chemical
compositions
Atmospheric, geothermal and magmatic inputs to the rivers
River suspended sediments and sands
The volcanic rocks of Iceland
REE and extended REE patterns of the suspended sediments and
sands
Differences between suspended sediment and sand chemical compositions
Local basalt chemical composition for each drainage basin
Steady state model of erosion
Determination of the suspended load concentrations
Calculation of the suspended sediment loads
Comparison of the calculated and measured suspended sediment
loads
Erosion rates and CO2 consumption by the
weathering
Low-temperature chemical erosion rates
Atmospheric CO2 consumption rates
Mechanical erosion rates
Coupling chemical and mechanical erosion rates
Conclusion
Acknowledgments
References