Transcript lect6_geomorphology

Establishing Timing in the Landscapes:
a)
closes
cont act
accelerometer
L
digit al clock
starts clock
T
stops clock
Clast Seismic Velocit y:CSV = L/ T
b)
2.2
C
2.0
B
1.8
1.6
A
1.4 D
1.2
1.0
0 24 6
Distance from Modern Dunes (km)
Figure 3.1: Clast seism ic velocity measurem ent s.
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Weathering often affects
only the surface of a rock
and a cross-section will
expose a very different
material inside.
Weathering affects seismic
velocity of clasts.
More weathering = slower
velocity -- therefore older
rock
A.
Lassen
(andesites)
McCall
(basalts)
3
2
1
0
Age (ka) New Zealand
2 4 68
0
10
6
B.
New Zealand
2
Bohemia
1
Yellowstone
0
05
0
100
150
200
Age (ka) Bohemia and Yellowstone
4
2
0
Figure 3.1: Weathering Rinds.
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Thin slabs or concentric sheets can peel off of this granite during
the weathering process known as exfoliation or spalling.
16
Bull Lake
moraines
lava
12
Pinedale
bedrock
8
Pinedale moraines
4
0
lava
0
10
100 200
Age (ka)
Figure 3.3: Hydration rind t hickness as a function of age.
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This obsidian cobble has a frosted surface due to
weathering, except where the large chip was knocked off.
Thickness of rinds can be used as an age indicator.
1.5
Lost River Valley, Idaho
1.0
0.5
0.0
05
10
15
20
Age (ka)
Figure 3.4: Carbonat e coatings as a function of deposit age, from soils
in the Lost River Valley, Idaho.
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Secondary carbonate
accumulation in the soil
profile is primarily due to
calcium carbonate
supplied by airborne dust,
dissolved in infiltrating
rainwater, and
precipitated in the soil.
Pedogenic carbonate
horizons typically are
approximately parallel to the
land surface, their upper
boundaries are within the
range of the depth of wetting,
and have distinct
morphology.
The age of the geomorphic
surface is related to the age
of the carbonate horizon.
Gravelly soils can develop
significant carbonate
accumulation and reduced
permeability within 10,000
years.
num ber of
moraines
6 0 4 2
100
Swedish Lappland Lichenometry
1570?
1650
80
80
1710
60
60
1780
40
100
1860
40
1890
20
0
1920
1950
1850
1750
20
1650
1550
1450
0
Years A.D.
Figure 3.5: Lichen diam et er as a function of age
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Maximum diameter of lichens can be used
as an age indicator.
Dendrochronology
Counting and measuring the
widths of the annual rings.
Good growing seasons
produce more growth and
thicker rings, while thin rings
occur in less favorable seasons.
The inner portion of a growth
ring is formed early in the
growing season, when growth is
comparatively rapid (hence
the wood is less dense) and is
known as "early wood" or
"spring wood".
The outer portion is the "late
wood" (and has sometimes
been termed "summer wood",
often being produced in the
summer, though sometimes in
the autumn) and is more
dense.
1. The species studied must only produce one ring per growing season or
year.
2. Only one dominant environmental factor can be the cause of hindered
or increased growth.
3. The dominant environmental factor should vary each year so we can
see the changes clearly in every ring.
4. And lastly, the environmental factor must affect a small or large
geographic area.
Douglas Fir Tree Rings
4
Tree 4
3
2
1
4
Tree 1
3
2
1
0
5
4
3
2
1
0
Tree 2
1800
1900
2000
Calendar Year
Figure 3.7: T ree-ring widt hs as a function of t im e for three D ouglas fir
t rees in the P acific NW of the Unit ed St at es.
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0.4
1481
0.2
P=0.26
0.0
-0.2
0.4
N=106 years
1489
0.2
P=0.14
0.0
-0.2
1480
N=114 years
1490
1500
1510
Calendar Year of Outermost Ring
Figure 3.8: Correlation of t ree-widt h t ime series wit h t he mast er t ree-ring
t ime series as a funct ion of chosen st art year.
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Dendrochronolgy has been useful for calibration of 14C ages with calendar
ages. Trees growing at latituded with seasonal variation in temperature will
produce distinct growth rings during the spring-summer (light color) and fallwinter dark color). The bristlecone pine record in the White Mountains, CA has
been extended back >10,000 years.
The width of tree rings depends
on their growth rate - in
particularly bad years, they may
not generate any rings.
Tree ring cores taken from 65
trees along the fault in
Wrightwood, Calif.
