Post on 15-Jan-2016
transcript
Dating Surfaces and Strata by U-series on Pedogenic
Carbonate
Dating Surfaces and Strata by U-series on Pedogenic
Carbonate
Warren Sharp
Kathryn Fletcher
Berkeley Geochronology Center
• Depends only on intrinsic properties of the sample (and well-known decay constants).
• Is largely model-independent.
• Offers internal checks on its major assumption of closed-system behavior.
• Depends only on intrinsic properties of the sample (and well-known decay constants).
• Is largely model-independent.
• Offers internal checks on its major assumption of closed-system behavior.
Strengths of U-series Dating
Therefore:
• Accuracy is comparable to measurement precision, commonly 1-2% (2).
• Accuracy is comparable to measurement precision, commonly 1-2% (2).
238U
The 238U Decay SeriesThe 238U Decay Series
234Th 234U234Pa
230Th226Ra222Rn218Po
214Bi214Pb 214Po 210Pb
210Bi210Po206Pb
4.5 Gyr 24 d
76 kyr1.6 kyr4 d
27 min 20 min 0.2 msec
Stable
138 d
7 hr
23 yr
245 kyr
3 min
50 d
230Th/238U evolution with time
230Th238U
Essentially zero initial 230Thin pure, authigenic material
Use 232Th as index for contamination
The 230Th/234U/238U Age EquationThe 230Th/234U/238U Age Equation
Must be solved numerically
• Isotope ratios for sample, and • Decay constants (known to ~0.3%)
• Isotope ratios for sample, and • Decay constants (known to ~0.3%)
Equation contains only:
Most minerals precipitated directly from near-surface waters contain ppm levels of uranium but (when pure) only ppb levels of thorium.
Most minerals precipitated directly from near-surface waters contain ppm levels of uranium but (when pure) only ppb levels of thorium.
• Reasonably free of incorporated detritus (and initial 230Th from other sources);
• Have behaved as closed systems;
• Are a few hundred to a few hundred thousand years old;
• Reasonably free of incorporated detritus (and initial 230Th from other sources);
• Have behaved as closed systems;
• Are a few hundred to a few hundred thousand years old;
So if these minerals are:
Then:Samples of 10-100 mg will yield 230Th/U dates with accura-
cies at the percent level or better.
Samples of 10-100 mg will yield 230Th/U dates with accura-cies at the percent level or better.
Basics of U-series
Pedogenic Carbonate
• Is widely developed in arid to semi-arid regions.
• Accumulates in gravelly sediments in a well-known morphological sequence.
• Is highly suitable for precise U-series dating if small, dense, visibly pure samples are analyzed.
• Is widely developed in arid to semi-arid regions.
• Accumulates in gravelly sediments in a well-known morphological sequence.
• Is highly suitable for precise U-series dating if small, dense, visibly pure samples are analyzed.
Morphology of Carbonate Accumulation in Gravelly Soils
Gile et al., 1966Time
Pedogenic Carbonate:LGM terrace: Wind River Basin, Rocky Mts.
Depth of Carbonate (Bk) versusMean Annual Precipitation (MAP)
MAP(mm)
Depth to Bk horizon (cm)
global, post-glacial soils
Retallack, 2005
U-series Dating of Pedogenic Carbonate
U-series Dating of Pedogenic Carbonate
Limitations when applied to landforms
• SINCE carbonate accumulation ensues after landform deposition; and
• U-series analyses require samples of finite thickness; i.e., > 200 microns,
• U-series ages on pedogenic carbonate will be minimum estimates of landform ages.
• This “time-lag” is variable and ranges from ~0.5 to 2 ka where determined thus far.
