『Abstract
Chemical weathering of basalts in the Putorana Plateau, Central
Siberia, has been studied by combining chemical and mineralogical
analysis of solids (rocks, soils, river sediments, and suspended
matter) and fluid solution chemistry. Altogether, 70 large and
small rivers, 30 soil pore waters and groundwaters and over 30
solids were sampled during July to August 2001. Analysis of multiannual
data on discharges and chemical composition of several rivers
of the region available from the Russian Hydrological Survey allowed
rigorous estimation of mean annual major element concentrations,
and dissolved and suspended fluxes associated with basalt weathering.
For the rivers Tembenchi and Taimura that drain monolithologic
basic volcanic rocks, the mean multiannual flux of total dissolved
cations (TDS_c = Ca + Mg + Na + K) corrected
for atmospheric input is 5.7±0.5 t/km2/yr. For the
largest river Nizhniya Tunguska - draining essentially basic rocks
- the TDS_c is 6.1±1.5 t/km2/yr.
The overall CO2 consumption flux associated
with basalt weathering in the studied region (〜700,000 km2)
achieves 0.08×1012 mol/yr, which represents only 2.6％
of the total CO2 consumption associated with
basalt weathering at the Earth's surface. The fluxes of suspended
matter were estimated as 3.1±0.5, 9.0±0.8, and 6.5±2.0 t/km2/yr
for rivers Taimura, Eratchimo, and Nizhniya Tunguska, respectively.
Based on chemical analyses of river solutes and suspended matter,
the relative dissolved versus particulate annual transport of
major components is Cinorg≧Corg＞Na
+ K＞Ca＞Mg＞Si＞Fe≧Mn≧Ti≧Al which reflects the usual order of element
mobility during weathering.
According to chemical and mineralogical soil and sediment analyses,
alteration of basalt consist of (1) replacement of the original
basaltic glasses by Si-Al-Fe rich amorphous material, (2) mechanical
desegregation and grinding of parent rocks, leading to accumulation
of “primary” hydrothermal trioctahedral smectite, and (3) transformation
of these trioctahedral (oxy)smectites and mixed-layer chlorite-smectite,
into secondary dioctahedral smectite accompanied by removal of
Ca, Mg, and Fe, and enrichment in Al. No vertical chemical differentiation
of fluid and solid phases within the soil profile was identified.
All sampled soil pore waters and groundwaters were found to be
close to equilibrium with respect to chalcedony, gibbsite, halloysite,
and allophanes, but strongly supersaturated with respect to goethite,
nontronite, and montmorillonite.
Over the annual cycle, the contribution of atmospheric precipitation,
permafrost melting, underground reservoirs, litter degradation,
and rock and soil mineral weathering for the overall TDS_c transport in the largest river of the region
(Nizhniya Tunguska) is 9.3±3, 10±5, 10.5±5, 25±20, and 45±30％,
respectively. In the summertime, direct contribution of rocks
and soil mineral weathering via solid/fluid interaction does not
exceed 20％. The main unknown factors of element mobilization from
basalt to the river is litter degradation in the upper soil horizon
and parameters of element turnover in the vegetation.』
1. Introduction
2. Studied area and methods
2.1. General setting of Putorana basalts and the territory
description
2.2. Sources of information and calculation methods
2.3. Sampling and analyses of rivers and soil pore waters
2.4. Analysis of solid phases
3. Results
3.1. Characterization of solid phases
3.2. Hydrochemistry
3.2.1. Dissolved and suspended matter fluxes
3.2.2. Composition of soil pore waters and groundwaters
4. Discussion
4.1. Sources of elements in rivers: Rocks, soil, litter,
underground reservoirs, and permafrost ice
4.2. CO2 consumption rates and the chemical
erosion intensity in the Siberian platform
5. Conclusions
Acknowledgments
References
Appendix 1