Postglacial landscape evolution of aeolian landforms, Savary Island, British Columbia
Libby C. M. Griffin1,2,3, Olav B. Lian1,2,3, John J. Clague2, Brent C. Ward2, Christina Neudorf1,3 and Jordan K. Bryce1
1
Department of Geography and the Environment, University of the Fraser Valley, Abbotsford, BC
2
Department of Earth Sciences, Simon Fraser University, Burnaby, BC
3
Hakai Institute, Calvert Island, BC, Canada
Introduction
Table 2. Optical ages of different samples (locations can be seen in Figures 1 and 2) were determined using the SAR protocols
P1 and P2 (Table 1). Ages were found to be consistently younger than associated radiocarbon ages even when corrected for
anomalous fading using the method of Huntley and Lamothe (2001).
Savary Island, southern coast of British Columbia (Fig. 1), is unique in the region in that it
is capped with a thick sequence of aeolian sand that rests unconformably on sediments deposited during the last (Fraser) glaciation (MIS 2). The aeolian sediments form large, stabilized, parabolic dunes that trend to the northwest (Fig. 2). The dunes consist of medium to
coarse sand with several paleosols that record periods of landscape stability separated by
times of sand accumulation. Using radiocarbon ages of charcoal from the B-horizons of paleosols, we test a new optical dating technique on Savary Island using potassium feldspar
(KF) grains so that a detailed chronology of landscape evolution can be established. Optical
(luminescence) dating is a radiometric technique applied to quartz and feldspar sand or silt
grains and is used to determine the last time the grains of interest were exposed to sufficient
sunlight or heat, which is usually their time of burial in a sedimentary landform.
A
B
A
Modern Soil
Site and Stratigraphy
Paleosol 3
9.21 ± .08
Incipient
paleosol
SIDS 4
SIDS 3
SIDS 8
20
6.81 ± 0.54
Aeolian
Sand
8.25 ± .07
125°W
Discovery Islands
10
5
15
20 km
10 mi
SIDS 2
Elevation (m)
5
SIDS 1
Bute
Inlet
0
0
Toba Inlet
Johnstone Strait
6.49 ± 0.53
9.52 ± .04
Paleosol 1
8,316 - 8,176
Cal yrs BP
7.04 ± 0.58
Gravel Lag
Vashon Till
10
Quadra Sand
Dis
cov
5
Quadra Island
ery
British
Columbia
Cortes Island
Desolation Sound
ag
ss
Pa
Vancouver Island
e
50°N
High Tide level
0
Ma
las
pin
50°N
aP
en
ins
ula
Pacific
Ocean
B
Gravel lag
Paleosol
25
126°W
25 m
Paleosol 2
9,555 - 9,478
Cal yrs BP
Figure 3. Top image shows the south facing cliffs on
Savary Island with location of section logs (shown in
the bottom image) outlined in red. The surficial sediments exposed on Savary Island are primarily composed of fluvial sand and gravel (Quadra Sand) at the
base. This unit is overlain unconformably by diamicton
(Vashon till). A gravel lag exists on top of the diamicton
at some localities and is interpreted to have formed by
wave action when sea level was higher than present at
the end of the Fraser Glaciation (Clague et al. unpublished). This glacial sequence underlies several metres
of aeolian sand, and overlies nonglacial sediments that
are thought to have been deposited during the Olympia nonglacial interval. Black dots indicate where optical dating samples were taken with ages in italics. Radiocarbon ages (not italics) of charcoal samples collected from paleosol B-horizons are consistently older
than the optical ages of sand collected immediately
below them. All ages are in ka cal yrs BP.
Savary Island
Strait of Georgia
Methods
USA
Optical dating
Figure 1. Savary Island is the southernmost island of the Discovery Islands in the northern Strait of Georgia, between
Vancouver Island and the mainland, approximately 145 km northwest of Vancouver, British Columbia. Savary is nearly
8 km long and varies in width from 0.5 to 1.0 km, and is oriented in an east to west direction. The red box indicates the
location of Figure 2.
Legend
Optical and radiocarbon dating samples
Optical dating sample
Soil pits; no optical dating sample collected
Radiocarbon dating
o Charcoal samples collected from paleosols were dated at the University of California
Irvine Accelerator Mass Spectrometry (UCIAMS) Facility and were calibrated using OxCal
4.2 and the IntCal 13 data set.
40 m asl
SIDS 5
Pedology
0 m asl
SIDS 11
SIDS 9
o Modern and ancient soils (paleosols) were described in accordance to common
soils science methods with the assistance of Dr. Paul Sanborn (UNBC).
