Transcript Dating and Understanding Desert Soils

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…..
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…..
north
south
Southern (summer rain) site
(coarse-loamy, mixed, hyperthermic Aridic Ustorthent
Northern (winter rain) site
(coarse-loamy, mixed, thermic Xeric Torriorthent
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….
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
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…
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 < 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)….
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
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)
Or recent time spans…..
•From 1800’s to 1950’s, 14C
was declining due to influx of
fossil fuel CO2 (no 14C) (the
Suess effect)
•After mid 1950’s to mid
1960’s, atmospheric CO2
doubled due to above ground
nuclear weapons testing…
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….
Calculating 14C ages:
d(F) /dt  F
 dF /F    dt
ln F  t  C
(whereF  fraction mod ernC)
@t  0,
C  ln F0  ln(1)  0
therefore :
t  1/  (ln F)  8033ln F
Measuring and reporting F
•14C commonly reported as D
values (relative to standard)
•These can be easily related to
F……..

Rsample 14C/Csample D14C
F

1
Rstandard 14C/Cair1950 1000
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 < atm
(radioactive
decay
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).
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…
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?
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
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).
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.
Carbonate depth patterns and age…
• approximate depth of
modern carbonate
movement
•Depth of Pleistocene
carbonate movement
• What was the magnitude
of climate change?
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)
hasn’t change much (always C3
in Mojave).
•O isotopes suggest warming and
increased evaporation (increasing
18O of remaining soil water
–Circulation likely hasn’t
changed, so temp and
evaporation are the main
causes….
Increase in both temp and
evaporation of soil water
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 !!!!!!!!