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 Late Quaternary glacial history of the Eastern Canadian Arctic 1. Brigham-Grette, 2001, QSR v. 20, p 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 England, 1998, JQS, v. 13, p Miller et al., 2002, QSR, v. 21, p vs. 3. Briner et al., 2006, GSAB, v. 118, p

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?

Surface Exposure Dating the basics

Gosse and Phillips, 2001 woah

Cosmo Isotope production versus depth Gosse and Phillips, 2001

The case of glacial erosion

Gosse and Phillips, 2001 quartz whole rock calcite parent

N=concentration P=production rate =decay constant T=time Exposure dating requires:

Production of cosmogenic radionuclides varies spatially Gosse and Phillips, 2001

Stone, 2000 Air Pressure

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

Application #1: exposure dating

Complication: degrading landforms

Result of moraine degradation

Complication: isotopic inheritanceApplication #2: glacial erosion

1.Know pre-existing cosmogenic isotope concentration 2.Measure what is left 3.Calculate depth of glacial erosion Solving for glacial erosion

Briner and Swanson, 1998, GEOLOGY

Low elevation

10 Be = 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

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 Shear zone Shear zone Ice Stream

Application #3: burial studies 11.4±0.5 ka 102.3±3.4 ka

Tor exposed at surface becomes saturated with 10 Be and 26 Al 10 Be and 26 Al accumulate in upper ~2 m of rock

Tor shielded by cold-based ice Once shielded: 10 Be and 26 Al radioactively decay differentially

With constant exposure ratio of isotope production eventually decreases

Upon burial or shielding ratio 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 dating 2. Glacial erosion 3. Burial history