SIDS 7
SIDS 6
SIDS 10
SIDS 8
o Quartz and feldspar grains were extracted from the samples using standard procedures
(Wintle 1997). Both 180-250 µm and 300-400 µm size fractions were used.
o Each aliquot consisted of 30-50 grains mounted on 1 cm diameter aluminum disks.
o Aliquots were measured using a Risø TL/OSL DA-20 reader equipped with a calibrated
90
Sr/90Y β source that delivered β particles to the sample at a rate of ~5.4 Gy/min.
o A duplicated sample (SIDS-1C) was collected beneath paleosol 1 and prepared under
dimmer light conditions to ensure that the intensity of the laboratory lighting was not
leading to age underestimates.
Figure 2. LiDAR image of Savary Island shows landforms that are not visible in traditional air photos. Parabolic dunes
occupy an area of roughly 1 km2 and indicate a southeasterly paleowind direction. Today the Savary Island dunes are
completely vegetated except for south facing erosional cliff faces.
Table 1. (A) Single-aliquot regenerative-dose (SAR) laboratory protocol (P1) developed by Neudorf et al. (2014) is applied
to KF in this study. A second protocol (P2), based on several studies done by others (i.e. Auclair et al. 2003), was also applied where a higher pre-heat temperature is used. (B) Example of a radial plot of equivalent doses (De) from 36 aliquots
of sample SIDS-1C. The De in grays (Gy) is read by extrapolating a line through the origin and the data point of interest to
the radial axis on the right. The grey band shows the weighted mean De, which is typically used in the age calculation.
Natural/ Regenerative Dose
2
Preheat (160°C, 10s)
Preheat (250°C, 60s)
3
IRSL (50°C, 200 s) --> Ln, Lx
IRSL (50°C, 200 s) --> Ln, Lx
4
Test Dose
Test Dose
5
Preheat (160°C, 10 s)
Preheat (250°C, 60 s)
6
IRSL (50°C, 200s) --> Tn, Tx
IRSL (50°C, 200s) --> Tn, Tx
7
Hotwash (180°C, 40 s)
Return to step 1.
Return to step 1.
16
12
10
Dose (Gy)
Natural/ Regenerative Dose
(B)
Standardized Estimate
SAR Protocol 2
1
8
N
1
SIDS-1
25
180-250
1
SIDS-1C
36
180-250
1
SIDS-1C
30
300-400
2
SIDS-1C
21
180-250
1
SIDS-2
24
180-250
2
SIDS-2
12
180-250
1
SIDS-5
15
180-250
1
SIDS-8
30
180-250
1
SIDS-9
15
180-250
Total dose rate
(Gy/ka)2
2.20 ± 0.15
1.99 ± 0.13
2.20 ± 0.15
1.99 ± 0.13
2.20 ± 0.15
1.99 ± 0.13
2.20 ± 0.15
1.99 ± 0.13
1.91 ± 0.13
1.72 ± 0.15
1.91 ± 0.13
1.72 ± 0.15
2.38 ± 0.16
2.08 ± 0.13
2.09 ± 0.14
1.86 ± 0.12
2.27 ± 0.15
2.01 ± 0.13
De (Gy)
(CAM)3
4
OD (%)
Recup5
(%)
7.47 ± 0.30
19.41
2.88
6.20 ± 0.36
6.62 ± 0.23
20.34
2.46
7.49 ± 0.10
10.00 ± 0.36 19.51
2.58
5.51 ± 0.11
7.11 ± 0.32
15.62
3.37
6.13 ± 0.17
7.32 ± 0.32
21.21
3.1
6.92 ± 0.10
8.28 ± 0.45
18.34
3.97
TBA
8.63 ± 0.26
11.68
2.18
TBA
7.86 ± 0.27
18.81
2.5
TBA
7.09 ± 0.33
17.8
3.28
TBA
Fading rate
(%/decade)6
Uncorrected age Fading-corrected
(ka)
CAM age (ka)
3.51 ± 0.28
3.88 ± 0.30
3.02 ± 0.23
3.42 ± 0.26
3.64 ± 0.26
4.00 ± 0.29
3.30 ± 0.25
3.74 ± 0.28
3.83 ± 0.40
4.27 ± 0.41
4.33 ± 0.48
4.82 ± 0.49
3.62 ± 0.64
4.15 ± 0.62
3.70 ± 0.32
4.17 ± 0.35
3.13 ± 0.25
3.523 ± 0.27
5.74 ± 0.47
6.37 ± 0.51
6.38 ± 0.51
7.29 ± 0.56
6.13 ± 0.46
6.76 ± 0.50
5.93 ± 0.48
6.76 ± 0.54
7.63 ± 0.82
8.55 ± 0.84
7.87 ± 0.89
8.81 ± 0.93
6.96 ± 0.53
8.03 ± 0.59
6.70 ± 0.61
7.13 ± 0.64
5.97 ± 0.49
6.78 ± 0.54
Number of aliquots dated
Shaded values indicate the age found using a dose rate calculation with a “saturated” water content value, while for non-shaded values the “as collected” water content was used; the latter is
likely a truer representation of the depositional environment.
