『Abstract
The aim of this study is to propose a method to assess the long-term
chemical weathering mass balance for a regolith developed on a
heterogeneous silicate substratum at the small experimental watershed
scale by adopting a combined approach of geophysics, geochemistry
and mineralogy. We initiated in 2003 a study of the steep climatic
gradient and associated geomorphologic features of the edge of
the rifted continental passive margin of the Karnataka Plateau,
Peninsular India. In the transition sub-humid zone of this climatic
gradient we have studied the pristine forested small watershed
of Mule Hole (4.3 km2) mainly developed on gneissic
substratum. Mineralogical, geochemical and geophysical investigations
were carried out (i) in characteristic red soil profiles and (ii)
in boreholes up to 60 m deep in order to take into account the
effect of the weathering mantle roots. In addition, 12 Electrical
Resistivity Tomography profiles (ERT), with an investigation depth
of 30 m, were generated at the watershed scale to spatially characterize
the information gathered in boreholes and soil profiles. The location
of the ERT profiles is based on a previous electromagnetic survey,
with an investigation depth of a about 6 m. The soil cover thickness
was inferred from the electromagnetic survey combined with a geological/pedological
survey.
Taking into account the parent rock heterogeneity, the degree
of weathering of each of the regolith samples has been defined
using both the mineralogical composition and the geochemical indices
(Loss on Ignition, Weathering Index of Parker, Chemical Index
of Alteration). Comparing these indices with electrical resistivity
logs, it has been found that a value of 400 Ohm in delineates
clearly the parent rocks and the weathered materials. Then the
12 inverted ERT profiles were constrained with this value after
verifying the uncertainty due to the inversion procedure. Synthetic
models based on the field data were used for this purpose. The
estimated average regolith thickness at the watershed scale is
17.2 m, including 15.2 m of saprolite and 2 m of soil cover.
Finally, using these estimations of the thicknesses, the long-term
mass balance is calculated for the average gneiss-derived saprolite
and red soil. In the saprolite, the open-system mass-transport
function τ indicates that all the major elements except Ca are
depleted. The chlorite and biotite crystals, the chief sources
for Mg (95％), Fe (84％), Mn (86％) and K (57％, biotite only), are
the first to undergo weathering and the oligoclase crystals are
relatively intact within the saprolite with a loss of only 18％.
The Ca accumulation can be attributed to the precipitation of
CaCO3 from the percolating solution due to
the current and/or the paleoclimatic conditions. Overall, the
most important losses occur for Si, Mg and Na with -286×106
mol/ha (62％ of the total mass loss), -67×106 mol/ha
(15％ of the total mass loss) and -39×106 mol/ha (9％
of the total mass loss), respectively. Al, Fe and K account for
7％, 4％ and 3％ of the total mass loss, respectively. In the red
soil profiles, the open-system mass-transport functions point
out that all major elements except Mn are depleted. Most of the
oligoclase crystals have broken down with a loss of 90％. The most
important losses occur for Si, Na and Mg with -55×106
mol/ha (47％of the total mass loss), -22×106 mol/ha
(19％ of the total mass loss) and -16×106 mol/ha (14％
of the total mass loss), respectively. Ca, Al, K and Fe account
for 8％, 6％, 4％ and 2％ of the total mass loss, respectively.
Overall these findings confirm the immaturity of the saprolite
at the watershed scale. The soil profiles are more evolved than
saprolite but still contain primary minerals that can further
undergo weathering and hence consume atmospheric CO2.』
1. Introduction
2. Field settings
3. Materials and methodology
3.1. Previous studies and sampling
3.2. Protolith/regolith geochemistry and mineralogy
3.3. Geophysical investigations
4. Results
4.1. Boreholes and soil profiles
4.1.1. Fresh gneiss and weathering products
4.1.2. Fresh and weathered amphibolite (BH6)
4.2. ERT profiles
5. Discussion
5.1. Determination of fresh and weathered materials
5.1.1. Determination of fresh gneiss, gneiss-derived saprolite
and red soil
5.1.2. Determination of the fresh amphibolite and amphibolite-derived
saprolite
5.2. Assessment of regolith thickness with ERT
5.3. Mass balance calculation
5.3.1. Selection of the inert element
5.3.2. Strain and elemental gain or loss in the average gneiss-derived
saprolite
5.3.3. Strain and elemental gain or loss in the average gneiss-derived
red soil
5.4. Long-term chemical weathering rate and minimum age of the
saprolite
5.5. Consequence of chemical weathering on the alkalinity production
potential on the Karnataka Plateau
6. Conclusion
Acknowledgments
References