30 trees in the Wrightwood area
but not on the fault
Dramatic and extended' growth
suppression in trees along the
fault beginning in 1813
Victim of the 1857 Fort Tejon
earthquake on the San Andreas fault,
this tree near Wrightwood had it's top
snapped off, causing lower branches to
grow vertically.
Something had happened
between the 1812 and 1813
growth seasons.
1812 EQ on SAF
Radiocarbon dating
Naturally occurring isotope
carbon-14 (14C) to determine
the age of carbonaceous
materials up to about 50,000
years
Raw (uncalibrated)
radiocarbon ages are usually
reported in radiocarbon
years "Before Present" (BP)
"Present" IS defined as AD
1950. Such raw ages can be
calibrated to give calendar
dates.
Radiocarbon methods
14C
----> 14N
5730 year ½ life
Useful between 100 and
about 50,000 years old
Can date things that
contain organic carbon
(Used to be living):
bones, shells, wood,
charcoal, plants, paper,
cloth, pollen, seeds)
Radiocarbon dating
The radiocarbon clock is based on
the known decay rate of the
unstable isotope of carbon, 14C,
which is formed when cosmic rays
interact with nitrogen in the
atmosphere.
The radiocarbon combines with
oxygen to form a radioactive form
of carbon dioxide.
When a living organism dies, the
carbon exchange stops.
Measuring the 14C concentration in
organic samples, and provided
they have not been contaminated
by younger material, one can
calculate the time elapsed since
the material was originally formed.
Time (ka)
01
02
03
0
15
Ao
T1/2 = 5735 years
10
activity = A o/2
5
activity = A o/8
0
01
3
2
4
5
Time (half-lives)
6
Time (years)
Figure 3.9: Decay of 14C concent rat ion wit h t ime follows classic
exponent ial curve.
Radiocarbon dating
A raw BP date cannot be
used directly as a calendar
date, because the level of
atmospheric 14C is not
constant in the past 50Ka.
The level is affected by
variations in the cosmic ray
intensity which is affected by
variations in the earth's
magnetosphere caused by
solar storms.
The level has also been
affected by human
activities, it was changed
during atomic bomb tests in
the 1950s and 1960s.
Radiocarbon dating
Raw radiocarbon dates, in
BP years, are calibrated to
give calendar dates.
Comparison of
radiocarbon dates of
samples that can be
independently dated by
other methods such as
examination of tree growth
rings, ice cores, deep
ocean sediment cores,
lake sediment varves, &
coral samples.
U-Th methods
Effect of sea level rise on
coral reefs.
The coral in the first
diagram is growing 5-7 m
below sea level.
As sea level rises, the coral
dies and a new, younger
coral grows 5-7 m below
the new sea level.
U/Th dating requires distinguishing between a sample’s radiogenic 230Th
(produced by in situ 238 U decay) and its non-radiogenic 230 Th (derived
from the surrounding environment).
Levels of non-radiogenic 230 Th (230 Th) are small or negligible
U-Th methods
Cross-section of a coral
microatoll.
X-rayed thin slab reveals a clear record of annual growth bands expanding
radially outward (from left to right) at about a cm per year.
The Highest Level of Survival (HLS) of the coral during the past 35 years is
recorded in the topography of the coral's upper surface.
The arrows track the rise of sea level in the 1960s and its subsequent fall.
238U s eries
235U s eries
238U
234U
235U
4. 49x10 9
2. 48x10 5
7. 13x10 8
234P a
1. 18m
232Th series
231P a
232Th
3. 43x10 4
1. 39x10 10
234Th
230Th
231Th
227Th
24.1d
7. 52x10 4
25.6h
18.2d
228Th
1. 91y
228Ac
6. 13h
228Ra
227Ac
22y
5. 75y
226Ra
1. 60x10 3
222Rn
3. 83d
stable:
206P b
207P b
208P b
Figure 3.11: Uranium and T horium decay chains.
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0
-20
-40
-60
-80
U/Th
14C
-100
-120
-140
69
12
15
18
21
Age (kybp)
Figure 3.12: P aired U/T h and radiocarbon ages of corals.
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Amino acid racemization
Life requires a certain
composition and shape of amino
acid molecules in order to
complete their function.
Living organisms on earth keep
their amino acids in the L position,
with a notable exception found in
certain bacterial cell walls, and
their sugars in the D position.
When the organism dies, control
ceases, and the ratio of D/L
moves slowly toward equilibrium
(racemic).
Measuring the ratio of D/L of a
sample can allow calculations of
how long ago the specimen died.
The rate at which racemization
proceeds depends upon the
type of amino acid, average
temperature, humidity, acidity,
alkalinity, and enclosing matrix.