3
CAM = central age model
4
OD = overdispersion
5
Recuperation
6
For samples that have not yet had their fading rates determined (listed as ‘TBA’), the average fading rate of those determined for the other samples was use.
1
2
Discussion
Our results show that the same optical age estimates, within error, were obtained for two
different grain sizes, and using two different laboratory protocols on the same sample (Table
2). This gives confidence that our procedures are valid, although our optical ages are
significantly younger that the radiocarbon ages of charcoal at the same site. We cannot rule
out the possibility that the charcoal in the paleosols is derived from organic material that
pre-dates the deposition of the dune sand. We know that strong winds must have been
present to transport and deposit the coarse sands in the dunes, these winds could have also
eroded and transported charcoal from older source areas over significant distances. Our
optical ages suggest that the dunes on Savary Island stabilized after the end of the Holocene
xerothermic interval (cf. Hebda 1995), when warm and dry conditions were replaced by moist
and cool climate.
Summary
• Optical age estimates from dune sands in this study post-date radiocarbon ages from
charcoal found in the B-horizons in the dune paleosols, suggesting that the charcoal may
be older material that was blown in from a different source.
•
Optical ages of dune stabilization are consistent with regional transition from warm and
dry (xerothermic) climate to moist and cool climate.
SIDS 3&4
SIDS 1&2
Results
(A) SAR Protocol 1 (Neudorf et al. 2014)
Sample
1
Grain
Size (µm)
SAR
protocol
8.3
2
0
-2
6.3
Relative Error (%)
0
4.3
5.00
2.50
1.67
1.25
20
40
60
80
Precision
o Quartz on Savary Island lacks a datable ‘fast’ component of its luminescence signal
therefore KF was used in all subsequent experiments.
o Preheat plateau and dose recovery tests show that the SAR protocol developed by
Neudorf et al. (2014) (Table 1) is also suitable for KF on Savary Island.
o SAR protocols with higher preheat temperatures yielded similar results but with
lower over- dispersion values (Table 2).
o Initial optical age estimates from 180-250 µm grains are consistently younger than
radiocarbon dated charcoal from paleosols B-horizons from the same sites.
o Optical age estimates from 300-400 µm grain size fraction yield similar results.
o Laboratory lighting does not adversely affect our optical ages values (see SIDS-1
and SIDS-1C in Table 2).
o Preliminary optical age estimates suggest the dune field stabilized between 6 and
7 ka ago.
References
Auclair, M., Lamothe, M., & Huot, S. (2003). Measurement of anomalous fading for feldspar IRSL using SAR. Radiation measurements,
37(4), 487-492.
Clague, J.J., Tupper, D., and Webb, S. Unpublished Manuscript. Early Holocene dune fields on the south east coast of British Columbia.
Huntley, D. J., & Lamothe, M. (2001). Ubiquity of anomalous fading in K-feldspars and the measurement and correction for it in
optical dating. Canadian Journal of Earth Sciences, 38(7), 1093-1106.
Hebda, R. J. (1995). British Columbia vegetation and climate history with focus on 6 ka BP. Géographie physique et Quaternaire,
49(1), 55-79.
Neudorf, C. M., Lian, O. B., Walker, I. J., Shugar, D. H., Eamer, J. B., & Griffin, L. C. (2015). Toward a luminescence chronology for coastal
dune and beach deposits on Calvert Island, British Columbia central coast, Canada. Quaternary Geochronology. In press.
Acknowledgements
Thanks to Brian Menounos for facilitating the LiDAR acquisition, and Rob Vogt for processing the LiDAR data. Thanks to
Travis Gingerich for help in the field and the laboratory, and Scotti-Lyn Griffin for help in the field. Paul Sanborn is gratefully
acknowledged for soil science advice in the field.