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Seventh Annual V. M. Goldschmidt Conference
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Coupling modern chemical and mechanical weathering rates of silicates using
rivers.
J. Gaillardet , P. Louvat, B. Dupré§ and C. J. Allègre
Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris - URA CNRS D1758 - UER
des Sciences de la Terre Université Paris VII Denis Diderot, 4 Place Jussieu, 75252 Paris cedex 05, France
§Laboratoire de Géochimie, CNRS OMP, Université Paul Sabatier, 38 rue des 36 ponts, Toulouse France.
On a global scale, the control of
modern weathering rates and hence of CO2
consumption by rock weathering, is a
subject of great debate. Among the factors
that may exert a role in determining the way
and the rate whereby continental material is
transferred from land to sea, temperature
and precipitation (hence climate) and relief
(hence tectonics) have been suspected as
being key parameters. Whether climate or
tectonic play the major role is still debated
(Berner, 1996; Edmond, 1996). However,
the role of physical erosion, by creating
surfaces and hence preparing chemical
weathering has attracted little attention.
In addition to experimental results,
field data are clearly needed to address this
issue. Rivers, and especially large rivers,
as they integrate large portions of the
continental crust are particulary well suited
to identify the parameters controlling the
modern weathering rates. Their dissolved
and suspended loads give insights into the
chemical and mechanical weathering
processes respectively. Although the
number of studies focusing on river
chemistry is increasing, these studies are
still focused on the chemical load rather
than on an integrated study of suspended
and dissolved load and, thus, the
relationships between chemical and
mechanical denudation.
We have made an effort to fill in this
gap, by systematically associating the
sampling of dissolved and suspended loads
in large basins such as the Congo, Niger,
Amazon, Mackenzie, S. Lawrence,
Huanghe, Yangtze, Red river, Mekong
river systems and in volcanic islands under
variable climatic conditions, such as the
Reunion, Azores, Iceland and Java.
The chemical composition of river
sediments in these settings together with the
scarce data from the litterature, show that
modern products of physical erosion are
characterized by a huge depletion in all the
most soluble elements compared to the bed
rocks from which they are derived. The
less depleted sediments are those from
volcanic islands. On a world scale, the
contrast between lowlands rivers (with
sediments strongly depleted in solutes) and
mountaineous or volcanic island rivers (thar
are much less depleted) is evident. The
influence of climate on the intensity of
solute solubilisation is therefore not
obvious.
Extending a formalism previously
described, the dissolved load of each river
can be used to derive chemical weathering
rates for silicates. Again, on a global scale,
the influence of climatic factors is
secondary and the lithology appears to be
the predominant factor.
Finally,
the
complementarity
between dissolved loads (once corrected
from non-silicate weathering inputs) and
suspended loads enables to calculate a
mechanical weathering rate for each river.
These rates are calculated according to a
steady-state hypothesis and are independant
from field estimations of mechanical
denudation. Such estimations suffer from
numerous problems such as for example
the deposition of suspended sediments
within the basins. A correlation between the
so-calculated mechanical erosion rates and
the chemical rates of silicate weathering is
observed in a log-log space, tending to
show the physical degradation of
continental rock has a major role on a
global scale on chemical weathering and
hence CO2 consumption.