Dating and Understanding the Formation of Calcic Desert Soils (Aridisols, Torriorthents)

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Dating and Understanding the Formation of Calcic
Desert Soils (Aridisols, Torriorthents)

• How does carbonate grow on gravels in desert
  soils?
• How do we measure how old the carbonate (and the
  soils) are?
• What chemical information does the carbonate have
  that tells us about climate?
• A brief overview of these questions..

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Why does carbonate grow as rings on bottom (or
top) of gravels?

• Scientists intuitively thought that carbonate
  forms on bottoms as water drips down and
  evaporates
• We, to our surprise, found that carbonate grows
  on tops of gravels in a study of soils in Baja
  California..

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north
south
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Southern (summer rain) site(coarse-loamy, mixed,
hyperthermic Aridic Ustorthent
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Northern (winter rain) site(coarse-loamy,
mixed, thermic Xeric Torriorthent
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Why is carbonate oriented (top vs. bottom) on
soil gravels?

• Carbonate solubility in water is affected by
  temperature
• Carbonate is more soluble in cooler water
• Thermal gradients (temp vs. depth) differ in
  winter vs. summer sites.

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Temperature vs. depth reversed in two climate
zones when rain falls.

• in winter sites, temp increases with depth
• Gravel bottoms warmer
• in summer sites, temp decreases with depth
• Gravel tops warmer
• Maxiumum temp gradients 0.0185 C/cm

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Summary

• Carbonate layers seem to adhere to rock position
  that is warmest
• Layers tend to grow with time, but the process is
  confounded by events that occasionally dissolve
  or break off layers
• Observable by stratigraphic relations
• Timing is determined by dating

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How do we measure how old these layers are?

• Measure concentrations of radioactive elements
  (isotopes of elements) in carbonate
• What elements (isotopes) can be used?
• 14C
• Formed by cosmic rays interacting with N
• Decay is relatively rapid, and only relevant for
  ages lt 40,000 yrs
• Incorporated into carbonate via CO2 from air
• U/Th isotopes
• Naturally occuring in rocks
• Form a chain of daughters that reach steady
  state
• U incorporated into CO3 structure
• Weathering rock to form carbonate disrupts steady
  state
• Th is insoluble and is not present in carbonate
  (thus, amount of Th in carbonate is guide to how
  old it is).

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Principles of 14C Dating

• The 14C content of source (atm) must be either
  constant or known over time
• The 14C content of the sampel must be same as atm
  (or known) at time of formation
• The decay rate of 14C must be known
• The sample must be closed and not exchange C
  after formation

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14C of atmosphere not constant over long..

• 0 o/oo is the 14C/12C ratio of CO2 relative to
  atmosphere of 1950.
• Large trends in 14C over time due to
• Solar activity and production
• Changes in global C cycle (that allow 14C to
  build up in atm)

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Or recent time spans..

• From 1800s to 1950s, 14C was declining due to
  influx of fossil fuel CO2 (no 14C) (the Suess
  effect)
• After mid 1950s to mid 1960s, atmospheric CO2
  doubled due to above ground nuclear weapons
  testing

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Summary of dating criteria

• Atmospheric CO2 14C not constant, but known.
• Relationship between soils and atmosphere now
  known (see next discussion)
• Decay rate of 14C known
• Carbonates CAN be closed systems.

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Calculating 14C ages
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Measuring and reporting F

• 14C commonly reported as D values (relative to
  standard)
• These can be easily related to F..

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How is the 14C of soil CO2 related to that of the
atmosphere?

• roots release recently acquired C, no
  radioactive loss of 14C
• Humus decomposition release soil C depleted in
  14C due to residence times of 103 years
• Humus 14C depletion increases as soil gets older,
  eventually reaching a s.s.

14C roots atm
CO2
14C humus lt atm (radioactive decay
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14C content of carbonate layers due to

• Proportion of soil CO2 from roots vs. humus
• Soil age (due to change in 14C of humus with
  time).

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Carbonate age interpretations

• Initial 14C reflects effect of humus C inputs
• 14C age of an individual layer related to initial
  14C age and time it has been undergoing decay
• Integrated 14C age less than total age but can be
  used to calculate total elapsed time using
  scenerios like this

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Understanding Mojave Desert Soils via Soil
Carbonates

• Some questions
• How does depth of leaching change with time?
• Can we identify soil horizons (Bk) formed at
  different times?
• What does the composition of the carbonate tell
  us about climate/vegetation change?

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Change in leaching depth with time

• Soil water holding capacity changes with time
  due to
• Increase in dust accumulation at surface
• Increase in clay in Bt horizons
• Reduction in infiltration rates enhances erosive
  processes.
• Amount of water available for leaching changes
  with time due to climate change

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Soil carbonate formation and history in Mojave
Desert Providence Mtns.

• Soils form on series of alluvial fans from
  grantic vs. limestone
• Part of soil/geological study of U of NM
  colleagues
• We focused on dating and climate history of
  youngest 4 (spanning late Pleistocene to
  Holocene).

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Carbonate Dating reveals how old soils and
alluvial features are..
Major soil age brackets of 11-8 ka, 8-7 ka, and
6-4 ka correspond to documented lake level highs
in Mojave desert This suggests that erosion
deposition cycles are driven (as hypothesized) by
climate oscillations.
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Carbonate depth patterns and age

• approximate depth of modern carbonate movement
• Depth of Pleistocene carbonate movement
• What was the magnitude of climate change?

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C and O isotope trends with time
Increasing atm vs biological CO2 (less plant
cover)

• C isotopes suggest decreasing vegetation cover
  with time
• We know from packrat midden studies the type (C3
  vs C4) hasnt change much (always C3 in Mojave).
• O isotopes suggest warming and increased
  evaporation (increasing 18O of remaining soil
  water
• Circulation likely hasnt changed, so temp and
  evaporation are the main causes.

Increase in both temp and evaporation of soil
water
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Summary of paleosols and carbonate isotopes

• Paleosols are important part of geologic record
  for learning about terrestrial climate
• C isotopes in carbonate a guide to plant type in
  most cases, and degree of plant cover in super
  dry climates
• O isotopes a somewhat complex parameter that
  reflects some combination of temperature, storm
  directions, and evaporation
• Carbonate can be dated using radioactive clocks
  like 14C
• Soil carbonate forms where water become saturated
  with Ca and CO3, and is temperature dependent
  (and orientation may reflect seasonality of
  precip).
• Remember concepts, not details !!!!!!!!