Silicate weathering and Climate Control West et al 2005

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Silicate weathering and Climate Control (West et
al 2005)
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Introduction

• Silicate weathering controls climate over long
  time scales through the consumption of
  atmospheric CO2 that is eventually stored in
  carbonates in the oceans.
• Weathering depends on mineral type reactivity,
  mineral supply, water and acids reactants, and an
  Arrhenius rate law (temperature).
• Acidity drives the dissolution reaction and is
  predominantly supplied from atmospheric CO2,
  either in soil waters or as organic acids
  produced by vegetation.

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Problem addressed by this paper

• For a given rock type and pCO2, weathering should
  depend on temperature, runoff and erosion rate.
• These factors are obviously codependent and their
  independent roles in weathering are unclear.
• Hence, it is unclear whether climate or erosion
  control global weathering fluxes.
• Contradictory results have made the development
  of a unified, quantitative description of global
  weathering challenging.

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Aims of this paper

• To show that the influence of erosion rate,
  temperature, and runoff on chemical weathering
  rates can be distinguished and quantified.
• Parameterization of silicate chemical weathering
  rates is achieved by considering the physical
  basis that controls chemical dissolution in Earth
  surface settings.

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What was done

• Data compilation of chemical weathering fluxes
  determined by solute export in surface waters,
  annual averages of temperature and precipitation
  for stable continental cratons and small
  catchments underlain by granites and meta-pelitic
  rocks. Large basins in Africa, Siberia, Guyana
  and Canada. Small catchments characterized by
  soil mantle, mixed vegetation, some agriculture
  in N and Central America, Europe and SE Asia as
  well as Alpine catchments (bare bedrock, partial
  glacial cover) in N America, Europe and S Asia.

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Data Problems and Solutions

• Large basin size means integration of multiple
  lithology types. Heterogeneity is unimportant
  because homogeneity of weathering environments
  means that the range of weathering fluxes is
  small at the scale of comparison in the data set.
• In more rapidly eroding settings, large basins
  integrate highly variable environments. Hence
  smaller catchments (lt103 km2) are used. In these
  settings variations are small and weathering
  conditions are well constrained.

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Calculations

• Weathering Flux values reported in the literature
  were used as published.
• Where not published they were calculated as the
  product of appropriately adjusted chemical
  concentration from spot sampling or annual
  averages, and annual discharge.
• Silicate derived Na was calculated by adjusting
  measurements for atmospheric deposition based on
  a universal precipitation Na/Cl ratio of 0.87,
  reflecting sea salt composition.
• Silicate derived Mg and Ca were calculated by
  adjusting measurements with adjusted Na
  concentartion using universal ratios of Ca/Na
  0.35 and Mg/Na 0.24.

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Calculations

• All dissolved K was assumed to derive from
  silicates.

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Results
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Results

• Transport and weathering are identified as rate
  limiting processes at both low and high erosion
  rates and should be considered in analysis of
  weathering dependencies.

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Transport Limitation

• When the supply of water and acid relative to the
  supply of silicate minerals is large, and the
  residence time in the weathering environment
  compared to reaction time is long, minerals are
  nearly completely altered before their removal.
• Hence, ? A e (? weathering rate, A weight
  fraction of soluble cations in silicate rock, e
  total erosion rate).

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Transport limitation line
Small catchments and basins with low residence
time
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On the other hand!!!

• Kinetic control of weathering, which is defined
  to be a linear combination of erosion rate (e),
  runoff (?) and temperature.

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Conclusions

• The study has enabled to distinguish between
  transport and kinetically limited silicate
  weathering in catchments with a wide range of,
  temperatures, runoff and denudation rates.
• Precise parameterization of silicate and chemical
  weathering from such a global compilation cannot
  be made since exact weathering mechanisms may
  vary in different climatic regimes.
• Further study of silicate weathering is still
  required for identifying weathering mechanisms
  and their relation to controlling parameters in
  order to make more precise parameterization.

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Conclusions

• Only in the weathering limited regimes will
  silicate weathering rates respond to changes in
  climate
• It might be a interesting to identify the areas
  of potentially untapped silicate weathering as a
  function of climate.