Cosmogenic exposure dating

-principles and applications

Quaternary glacialhistory of Beringia-overview with casestudies

Late Quaternary glacial historyof the Eastern Canadian Arctic

-the Clyde River Project

Firstly, it is great to be here!

My biased reading suggestions:(be familiar with lots more, but be sure to read these)

Quaternary glacial history of Beringia

Late Quaternary glacial history of the Eastern Canadian Arctic

1. Brigham-Grette, 2001, QSR v. 20, p. 15-24.2. Briner and Kaufman, submitted, Journal of Quaternary Science.Read this for discussion:3. letter to the editor debate on ‘Beringian Ice Sheet’ - Brigham-Grette and Gualtieri et al., 2004; Grosswald and Hughs, 2004, QR, v. 62. 

1. England, 1998, JQS, v. 13, p. 275-280.2. Miller et al., 2002, QSR, v. 21, p. 33-48.

vs.3. Briner et al., 2006, GSAB, v. 118, p. 406-420.

Cosmogenic Exposure Dating

3 Questions to consider:

1. How would you explain cosmogenic exposure dating to your Dad (elementary school teacher) and Mom (engineer)?

2. What are three ways that cosmogenic radionuclides are used?

3. How would you critique a dataset of cosmogenic exposure ages?

GEOREF hits of "cosmogenic" and "glaci*"

0.00

5.00

10.00

15.00

20.00

25.00

30.00

1930 1940 1950 1960 1970 1980 1990 2000 2010

Year

Number of records

Series1

Surface Exposure Datingthe basics

lava flowold landscapelandslidemoraineCosmic Radiation

Gosseand Phillips,2001

woah

CosmoIsotopeproductionversus depth

Gosseand Phillips,2001

The case of glacial erosion

Gosse and Phillips, 2001

quartz

whole rock

calcite

parent

N =

P

λ

( 1 − e

− λ t

)

t =

ln 1 −

N λ

P

⎛

⎝

⎞

⎠

− λ

.

N=concentrationP=production rateλ=decay constantT=time

Exposure dating requires:

Production of cosmogenic radionuclides varies spatially

Gosseand Phillips,2001

Stone, 2000

Air Pressure

Complication: Surface erosion

Steig et al.,1998

Shielding of cosmic rays by surrounding topography

Complication:Seasonal snow cover

Gosseand Phillips,2001

Use CRONUS-Balco age calculator

http://hess.ess.washington.edu/math/

Application #1: exposure dating

Complication: degrading landforms

Result of moraine degradation

Complication: isotopic inheritanceApplication #2: glacial erosion

1.Know pre-existing cosmogenic isotope concentration2.Measure what is left3.Calculate depth of glacial erosion

Solving for glacial erosion

Briner and Swanson, 1998, GEOLOGY

Low elevation

10Be = 9.4±0.4 ka

Low elevation

Intermediate elevation

22.0±0.7 ka

Intermediate elevation

High elevation

84.4±2.0 ka

High elevation

Low-elevation bedrock(n=10)

Intermediate-elevation bedrock(n=11)

High-elevation bedrock(n=12)

Relative Probability

High elevation

High elevation

102.3±3.4 ka

High elevation

11.4±0.5 ka

102.3±3.4 ka

Erratics from intermediate and high elevation bedrock(n=27)

Low-elevation bedrock(n=10)

Intermediate-elevation bedrock(n=11)

High-elevation bedrock(n=12)

Relative Probability

Briner et al., 2006, GSAB

Cold-based

Cold-based

warm-based

Shearzone

Shearzone

Ice Stream

Application #3: burial studies

11.4±0.5 ka

102.3±3.4 ka

Tor exposed at surface becomes saturated with 10Be and 26Al

10Be and 26Al accumulate in upper ~2 m of rock

Tor shielded by cold-based ice

Once shielded:

10Be and 26Al radioactively decay differentially

With constant exposureratio of isotope production eventually decreases

Upon burial or shieldingratio decreases below the constant exposure line

84.4±2.0 ka

High elevation

Al/Be burial age:~420 ka

High elevation

102.3±3.4 ka

Al/Be burial age:~475 ka

High elevation

11.4±0.5 ka

102.3±3.4 ka

Al/Be burial age:~475 ka

Overview: 1. Exposure dating2. Glacial erosion3. Burial history