Also, D/L concentration
thresholds appear to occur as
sudden decreases in the rate of
racemization.
These effects restrict amino
acid chronologies to materials
with known environmental
histories and/or relative
intercomparisons with other
dating methods.
0.6
forward and backward
reactions roughly equal
1.2
0.4
therm al effect
on 125 ka
deposits
0.6
0.2
0.0
0
forward (L-D)
reaction
dominates
200
0.0
-10
10
30
MAT ( C)
400
600
800
1000
a fte r Ka uf m an a nd Miller , 1992
1;
Time (ka) fig
inse t a fter Hea rty a nd Miller , 1987, fig 2 in
K& M 1992
Figure 3.13: T heoret ical curve of amino acid racemization through time.
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Luminescence dating
 Measures the energy of photons being released.
 In natural settings, ionizing radiation (U, Th, Rb, & K ) is absorbed and
stored by sediments in the crystal lattice.
 This stored radiation dose can be evicted with stimulation and
released as luminescence.
Luminescence dating
The calculated age is the time since the last exposure to sunlight or
intense heat.
The sunlight bleaches away the luminescence signal and resets the
time 'clock'. As time passes, the luminescence signal increases
through exposure to the ionizing radiation and cosmic rays.
1500
75000
(a)
1000
50000
500
25000
0
0
100
200
300
Temperature (C)
400
0
(b)
0 1
5 1
0 2
5
Time (seconds
02
5
Figure 3.14: T hermal and optically st im ulat ed luminescence.
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The intensity of the luminescence is
calibrated in the laboratory to yield an
equivalent dose, which is divided by an
estimate of the radioactivity that the
sample received during burial (dose rate,
Dr) to render a luminescence age.
The selection and sampling of
sediment is crucial.
A luminescence age is a measure
of the time since the last sunlight
exposure (or heating) event of the
sediment.
The reduction in luminescence,
usually by sunlight, must be
related to an event (e.g. faultgenerated colluvium) for a
luminescence age to be
meaningful.
The luminescence dating of
sediment that was exposed to
little or no light during a specific
event will yield a spurious age.
The preferred sediment for OSL dating has
had > 1 hr sunlight exposure, accumulated
as relatively homogeneous stratigraphic
unit, >30 cm thick and has not undergone
significant water-content variations or
diagenetic changes during burial.
In many sedimentary
environments, particularly
eolian and littoral, coarsegrained (100-300 µm)
quartz grains is best; it is
an abundant particle size
that is easily solar reset.
In other sedimentary
settings the fine-grained (411 µm) polymineral or
quartz fraction is best,
particularly for loess,
colluvial, eusturine, and
many lacustrine and fluvial
environments.
Cosmogenic Surface Exposure Ages
 Cosmogenic isotopes are
created when elements in
the atmosphere or earth are
bombarded by high energy
particles that penetrate into
the atmosphere from outer
space.
 Some cosmic ray particles
reach the surface of the
earth and contribute to the
natural background
radiation environment.
Cosmogenic Surface Exposure Ages
 Cosmic ray interaction with
silica and oxygen in quartz
produced measurable
amounts of the isotopes
Beryllium-10 and Aluminium26.
 The accumulation of these
isotopes within a rock surface
could be used to establish
how long that surface was
exposed to the atmosphere.
With constant rate of
production, the number of
atoms of Be-10 and Al-26
that accumulate in a rock
surface will be proportional
to the length of time the
rocks were exposed to
cosmic ray bombardment.
The amount of each nuclide
would be an estimate of the
minimum time that the
particular surface had been
exposed.
Cosmogenic Surface Exposure Ages
 Rocks exposed to cosmic rays contains “exotic” short-lived
isotopes.
 Only rocks near the surface (upper few meters) effected.
 The older the surface, the higher the concentrations of
CRN isotopes.
CRN’s produced in quartz grains by cosmic-ray
bombardment of Si, O nuclei
 Production rate variable with altitude, latitude
 Cosmic-ray flux decreases exponentially with depth below
the surface.
 If a previously exposed surface is buried, nuclide
production ceases.
post-depositional
production
inheritance
0
0.5
1
1.5
2
2.5
10
Be age wit h inheritance: ~ 26 ka
Be age w/o inheritance: ~ 15ka
10
3
0.0
0.10
0.20
0.30
0.40
0.50
0.60
Be Concentration (atoms/ g qtz)
10
Figure 3.16: Use of cosmogenic radionuclide concentrat ion profile to
deduce both inherit ance and age of t he surface.
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0.70