conclusions of small-scale geomorphology
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Conclusions of small-scale geomorphology. Off road vehicles have lasting affects on desert environment Channels in walkway plots are bimodal and follow the orientation of rock alignments and the piedmont’s steepest gradient - PowerPoint PPT PresentationTRANSCRIPT
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Conclusions of small-scale Conclusions of small-scale geomorphologygeomorphology
• Off road vehicles have lasting affects on desert environment
• Channels in walkway plots are bimodal and follow the orientation of rock alignments and the piedmont’s steepest gradient
• After 50 years the channels in walkway and road plots have not attained the characteristics of channels in control plots, mainly due to compaction and the rock alignments
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• Cosmogenic isotopes
• Tracers as sediment moves down piedmont
• Tracers at near surface to determine deposition rates
Switch from human-induced Switch from human-induced rates to long-term rates of rates to long-term rates of
landscape changelandscape change
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Cosmogenic IsotopesCosmogenic Isotopes
Si
O
Ca, K, Cl
26Al, 21Ne, 3He
10Be, 14C, 3He
36Cl, 3He
The isotopes are like a suntan.
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Objectives1. Sediment residence time in low
terraces
2. Depositional history of the piedmont
3. Sediment generation rates of source basins
4. Average sediment transport velocities
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Three types of cosmogenic Three types of cosmogenic isotopes samplesisotopes samples
• Integrated valley samples collected from streams that exit steep narrow basins of the Iron and Granite Mountains
• Integrated soil profiles from two pits on the Iron Mountain piedmont
• Integrated transect samples collected at 1 km intervals away from the Iron and Granite Mountain rangefronts
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Sampling Map Sampling Map
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Valley Valley SampleSample
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Transect SampleTransect Sample
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Soil Pit SamplesSoil Pit Samples
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Sediment generation rates
mP
C
valley data determine mountain mass erosion and sediment generation rates (Bierman and Steig, 1996):
Average sediment generation rates are 0.127 m3 y-1 m-1 for Iron Mountains and 0.098 m3 y-1 m-1 for Granite Mountains.
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0
100
200
300
400
0 1 2 3 4
Nuclide abundance (105 atoms g-1)
Dep
th (
cm)
Deposition
70 m Ma-1
Stable surface
Erosion
38 m Ma-1
Predicted depth profiles for stable,erosional, and depositional surfaces
P P ex o(x / )
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0
20
40
60
80
100
0 1 2 3 4 5 6
Dep
th (c
m)
15 m Ma-1
40 m Ma-1
0
20
40
60
80
100
0 1 2 3 4
15 m Ma-1
40 m Ma-1
26Al abundance (106 atoms g-1)
10Be abundance (105 atoms g-1)
Pit 1 Pit 2
N C1
sP e dzi 0
( z / )
0
h
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Unconformity• Lower nuclide abundances =
less dosed sediment
• Nuclide difference represents a period of time
• Soil pit 1 10Be difference represents ~15 ka and 26Al difference represents ~6 ka
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Iron Mountain transect samples
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0.E+00
1.E+06
2.E+06
3.E+06
IMT-1 IMT1-CHAN
IMT1-CRIT
IMT1-TERR
IMT-4 IMT4-CHAN
IMT4-CRIT
IMT4-TERR
Nu
clid
e ab
un
dan
ce (
ato
ms
g-1
)Channels “firehose” across
the surface
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Slope = 0.36x
r2 = 0.98
Slope = 0.32x
r2 = 0.991.0
2.0
3.0
4.0
-1 0 1 2 3 4 5 6 7Distance from rangefront (km)
10B
e ab
unda
nce
(10
5
atom
s g
-1)
Iron Mountains
Granite Mountains
Incredible Linearity
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Slope = 0.21x
r2 = 0.93
Slope = 0.26x
r2 = 0.97
0.0
1.0
2.0
3.0
-1 0 1 2 3 4 5 6 7Distance from rangefront (km)
26A
l ab
un
dan
ce (1
06
ato
ms
g-1
)
Iron Mountains
Granite Mountains
More linear trendlines!
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Three endmember processes
1. Depositional surface
2. Surface of transport
3. Erosional surface
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Model EquationModel Equation
Sediment inSediment in Sediment outSediment out
Sediment depositionSediment depositionSediment erosionSediment erosion
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Constraints on piedmont deposition and active layer• Deposition rates are not uniform across
piedmont (15 m Ma-1 to 40 m Ma-1)
• Deposition rates are not uniform through time (unconformity from ~5 to ~15 ka at soil pit 1)
• Active layer is well-mixed and thickness (20 to 30 cm) is spatially uniform
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Mixed layer or active layer is uniform thickness (20 to 30
cm) as determined from• measurement of B-horizon
depth
• measurements of maximum channel depths
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1.0E+05
2.0E+05
3.0E+05
4.0E+05
5.0E+05
6.0E+05
7.0E+05
0 2 4 6
Distance from rangefront (km)
10B
e ab
un
dan
ce (
ato
ms
g-1) 1. Deposition 15 m Ma -1
6 to 28 cm y -1
3. Erosion 5 m Ma-1
28 to 48 cm y-1
Transect data
2. Surface of Transport
32 cm y -1
1.
2.3.
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Mixed model assumesPre-unconformity
• 40 cm ATL• 25 m Ma-1 erosion
rates• deposition of 15 m
Ma-1
• deposition of 40 m Ma-1
Post-unconformity• 30 cm ATL• 38 m Ma-1 erosion
rates• surface of transport• deposition of 40 m
Ma-1
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1
2
3
4
5
-2000 0 2000 4000 6000
Distance from rangefront (m)
10B
e a
bu
nd
an
ce
(10
5 ato
ms
g-1
)Mixed Model
Velocities are 35 ±10 cm y-1
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Remember First estimates of:
•Consistent average sediment transport rates•Sediment velocities (35 cm y-1)•Sediment residence times in low terraces (< 1000 years)
Big picture using 10Be and 26Al:•Sediment generation rates and flux of sediment across piedmonts•Constrain complex surface histories of piedmonts
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Acknowledgements• We would like to thank Darrin Santos and
Christine Massey for field assistance
• U.S. Army Research Office for funding this research
• Ben Copans and Susan Nies for laboratory assistance
• John Southon and Erik Clapp for AMS assistance