Cosmogenic exposure dating of glacial (and other ... · •Cosmogenic dating is easy to apply because rocks (quartz) are everywhere (common) •It requires much lab work (expensive)

Cosmogenic exposure dating of glacial (and other)

landforms – potentials and problems

Jakob Heyman

Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, USA Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden

Outline Basics of cosmogenic dating Glacial exposure dating and geological uncertainties Other cosmogenic nuclide techniques - Burial dating - Erosion rate quantification

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Publications on cosmogenic glacial dating ISI Web of Science search: cosmogenic AND glaci* n = 754

History

Earth is constantly bombarded by cosmic rays

Interaction in the atmosphere and production of secondary cosmic rays

Basics

Production of specific cosmogenic nuclides in the earth surface

Isotope Half-life (years) Target mineral

10Be 1 387 000 Quartz

26Al 705 000 Quartz

36Cl 301 000 Several rock types

14C 5 730 Quartz

21Ne Stable Quartz, Olivine, Pyroxene

3He Stable Olivine, Pyroxene

Cosmogenic nuclides produced in the earth surface when exposed to cosmic rays

Isotope Half-life (years) Target mineral

10Be 1 387 000 Quartz

26Al 705 000 Quartz


14C 5 730 Quartz



Cosmogenic nuclides produced in the earth surface when exposed to cosmic rays

Most commonly used isotope for dating studies: 10Be

Cosmogenic isotope surface production

COSMIC RAYS

Cosmic rays Production of 10Be in quartz The production rate decrease exponentially with depth below the surface At depths of 2-3 m or more is the production rate very low

Important for surface exposure dating

Glaciers erode! 10Be produced in the surface is removed

At depths of 2-3 m or more is the production rate very low

Important for surface exposure dating

At the time of deglaciation (T = 0) the 10Be concentration is zero As time passes after deglaciation the sample will be exposed to cosmic ray bombardment and the 10Be concentration will increase

10B

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T = ln (1 – Nλ / P) / -λ

T = Time (age) N = 10Be concentration λ = decay constant -ln 0.5 / 1 387 000 P = production rate

Sampling

Quartz separation and 9Be addition

AMS measurement of 10Be

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Buildup of 10Be

In theory, dating of several million years old events is possible

Schaefer et al. (2009)

Also, high measurement precision has enabled dating

of very young surfaces

Ideal case New Zealand

From Kaplan et al. (2010)

From Kaplan et al. (2010)

Ideal case New Zealand

From Heyman et al. (2011)

Non-ideal case Tibetan Plateau

45.5 ka

94.7 ka

19.8 ka

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Glacial exposure dating – problems

All boulders < 4 ka exposure age

No large prior exposure! Heyman et al. (2011)

How common is prior exposure? - in young boulders?

How common is prior exposure?

BIIS/FIS: Gyllencreutz et al. (2007) LIS: Kleman et al. (2010) / Dyke et al. (2003)

Deglaciation reconstructions based on 14C dates Independent deglaciation ages for comparison with exposure ages

In boulders from the last deglaciation?

More common than in young boulders

but still limited

Relict boulders preserved under non-erosive ice

Glacial boulders from glacially modified landscapes

Kleman et al. (2008)

Exposure age scatter in boulders from the Tibetan Plateau

Caused by prior exposure or

incomplete exposure?

Exposure age modeling (Monte Carlo model)

Glaciation Shielding from cosmic rays Exposure

Minimum age

Incomplete exposure is typically more important than prior exposure

However, prior exposure (inheritance) does happen every now and then and cannot be ignored

Reduced chi-square statistics: Test if exposure age scatter can be explained by analytical uncertainty

Reduced χ2 value around 1 indicates that the entire exposure age scatter can be explained by the analytical uncertainty

Reduced χ2 value >2 Geomorphological factors…

(Global) glacial 10Be exposure age compilation

New Zealand 266 samples

Tibetan Plateau 1789 samples

Europe 662 samples

North America 835 samples

South America 562 samples

Σ 4114 samples

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New Zealand Tibetan Plateau

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Exposure ages

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Boulder sample groups Total: 561 groups Rχ2 < 2: 111 groups 20%

Bedrock sample groups Total: 59 groups Rχ2 < 2: 21 groups 36%

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Bedrock sample groups Total: 59 groups Rχ2 < 2: 26 groups 44%

Boulder sample groups Total: 561 groups Rχ2 < 2: 195 groups 35%

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Error weighted mean exposure ages for groups with Rχ2 < 2

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Error weighted mean exposure ages for groups with Rχ2 < 2

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Many (perhaps most) glacial exposure ages do not show the deglaciation age

Sample groups with good clustering are mostly from the last major deglaciation 5

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Burial dating with multiple isotopes

Isotope

Half-life (years)

Target mineral

10Be 1 387 000 Quartz

26Al 705 000 Quartz


14C 5 730 Quartz



Different half-lives enables quantification of burial time (shielding from cosmic rays)

10Be and 26Al concentrations yield burial age of 770 000 ± 80 000 years Prior estimate: 230 000 – 500 000 years

Shen et al. (2009)


Dating of Peking man

Quantification of burial under ice

79 ± 17 ka

64 ± 14 ka

37 ± 8 ka

Stroeven et al. (2002)

600 000 years burial under ice


Bierman and Caffee (2001) Namib desert

Bedrock samples

Erosion rate quantification

Average catchment-scale erosion rates quantified based on river sediments

von Blanckenburg (2005)

Catchment-scale erosion rate quantification



• Cosmogenic dating is easy to apply because rocks (quartz) are everywhere (common)

• It requires much lab work (expensive)

• Cosmogenic dating can give an absolute measurement of exposure to cosmic rays

• To convert exposure to cosmic rays into an age we have to make several crucial assumptions regarding the exposure history

Summary

Thank you!

Documents

Cosmogenic exposure dating of glacial (and other ... · •Cosmogenic dating is easy to apply because rocks (quartz) are everywhere (common) •It requires much lab work (expensive)