Organic matter, ice content and structure determined in a permafrost peatland through
GPR profiling and computed tomography scanning, Salluit, Nunavik, Canada
1
Ducharme ,
1
Allard ,
1
L’Hérault
M.-A.
M.
E.
1Geography Department and Centre d’études nordiques, Université
Laval, Québec, Canada
SCIENTIFIC RATIONALE
RESULTS AND DISCUSSION
INTRODUCTION
Polygonal grounds of northern Quebec were studied under several periglacial environments such as sand and organic matter in
continuous permafrost (Kasper and Allard, 2001), tills and sands over marine clay in discontinuous permafrost (Jetchick and
Allard, 1990). However, they remain poorly studied in organic deposits. For this study a polygonal peatland located in the
continuous permafrost zone in Salluit, Nunavik, Canada (figure 1) was targeted. A new multiscale approach to characterize
permafrost (geocryology) was developped combining high resolution mapping, GPR surveys (Fortier and Allard, 2004) and
computed tomography analyses (Calmels and al., 2004, 2008, 2010). This new approach provides multiscale views in order to
better visualize, analyze and define permafrost structural characteristics for understanding its genesis and dynamic.
MAIN OBJECTIVES
1. Characterizing the cryostructure (disposition of ground ice within the frozen organic sediments)
2. Determining the structural characteristic of the ice-wedges polygons network in relation with the organic and cryostructure
sequences
In total, nine polygons (figure 2) were identified and mapped. The perimeter of the polygons
varies between 38.3 m and 47.2 m . Most of the furrows were flooded .
Ground-penetrating radar clearly illustrates that the polygons form a bowl pattern surrounded
by ice wedges. The GPR profiles (figure 3) conducted at 225MHz suggest three major radar
reflectors and one facies : (1) ice wedges (hyperbolic), (2) permafrost table located at
approximately 30 cm depth, (3) stratigraphic contact between peat and till located at
approximately 2 m depth, (4) occurrence of thick ice lenses which form the bowl pattern.
Furthermore, theses lenses can also be found in the CT-Scan images.
The analyses of tomography images (figure 4) have allowed the observation of (1) a poor ice
content active layer, (2) a permafrost table characterized by an ice-rich layer, (3) alternating
segregation ice and pore ice, (4) relic permafrost table, (5) old ice veins connected to seasonal
thermal contraction cracks and (6) the presence of ice increasing with depth. All these
observations would have been impossible with the unaided eye.
Carbon 14 dates obtained from samples taken at the stratigraphic contact between the peat and
the underlying till yielded ages of 3740 ± 20 BP and 4615 ± 30 BP. The vertical accretion
resulting from combined peat accumulation and the development of an ice rich syngenetic
permafrost took place at a rate of approximately 5 cm per century. The 14C dates at the surface
of the peatland revealed modern ages meaning that paludification is still active.
SAL_L01
Figure 1: Study site localisation: (A) Permafrost zones (modified from Canadian Geographic, 2008), (B) Salluit valley, (C) Study area
METHODS
SAL_L02
1. High precision mapping of the polygons network
(scale 1: 800) using high resolution aerial
photography (15 cm)
SAL_L03
(niveo-aeolian)
2. Ground penetrating radar surveys
•
225 MHz Frequency
•
Ziplevel elevation measurements (integration
to the GPR profiles, resolution of 10 cm)
Depth (cm)
3. Sampling of undisturbed permafrost cores using a
portable earth drill
4. Computed tomography
•
5.
CT-Scan (spatial resolution of 0.23 x 0.23 x
0.6 mm)
14C
General cryology description of
SALPOL_F1
12 - 30
Interstitial ice
30 - 36
Ice-rich layer. Indicates the permafrost table at
32 cm.
36 - 64
Interstitial ice.
64 - 70
Inclined ice segregation lenses. Presence of air
bubble tracks which indicate the direction of
aggradation ice.
dating of organic matter
70 - 105
From non-visible interstitial ice to visible
Two segregations lenses about 5 cm thick.
Between these two lenses there are finer layers
105 - 128
of corrugated parallel lenticular ice about 5
mm thick.
Figure 2: Study area map
128 - 162 Non-visible interstitial ice
164 - 181
Massive ice with lenticular cryostructure about
1 mm thick. Presence of ice film at the base.
Depth (cm)
General cryology description of SALPOL_F2
11 - 35
Small lenses of segregation ice
Ice-rich area that indicates the limit of permafrost - 35
cm.
Non-visible interstitial ice
Interstitial ice and air bubble tracks.
Segregation parallel lenses about 2 mm thick.
Segragated ice with air bubble tracks and suspended
organic matter.
Oblique, parellel ice lenses.
Segregation ice.
Wavy parallel lenticular ice.
Segregation ice and air bubble tracks.
Non-visible interstitial ice. Ice film at the base.
35 - 38
38 - 80
80 - 146
146 - 160
160 - 175
175 - 177
177 - 181
181 - 186
186 - 190
190 - 243
Figure 3: GPR profiles and stratigraphic / cryogenic interpretation
ADVANTAGE OF MULTI-SCALING
The findings suggest that more probing in frozen peatlands across the Arctic could be useful to
improve the precision of assessments of stored carbon in the permafrost. The approach combining high
precision mapping, GPR, coring, computed tomography and C-14 dating yields a good understanding
of past permafrost dynamics, especially when applied in a geomorphological context already relatively
well understood. CT-scan allowed us to observe and analyze permafrost composition and structural
organization on numerous samples extracted from the field. This new technology offers great
possibilities for imaging and characterizing permafrost composition and cryostructure.
Figure 4: Cores samples and CT-Scan images with a general cryology description
REFERENCES
GPR
ZipLevel
Portable drill
CT-Scan
Calmels, F., Allard, M. 2004. Ice segragation and gas distribution in permafrost using tomodensitometry analysis. Permafrost and Periglacial Processes 15: 367 – 378
Calmels, F., Allard, M. 2008. Segregated Ice Structures in Various Heaved Permafrost Landforms Through CT Scan. Earth Surf. Process. Landforms 33, 209 – 225
Calmels, F., Clavano, W.R., Froese, D.G., 2010. Progress on X-ray computed tomography (CT) scanning in permafrost studies. GEO2010: 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost
Conference. Canadian Geotechnical Society: Richmond, B.C. 1363 – 1368.
Fortier, D. and Allard, M., 2004. Late Holocene syngenetic ice-wedge polygons development, Bylot Island, Canadian Artic Archipelago. Can. J. Earth Sci. 41: 997–1012, NRC Canada, 17 pp.
Jetchick, E, and M. Allard (1990). Soil wedge polygons in northern Québec: description and paleoclimatic significance. Boreas, 19 : 353-367.
Kasper, N.K. and Allard, M., 2001. Late-Holocene climatic changes as detected by the growth and decay of ice wedges on the southern shore of Hudson Strait, northern Québec, Canada. The Holocene July 2001
11: 563-577, 16 pp.