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 = λF

∫ dF /F = λ ∫ dt

lnF = λ t +C

(whereF = fractionmodernC)

@t = 0,

C = lnF0 = ln(1) = 0

therefore :

t = −1/λ (lnF) = −8033lnF

Measuring and reporting F

•14C commonly reported as D values (relative to standard)

•These can be easily related to F……..

€

F =RsampleRs tan dard

=14C /Csample14C /Cair1950

=1+D14C

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

14C humus < atm (radioactive decay

CO2

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…

• 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….

Increasing atm vs biological CO2 (less plant cover)

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 !!!!!!!!