Discordant Cosmogenic and U-series Ages for the Biskra Palms Fan
Discordant Cosmogenic and U-series Ages for the Biskra Palms Fan
U-series, Soils: Kate Fletcher1, Katherine Kendrick2, Warren Sharp1
New 10Be Dating: Whitney Behr3, Dylan Rood4,
Tom Hanks2, Ken Hudnut2, Bob Finkel4
U-series, Soils: Kate Fletcher1, Katherine Kendrick2, Warren Sharp1
New 10Be Dating: Whitney Behr3, Dylan Rood4,
Tom Hanks2, Ken Hudnut2, Bob Finkel4
1. Berkeley Geochronology Center
2. U.S. Geological Survey
3. University of Southern California
4. Lawrence Livermore National Lab
1. Berkeley Geochronology Center
2. U.S. Geological Survey
3. University of Southern California
4. Lawrence Livermore National Lab
San Andreas fault system, southern California San Andreas fault system, southern California
~50 mm/yr
BiskraPalms
fan
Biskra Palms Fan, T2 Surface
from van der Woerd et al., 2006
• 10Be on pavement-clasts(van der Woerd et al., 2006)
• 10Be on boulder-tops(Behr et al., 2007; in prep.)
• U-series on pedogenic carbonate(Fletcher et al., 2007; in prep.)
• 10Be on pavement-clasts(van der Woerd et al., 2006)
• 10Be on boulder-tops(Behr et al., 2007; in prep.)
• U-series on pedogenic carbonate(Fletcher et al., 2007; in prep.)
Compare three geochronological datasets for Biskra fan T2 surface:
Compare three geochronological datasets for Biskra fan T2 surface:
Relict bar and swale, Biskra T2 fan surface
van der Woerd et al. (2006)
swale
San Andreas fault
bar
F10Be dating of pavement cobbles; typical sample
(van der Woerd et al., 2006)
10Be Age(ka)
box heights show2 sigma internal errors
10Be ages for pavement cobbles from T2 surface10Be ages for pavement cobbles from T2 surface
data from van der Woerd et al. (2006)
24 28 32 36 40 44 48 52 56
Age (ka)
RelativeProbability
10Be ages of pavement clasts from T2 surface,(van der Woerd et al., 2006)
35 ka
10Be Age(ka)
box heights show2 sigma internal errors
interpretation of van der Woerd et al. (2006)
mean age =35.5 ± 2.5 ka
10Be ages for pavement cobbles from T2 surface10Be ages for pavement cobbles from T2 surface
Highest T2 boulder
HanksBehr
Hudnut
Platt
10Be Age(ka)
10Be ages for samples from T2 surface
box heights show2 internal errors
pavement-cobbles,van der Woerd
et al., 2006
mean ageof cobbles
boulder-tops,Behr et al., 2007
for all 10Be ages,MSWD = 11.3
probability of fit = 0.000
Erosion/Exhumation
Inheritance
U-series dating of pedogenic carbonateFletcher et al. (2007)
Kendrick
2 m
datedhorizon
soil profile,T2 alluvium
graniticclast
carbonatecoating
Carbonate is highly suitablefor U-series dating…median values, n = 21
U ~ 6 ppm232Th ~ 0.2 ppm
(230Th/232Th)Act. ~ 40
Pedogenic carbonatefrom Biskra Palms
detrital grain
Age(ka)
U-series ages of T2 carbonate
Lower fan Upper fanMid.fan
Fletcher et al., 2007
ellipses enclosesub-samplesfrom a single clast
errors are2sigma
Age(ka)
U-series ages for pedogenic carbonate from T2 surface
errors are2 sigma
mean age =45.3 ± 0.5 ka
four sub-samples from asingle, ~200 micron-thickclast-coating
Fletcher et al., 2007MSWD = 1.3
No excess scatter
Age(ka)
U-series ages of T2 carbonate
Lower fan Upper fanMid.fan
minimum age,45.3 ± 0.5 ka
Fletcher et al., 2007
ellipses enclosesub-samplesfrom a single clast
errors are2sigma
box heights showexternal errors
pavement-cobbles,van der Woerd
et al., 2006
Age(ka)
boulder-tops,Behr et al., 2007
minimum depositional ageof T2 from U-series, 45 ka
Ages for T2 surface
Erosion/Exhumation
10BeModelAge(yr)
Erosion rate (mm/ka)
Modeling the effects of rock-erosionon apparent 10Be ages
minimum T2 agefrom U-series
youngest boulder-top
youngest pavement-clast
external errors,1
Soil profile,Indio Hills,courtesy ofTom Rockwell
~20-cm thickremnant A-layer
• At Biskra Palms, the A-layer is missing and was likely eroded, thereby exhuming partially shielded cobbles.
