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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

Soils and Geomorphology

Bob Anderson

October 9th 2007

slide4

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.

slide5

Regolith balance

climate

Q = -k dz/dx

But climate and all

other interesting

physics hides in k

slide7

Occurs throughout the critical zone

Transforms the hydrologic behavior of the landscape

frost cracking
Frost cracking
  • Time spent within the “frost-cracking” window ~ -3 - -8°C
  • Poses a very specific question of temperatures in the subsurface
k a exp e a rt

k = A exp( -Ea / RT)

Arrhenius equation

slide11

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.

slide12

Cosmogenic radionuclides

e.g. 10Be, 26Al

with half-lives of

order 1 Myr

slide13

Dating a Baffin fjord

Bedrock surface using 10Be

slide15

Bedrock lowering rates based upon 10Be concentrations

Bottom line: they are VERY slow rates…

slide16

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

slide19

Transport of regolith, Q

One example:

Rainsplash

Rain bombs!

Courtesy David Furbish

slide20

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

slide23

But reality is MUCH more complicated and interesting

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.

slide24

Vegetable matter Vegetation matters.

Interception

Evaporation

Infiltration capacity

Root strength…

The pre-land plant world would have operated

In a very different way. Ditto Mars.

slide25

High summit surfaces of the Laramide province

Osborne Mountain, Wind River range

slide26

Sampling tors for cosmogenic radionuclides

Scale for w = 5 microns/yr!!

slide27

High surfaces

Model rules

Cosmogenic radionuclide

Results:

Surface lowering

rates are 5-10 microns/yr

Or 5-10 m/Ma

slide28

Late Cenozoic features:

Ornamentation of the crests

differential lowering of high surfaces vs glacial canyons

Ornamentation of the front

transient incision of the fluvial system

Boulder

Golden

slide31

James

Peak

Front Range high surface

slide36

A few landscapes behave themselves…

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

slide40

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

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

h = 1m, w = 5m/Ma

T=1/5 Ma or 200ka

slide41

Summary

• 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