1. Using Geophysical Techniques in the Critical Zone to Determine the Presence of Permafrost, Boulder Creek, CO -Gabriel Lewis Williams College
GSA Annual Meeting
November 6, 2012

2. Introduction
Electrical Resistivity Tomography was used to verify the existence of permafrost in previously modeled and observed regions on Niwot Ridge and Green Lakes Valley
My study dug pits and measured soil and water temperature profiles along geophysics lines to ground check resistivity values and permafrost probabilities
Image from Janke, 2005

3. Introduction to Colorado
During LGM temperatures were ~4.5-7°C cooler than today
Permafrost was possible at elevation of Gordon Gulch
Wide variety of elevation, temperatures, and precipitation among Green Lakes Valley, the 4th of July Valley, and Gordon Gulch
Climate data show
post 1980 warming
of summer
temperatures with
constant winter
temperatures
Implications for
permafrost
4th of July and GLV
Gordon Gulch
Boulder
Image from Birkeland et al., 2003

4. Subsurface Temperature Variations
Surface temperatures oscillate roughly sinusoidally at station D1 on Niwot Ridge

5. Subsurface Temperature Variations
Surface temperatures oscillate roughly sinusoidally at station D1 on Niwot Ridge
Amplitude of oscillations decreases with depth
Lag of temperature variations increases with depth
For permafrost to occur, ground temperatures must all be below 0°C
Permafrost at 5m
Fix equation

6. Periglacial Transport
Solifluction lobes and stone-banked terraces demonstrate transport
Active today even without permafrost
Possibly due to seasonal freeze-thaw
Gives incentive for studying
9.4mm/a from 1963 to 1967
60cm
Images from Benedict, 1970 & 1976

7. Electrical Resistivity Tomography
I PROMISE I’m wearing shorts!

8. Electrical Resistivity Tomography Lines
Gordon Gulch
Ran 15 different lines

9. Electrical Resistivity Tomography Lines
Gordon Gulch

10. Niwot Ridge ERT-6
Horizontally continuous extremely high (>80,000Ωm) resistivity values suggests location of permafrost
MAAT of -2.13°C at site is colder than threshold for permafrost
Cold (1.7°C) spring immediately downslope indicates partial melt-out of frozen water
110cm

11. Electrical Resistivity Tomography Lines
Gordon Gulch

12. Gordon Gulch Low resistivity pocket
Very low resistivity on shallow slope
High resistivity underneath

13. Gordon Gulch
220 cm ≈1.4 Ian’s
Five pits along this line reveal subsurface structure and composition to field-check resistivity values

14. Gordon Gulch Weathered pocket of saprolite Toeslope deposits Bedrock
Resistivity interpretation values from Leopold et al, Under Review

15. Looking Forward
Complete processing the other 14 ERT lines and GPR data within the CZO
Define subsurface features such as permafrost and semi-permanent ice lenses along ERT lines in the alpine
Compare permafrost extent along Niwot Ridge to modeled probabilities

16. Conclusions
Evidence for permafrost in the 1970’s; no evidence at present
Late Pleistocene toeslope deposits at lower elevation suggest periglacial transport
Future work to model permafrost extent
Thank you Any questions?