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Soils and Geomorphology Bob Anderson October 9th 2007 Hillslopes Convex hilltops G. K. Gilbert’s view of a convex hilltop (1909) Need to address both the source of regolith and its transport. Both are climate-dependent. Regolith balance climate Q = -k dz/dx But climate and all

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Soils and geomorphology l.jpg

Soils and Geomorphology

Bob Anderson

October 9th 2007




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G. K. Gilbert’s view of a convex hilltop (1909)

Need to address both the source of regolith and its transport.

Both are climate-dependent.


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Regolith balance

climate

Q = -k dz/dx

But climate and all

other interesting

physics hides in k



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Occurs throughout the critical zone

Transforms the hydrologic behavior of the landscape


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Frost cracking

  • Time spent within the “frost-cracking” window ~ -3 - -8°C

  • Poses a very specific question of temperatures in the subsurface


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k = A exp( -Ea / RT)

Arrhenius equation



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How do we measure “regolith production” or lowering of the regolith-bedrock interface?

Basin-wide averages from sediment and solute output

But this requires assumptions about steady state…

At a point:

You wait a really really really long time (>>PhD timescale)

…Or…

You use a long term integrating tool, and measure the concentration of cosmogenic radionuclides.


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Cosmogenic radionuclides the regolith-bedrock interface?

e.g. 10Be, 26Al

with half-lives of

order 1 Myr


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Dating a Baffin fjord the regolith-bedrock interface?

Bedrock surface using 10Be


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Dating stable (noneroding) depositional surfaces the regolith-bedrock interface?


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Bedrock lowering rates based upon 10Be concentrations the regolith-bedrock interface?

Bottom line: they are VERY slow rates…


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But what sets these rates is still up for debate… the connections to climate and tectonic settings are still fuzzy, entangled


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Now on to transport, the Q in the regolith balance: connections to climate and tectonic settings are still fuzzy, entangled

Water in the landscape


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Blue Hills badlands, Utah. On average it rains < 1 hr/yr… connections to climate and tectonic settings are still fuzzy, entangled


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Transport of regolith, Q connections to climate and tectonic settings are still fuzzy, entangled

One example:

Rainsplash

Rain bombs!

Courtesy David Furbish


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Another example: Frost creep due to repeated freeze-thaw cycling

Single frost event:

• Displacement ~ slope

• Discharge ~ square

of frost depth

Simulation of frost creep

Green = maximum heave; red = post-thaw

Multiple frost events:

• Concave up profile

Climate!

RSA 2002




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But reality is MUCH more complicated and interesting cycling

I = f(S), the saturation state of the soil

So we must allow S to evolve

dS/dt = f(S,P,T) -- i.e. climate again

The California case:

Early storms yield <10% runoff

Late storms yield > 60% runoff

So we need to know the sequence of rain input: the rainfall intensity, the duration of the storm, the interval between storms, and the number of storms per year.


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Vegetable matter Vegetation matters. cycling

Interception

Evaporation

Infiltration capacity

Root strength…

The pre-land plant world would have operated

In a very different way. Ditto Mars.


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High summit surfaces of the Laramide province cycling

Osborne Mountain, Wind River range


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Sampling tors for cosmogenic radionuclides cycling

Scale for w = 5 microns/yr!!


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High surfaces cycling

Model rules

Cosmogenic radionuclide

Results:

Surface lowering

rates are 5-10 microns/yr

Or 5-10 m/Ma


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Late Cenozoic features: cycling

Ornamentation of the crests

differential lowering of high surfaces vs glacial canyons

Ornamentation of the front

transient incision of the fluvial system

Boulder

Golden




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James cycling

Peak

Front Range high surface





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High surface profiles, cycling

Front Range


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A few landscapes behave themselves… cycling

Note timescale for achieving steady state is several Ma, so must average over glacial-interglacial cycles… (gulp)





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Residence time of regolith (or soil) on a landscape: cycling

Estimated by T=h/w. In the case of the high surfaces,

h = 1m, w = 5m/Ma

T=1/5 Ma or 200ka


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Summary cycling

• The majority of any landscape is hillslopes

• Most of them are cloaked with soils

• The evolution of soil thickness is modulated by both production rate of regolith and its transport

• We can measure soil production using cosmogenic radionuclides

• In high alpine settings transport is dominated by periglacial processes

• The high surfaces of the Rockies are likely steady state surfaces, and residence time is long relative to changes in climate


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