Dating and Understanding the Formation of Calcic Desert Soils (Aridisols, Torriorthents) - PowerPoint PPT Presentation

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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 ... – PowerPoint PPT presentation

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


1
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..

2
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..

3
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4
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5
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6
north
south
7
Southern (summer rain) site(coarse-loamy, mixed,
hyperthermic Aridic Ustorthent
8
Northern (winter rain) site(coarse-loamy,
mixed, thermic Xeric Torriorthent
9
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.

10
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

11
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

12
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13
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).

14
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

15
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)

16
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

17
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.

18
Calculating 14C ages
19
Measuring and reporting F
  • 14C commonly reported as D values (relative to
    standard)
  • These can be easily related to F..

20
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
21
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).

22
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

23
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?

24
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

25
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).

26
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.
27
Carbonate depth patterns and age
  • approximate depth of modern carbonate movement
  • Depth of Pleistocene carbonate movement
  • What was the magnitude of climate change?

28
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
29
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 !!!!!!!!
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