Chemical Weathering, Part II:

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• Chemical Weathering, Part II
• Weathering River Chemistry

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Weathering Basics Review Structure

• dissolution of ionic salts
• carbonation of carbonates
• oxidation of reduced minerals
• silicate hydrolysis

Types of weathering processes
Typical chemical reactions associated with each
process
e.g. simple dissolution for halite more
complicated for pyrite oxidation (well cover
mechanism of silicate hydrolysis next week)
Typical mechanisms for each reaction to take place
Why each process is important in Earths
environment
e.g. global cycles of C O heavy metal release
formation of soils other features like karst
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Whats the interesting science here?
What controls weathering processes? Why do they
happen at specific times places, and which
reactions and mechanisms dominate? How quickly
do they happen what determines this? Can we
quantitatively predict these things, in the past,
present, and future?
What do basic chemical principles tell us should
be the case? principle of equilibrium kinetic
theory - next week
What do we observe in the natural world how
does this relate to our theoretical
predictions? field laboratory studies
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Why rivers?

• arteries of the continents - integrate global
  terrestrial processes
• major input to the oceans - regulate ocean
  chemistry global geochemical cycles
• surface water supply - critical natural resource
  for human civilization

• How would you measure the chemistry of a river?
• collect a sample in the field bring it back
  while preserving it and without contaminating it
• analyze it in the lab - but how do you determine
  the chemical composition of something like a
  river water?

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Ion Chromatography
Pass liquid sample through a ion-exchange resin
Elute with a solution that displaces retained ions
Detector
Measure conductivity of output solution over time
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Plasma Spectrometry
Inject liquid sample into high-T plasma (Argon
plasma) to excite ions

• Mass Spectrometry
• use high energy to force ions through a flight
  tube in vacuum
• deflect ions of different mass with
  high-strength magnetic field
• count all the ions of a given mass from a sample

• Optical Emmision Spectrometry (OES)
• excited ions emit light of characteristic
  frequency
• emission intensity related to amount of each ion
• measure emitted light of given frequency with
  optical detector

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Plasma Spectrometry, cont.
Element2 Mass Spectrometer, upstairs in this
department Fast scanning across a wide mass range
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Standardization
Chromatography spectrometry give a measure of
intensity for a specific chemical species
Requires comparison to the intensity derived from
a substance of known composition to determine
real concentration in an unknown sample
Inferred concentration
Measured sample
Pure halite Standard (NaCl)
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What does river chemistry look like?
Rivers
Si
Ca
Na
Different from rain - increase in Si much more
concentrated (gt5 times more total ions)
K
Mg
Rain
Na
Mg
Must be explained by input from weathering to
rivers
Ca
K
Rivers also very different from rocks (e.g.
extremely low Al, Fe)
Rocks
Al
Ca
Si
Fe
Must be explained by incongruent weathering
processes
K
Mg
Na
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Explaining the general character of river
solutions The inverse of soil

1. resistant minerals (eg, quartz) dont dissolve
2. aluminosilicates alter to clays
3. soluble elements removed in waters
4. some precipitate lower in soil or in sediments
   (Fe-oxides, carbonate)

O-Layer Organic Debris
A-Layer Organics Ion-Depleted Clays Resistant
Minerals
B-Layers Primary Secondary Minerals

• Whats left behind?
• regolith accumulation of fine rock material
• soil regolith plus organic matter - often
  vertically stratified

C-Layer Actively Weathering Rock (Saprolite)
Fresh Rock
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• Variability of River Chemistry
• Kalix River, Sweden
• Difference between filtering methods, more
  significant for Al
• Difference between seasonal pattern of AlMg

µM
3
Al
Bulk
2
1
lt0.22µm
lt10kDa
0
Jan
Feb
Mar
Apr
May
June
µM
60
Mg
Bulk
50
40
30
lt0.22µm
20
lt10kDa
10
0
Jan
Feb
Mar
Apr
May
June
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Geochemical Contributions to the Aquatic System

• different processes control sediments solutes
• systems related, but cant be treated together
• problem of timescale

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Himalayan Small Catchment Solutes
Malemchi Khola
In-depth study of weathering rates across a
climate gradient in central Nepal (West et al.,
Geology 2002)
Silicate cation weathering rates, dissolved,
kmols.ha-1.yr-1 High Mountains 1-2 Middle
Hills 3-4 Higher weathering rate attributed to
higher temperature, despite lower runoff
erosion rates
Langtang Lirung
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• Sediment Chemistry
• no signal in High Himalayas - erosion too fast
• more intense weathering in Middle Hills - as
  expected
• consistent with soil chemistry

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Quantitative Treatment of Sediment Chemistry
Total Denudation Physical Erosion (ie,
sediments) Chemical Weathering (Louvat,
Gaillardet _at_ Paris)
Total Denudation Soil Formation Chemical
Weathering (Riebe _at_ UC Berkeley)
Steady State weathering determined from
sediment/soil chemistry is equivalent to
weathering determined from dissolved chemistry
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2 2 ? 4 for Weathering Fluxes !!

• series of possible explanations - both natural
  analytical
• dissolved particulate loads not a single
  system, at least as measured
• can we better understand the sediment signal?

Same observed from soil data
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The Potential of U/Th

• define time scale of sediment reaction -
  independent measure of particulate behavior
• U Th fractionate during weathering
• return to equilibrium governed by decay
  constants, providing time information

Model U/Th fractionation compare to observation
from sediments to determine weathering time
(Vigier et al., 2001)
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Applying the U/Th Model
Sediment Transfer Time in Large Himalayan Rivers
Chabaux et al., 2006
Andes System, Dosetto et al., 2006
Do U/Th Times Link to Sediment Chemistry? This
is a new unexplored research question . .
. Important across Environmental Geochemistry
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U/Th of Himalayan Range Soils Sediments

• most samples very close to equilibrium
• significant differences in initial U/Th ratios

Not possible to quantify timescale of early-stage
erosion!!

• no trend through soil profiles with depth
• sediments show more disequilibrium than soils

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A Discrepancy

• soils sediments have similar depletion of
  weatherable elements - yet sediments have greater
  U fractionation
• soils sediments both significantly weathered
  despite overall little U/Th disequilibrium

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U-Series Timescales
After Dosetto et al., 2006
Little U/Th fractionation during early stages of
chemical weathering - U-series may operate
differently from other geochemical processes

• What does the U-series weathering (or transport)
  timescale really represent?