Cosmogenic exposure
dating
-principles and applications
Quaternary glacial
history of Beringia
-overview with case
studies
Late Quaternary glacial history
of 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
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.
Late Quaternary glacial history of the Eastern
Canadian Arctic
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*"
30.00
Number of records
25.00
20.00
15.00
10.00
5.00
0.00
1930
1940
1950
1960
1970
Year
1980
1990
2000
2010
Surface Exposure Dating
the basics
woah
Gosse
and Phillips,
2001
Cosmo
Isotope
production
versus
depth
Gosse
and Phillips,
2001
The case of glacial erosion
whole rock
calcite
quartz
parent
Gosse and Phillips, 2001
Exposure dating requires:
N
N=concentration
P=production rate
=decay constant
T=time
P

(1  e
t
)
 N 
ln 1 

P 
.
t

Production of cosmogenic radionuclides varies spatially
Gosse
and Phillips,
2001
Air Pressure
Stone, 2000
Complication: Surface erosion
Steig et al.,
1998
Shielding of cosmic rays by surrounding topography
Complication:
Seasonal snow cover
Gosse
and 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:
Application #2:
isotopic
glacialinheritance
erosion
Solving for glacial erosion
1. Know pre-existing cosmogenic isotope concentration
2. Measure what is left
3. Calculate depth of glacial erosion
Briner and Swanson, 1998, GEOLOGY
Low elevation
Low elevation
10
Be = 9.4±0.4 ka
Intermediate elevation
Intermediate elevation
22.0±0.7 ka
High elevation
High elevation
84.4±2.0 ka
Relative Probability
Low-elevation bedrock
(n=10)
Intermediate-elevation bedrock
(n=11)
High-elevation bedrock
(n=12)
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
Relative Probability
(n=27)
Low-elevation bedrock
(n=10)
Intermediate-elevation bedrock
(n=11)
High-elevation bedrock
(n=12)
Briner et al., 2006, GSAB
Shear
zone
Shear
zone
warm-based
Coldbased
Coldbased
Ice Stream
Application #3:
burial studies
11.4±0.5 ka
102.3±3.4 ka
10Be
and
26Al
accumulate in upper ~2 m of rock
Tor exposed at surface becomes saturated with
10Be
and
26Al
Tor shielded by cold-based ice
Once shielded:
10Be
and
26Al
radioactively decay differentially
With constant exposure
ratio of isotope production eventually decreases
Upon burial or shielding
ratio decreases below the constant exposure line
High elevation
84.4±2.0 ka
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 dating
2. Glacial erosion
3. Burial history