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App 4: Erosion rates (Idaho Study (Kirchner 2001) (Multiple methods…
App 4: Erosion rates
Determine erosion rates
Cosmogenic nuclide concs
can shed light on
steady state erosion rates of rock surfaces
If steady state
nuclide loss = proportional to erosion rate
Flip exposure around
Exposure dating
∴ calculate surface exposure age
Assume erosion = negligible
Erosion rates quant
Surface undergoing steady state erosion
∴ use conc to infer s.s. erosion rate
Equation s.s. erosion rate
ε = (P0/N0 = λ1) / (λ/ρ)
where
λ2 = attenuation length
P0 = prod rate
N0 = nuclide conc
λ = decay constant
Method quant erosion rate
Outcrop erosion rate
Old surfaces in
Assume conc in s.s. w erosion
Antarctic erosion rates (Marrero)
Erosion rates compiled wrt geology/proximity to ocean etc
Really low erosion (min 0.1-0.2 mm/ka) - interesting
Sandstones = anomalous
Straddle great range erosion rates
Strong wind erosion
Grains made available thus lotsa tools for erosion
Catchment-scale erosion rates
River provides
Well mixed representative sed sample
Compare average prod rate / nuclide conc / ρ etc
Provides spatially averaged erosion rate
--> for entire catchment / all source areas
Assumptions rqrd for spatial averaging
Erosion is steady state
i.e prod rate nuclides = export nuclides
Homogeneous lithology
e.g. generally qtz-bearing
Similar grain sizes
More conditions rqrd for spatial averaging
Denudation via surface lowering
(not chem weathering)
Minimal storage in system
(often false, esp. in glacial catchments)
Denudational tscale
< radioactive decay tscale
otherwise nuclides decayed before reach stream
problematic if wanted to use C-14
Ways to estimate erosion rate
Point msmt
From different areas landscape
Collect stream sediment
should be representative of everything in catchment (if s.s.)
Nuclide of choice
Nuclide used & erosion rate of catchment
--> dictate erosion rate that nuclide = sensitive to
Erosion rates
Be-10 = sensitive to
0.3 < Be-10 denudation range < 10 m/Ma
Catchment-scale erosion rates
Average out short-term variations due to
long averaging timescales
insensitivity of cosmogenic nuclides
Small tscale - big flood not picked up
Glacial-interglacial (100kyr tscale) = picked up
Proof of Concept
Fort Sage Mtns
Compare 2 small catchments
Calculate volume
Catchment contributing area
Alluvial conc (sediment deposited)
Erosion rate
alluvial fan = contributing area
erosion rate from fan accumulation rate ~ 5-7 cm/ka
= consistent w cosmogenics
Idaho Study
(Kirchner 2001)
32 catchments (diff sizes/steep, fluvial)
Multiple methods
Stream gauging data (historical)
Cosmogenics
Thermochronology
to investigate erosion rates @ different tscales
Denudation Rates
Short term
v. low erosion rates (stream gauge)
Long term
much faster erosion on longer term - order mag higher (cosmo & AFT)
Cosmo ~ 17 times > short term
Interp
Periodic catastrophic storm events
--> dominate system w peak erosion
Trigger: convective storms
Sed gauge data
missing much of this erosion data
Why not climate change cause?
Episodic nature of sediment yield
Large, but rare, erosion rates
Significance
Short term sed yields
not representative average sed yield
70-97 yiedl not accounted for
Reservoir lifetime
will be overestimated