dating surfaces and strata by u-series on pedogenic carbonate warren sharp kathryn fletcher berkeley...

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