• Stripping of ~50 cm from the fan surface at the Pleistocene/Holocene climate transition, for example, would yield the observed array of pavement-cobble ages of 25-45 ka.
• At Biskra Palms, the A-layer is missing and was likely eroded, thereby exhuming partially shielded cobbles.
• Stripping of ~50 cm from the fan surface at the Pleistocene/Holocene climate transition, for example, would yield the observed array of pavement-cobble ages of 25-45 ka.
ConclusionsConclusions
• U-series dating of pedogenic carbonate can provide reliable and precise minimum landform ages.
• Biskra Palms T2 surface is > 45 ka, rather than 35±2.5 ka, with attendant reduction in slip rate.
• Effects of erosion/exhumation on cosmogenic surface exposure ages must be considered even on well-preserved, late Pleistocene landforms.
• U-series dating of pedogenic carbonate can provide reliable and precise minimum landform ages.
• Biskra Palms T2 surface is > 45 ka, rather than 35±2.5 ka, with attendant reduction in slip rate.
• Effects of erosion/exhumation on cosmogenic surface exposure ages must be considered even on well-preserved, late Pleistocene landforms.
age averaging results in scatter towards younger ages
a b c
Clast
1 mm
> 3 mm3 of sample needed to yield ages precise to 1-2%
0.6 meterdepth
stone pavement
accretionarymantle
basalt bedrock
stratigraphic age
3Heage(ka)
100
75
50
25
0
pavement
flow surface
cone scoria
Wells et al. (1995) pavement-clast ages
Secular variation in slip rates within the San Andreassystem? (Bennett, Friedrich, & Furlong, 2004)
(mm/yr)
30
20
10
0
1 10 100 1000
San Andreas
San Jacinto
Time (ka)
SlipRate
Biskra Palms,this study
Geochemistry of Uranium and Thorium in the Near-Surface Environment
• Uranium in oxygenated water (such as almost all natural, near-surface water) readily complexes to form the uranyl ion, (UO2)2+; hence uranium and has appreciable solubility at low temperature.
• Thorium occurs only in the +4 oxidation state in natural waters and does not generally complex; hence thorium is extremely insoluble at low temperature.
• Uranium in oxygenated water (such as almost all natural, near-surface water) readily complexes to form the uranyl ion, (UO2)2+; hence uranium and has appreciable solubility at low temperature.
• Thorium occurs only in the +4 oxidation state in natural waters and does not generally complex; hence thorium is extremely insoluble at low temperature.
Southern San Andreas Slip RatesSouthern San Andreas Slip Rates
Geodetic Rates (Coachella Valley, past 30 yr)
• Fay & Humphreys (2005): 21.4 ± 0.5 mm/yrFay & Humphreys (2005): 21.4 ± 0.5 mm/yr
• Meade & Hager (2005): 23.2 ± 0.5 mm/yrMeade & Hager (2005): 23.2 ± 0.5 mm/yr
Geological Rate (Biskra Palms, past 50 kyr)• van der Woerd et al. (2006): 13-19 mm/yrvan der Woerd et al. (2006): 13-19 mm/yr
• Behr et al. and Fletcher et al. (in prep.): Behr et al. and Fletcher et al. (in prep.): << 15 mm/yr 15 mm/yr