Pedogenic carbonate

Pedogenic carbonate

Formation of calcic horizons
• Requires dry climate
– Rain=leaching=acid=carbonate soluble
• In US, most soils west of prairie-forest
transition (not mountains) have carbonate

Depth to carbonate horizon
• Carbonate comes from…
– Ca: dust (windblown)
– C- decomposition of organic
matter/atm/resp
• Soluble, so depth is a
function of rainfall…
– Reported relationship
• Depth to top of calcic horizon
correlated to MAP
• R2=.62, stdev=149mm
• Reanalysis, more data r2=0.31

Other problems
• Modern relationship masks control of atm CO2
(also correlated to depth!)
• What if it’s truncated?
• What if it’s compacted?
• Mississippi floodplain soils
– Carbonate formed by upward (capillary) movement of
groundwater; depth reflects water table…
– Retallack equation calculates 545 and 908 mm, actual
1500
• Used rather extensively…

Isotopes…
• C from OM/CaCO3
• O from CaCO3
• Check- offset in C
right?
• C-veg. or ?
• O…rain?

Atmospheric CO2?
• “Paleosol barometer”
– 2 component mixing: atmospheric and soil CO2
– Ca is atmospheric CO2 concentration (ppmV), Sz is
soil-respired CO2 concentration (ppmV), and
δ13Cs, δ13Cr, and δ13Ca are isotopic
compositions of soil CO2, soil-respired CO2, and
atmospheric CO2, respectively.
• Cerling, T.E., 1999, Stable carbon isotopes in paleosol carbonates, in Thiry, M., and Simon-Coincon, R., eds.,
Palaeoweathering, palaeosurfaces and related continental deposits: International Association of
Sedimentologists Special Publication 27, p. 43–60.

Where to get variables?
• No, or known C4
• Atmospheric d13C?
– Find marine rocks of similar age.
– Find shallow-living microfossils making shells from
carbonate
– Assume equilibrium between dissolved CO2 in ocean
and atmosphere
–Calculate
• Soil respired carbon d13C
– Measure from OM? Find nearby coals?
• Soil respired C concentration
– Estimate from modern analogue
• Soil CO2- measure from pedogenic carbonate…

Or…
• Estimate a simpler relationship from
different soil types

Other?
• Total Fe content of Fe-Mn nodules in
vertisols correlates to MAP (r2=.92)
– Not dependent on soil depth! (better than
carbonate!)
• Oxygen/hydrogen isotopes of sheet
silicates…(?)
– Must be authigenic (formed in soil)

Or…
• Estimate a simpler relationship from
different soil types

Depth to carbonate horizon
• Carbonate comes from…
– Ca: dust (windblown)
– C- decomposition of organic
matter, respiration, atm. Etc.
• identified using 10% HCl
– degree of effervescence
(audible only, visible as
individual bubbles, or foamlike)
• Depth is a function of
rainfall…
– Tied to pCO2, evap

For what soils does this work??
• Soils of “moderate development”
– Carbonate “nodules”, not “wisps” or “layers”
• Unconsolidated parent material
• Seasonal warm climates *if*
– Depth measured to “abundant” not “solitary”
nodules
– *and* “within the low points of gilgai
microrelief”

Gilgai microrelief
• Gilgai: Australian aboriginal term for small
water hole
• Usually associated with vertisols/shrink
swell

Does not work for…
• “Petrocalcic horizons”
• Calcretized bedrock
• Dolocretes
• Carbonate collars
• Erosion-susceptible hillslope soils
• “Nevertheless, the relationship…has
wided applicability to soils of tropical to
frigid, monsoonal to mildly seasonal, and
desert to forest climates”

Other precipitation indicators
• In general;
wetter=more
clay, more red,
less weatherable
stuff
• Histosols=wet
• Aridisols=dry
– evaporites

Clays
• Clays: Hydrated layer silicates
of Al, Fe, and Mg
• Weathering products:
authigenic (formed in soil)
– Aluminum/octahedral,
silica/tetrahedral
– 1:1 group (TO; kaolinite)
• CEC low
– 2:1 groups (TOT)
• smectite (montmorillonite)
– CEC high, shrink/swell
• Mica (illite)
– CEC low; inter unit cations
– 2:1:1 group ((TOT)O;chlorite)
• 2:1 plus octahedral, CEC
moderate-low
– Mixed layer clays

Clays vs. climate and time
• Weathering extent
sequence of clays
– Muscovite/illite/chlorite
–Smectites
– Kaolinite
– Oxides
• Depends on other soil-forming
factors
– Grain size/mineral composition
of parent
– Temperature, seasonality of
rainfall, time

• Palygorskite, Sepiolite
Indicator clays
– Very dry (Morocco: all clays in soils

• Kaolinite (clay), boehmite, gibbsite
(aluminum oxides)
– Very humid
– (tropics)

How to use?
• Ideally, use soils developed on igneous or
metamorphic bedrock
• Petrographic work
– “Bright clay fabric” = soil clays vs. alluvial clays
• Grain size
– Authigenic finer grained
• Crystallinity (XRD)
– More finely crystalline= authigenic
• Also check for burial alteration
– “Illitization”
– Lots of paleosols=illite dominant
– Alteration of smectite to illite during burial?

Burial illitization
• Not necessarily complete!
– Smectite often left
– Illitization spatially limited
• Check:
• (Maybe you’re ok)
– Crystallinity
• “metamorphic” illite= more crystalline
– Chemical variation
• Long story:
– Coupled variations in Na/Ca/Al/K different for weathering
process vs. illitization

Soil chemistry
• Idea: more water, fewer bases
• Won’t work for:
– Dry climate soils (evaporites)
– Wet climate soils (lots of kaolinite clay; Al
only, can’t change)
– Very weakly developed
– Very strongly developed
– Altered by burial illitization

• Clays!
Oxygen/hydrogen isotopes
– Must be authigenic (formed in soil)
– Temperature dependent fractionation- water to
clay
– Unknown water
– Possibility for post depositional alteration by
hydrothermal fluids
• Seems to work?

• Total Fe content of Fe-Mn nodules in
vertisols correlates to MAP (r2=.92)
– Not dependent on soil depth! (better than
carbonate!)
Geology: Vol. 29,
No. 10, pp. 943–946.
Pedogenic ironmanganese
nodules
in Vertisols: A new
proxy for
paleoprecipitation?
Cynthia A. Stiles, *
Claudia I. Mora, and
Steven G. Driese