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Géographie physique et Quaternaire
Relative Sea-Level Change in the Northern Strait of Georgia,
British Columbia
Changement du niveau marin relatif de la partie nord du
détroit de Géorgie, Colombie-Britannique
Thomas S. James, Ian Hutchinson, J. Vaughn Barrie, Kim W. Conway et Darcy
Mathews
Volume 59, numéro 2-3, 2005
URI : https://id.erudit.org/iderudit/014750ar
DOI : https://doi.org/10.7202/014750ar
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Résumé de l'article
Vingt-quatre nouvelles dates au 14C provenant de carottes sédimentaires de
bassins isolés, d’excavations, de coupes naturellement exposées et
archéologiques permettent de cerner les changements du niveau marin relatif
depuis la dernière glaciation dans la partie nord du détroit de Géorgie, en
Colombie-Britannique. Le niveau marin relatif a rapidement passé de 150 m à
45 m d’altitude entre 11 750 et 11 000 ans BP (13 750-13 000 cal. BP), le taux
d’abaissement ayant ralenti par la suite. L’émersion initiale rapide correspond
à la fin de la sédimentation glaciomarine distale qui accompagnait le retrait du
front glaciaire. Un bas niveau de quelques mètres sous le niveau actuel a pu
s’instaurer durant l’Holocène inférieur, jusqu’en 2000 ans BP. Au début, le
soulèvement isostatique a accusé un retard de quelques siècle sur celui de la
zone du détroit à 80 km plus au sud. Le bas niveau de la partie nord du détroit
est survenu plus tard et fut moins prononcé. Les données indiquent un
enfoncement isostatique de type exponentiel avec des constantes de
désintégration de 500 et de 2600 ans 14C. Le taux le plus rapide correspond à
une viscosité du manteau terrestre d’environ 1019 Pa s, ce qui concorde avec
les résultats de la modélisation glacio-isostatique. Le taux de soulèvement
glacio-isostatique actuel résultant de l’inlandsis de la Cordillère s’établit à
environ 0,25 mm/a. Ce taux ne peut contrer les effets du rehaussement du
niveau marin prévu pour cette région en raison des faibles mouvements de la
croûte terrestre qui y sont envisagés.
James, T. S., Hutchinson, I., Vaughn Barrie, J., Conway, K. W. & Mathews, D.
(2005). Relative Sea-Level Change in the Northern Strait of Georgia, British
Columbia. Géographie physique et Quaternaire, 59 (2-3), 113–127.
https://doi.org/10.7202/014750ar
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Géographie physique et Quaternaire, 2005, vol. 59, nos 2-3, p. 113-127, 10 fig., 1 tabl.
RELATIVE SEA-LEVEL CHANGE
IN THE NORTHERN STRAIT OF GEORGIA,
BRITISH COLUMBIA*
Thomas S. JAMES**, Ian HUTCHINSON, J. Vaughn BARRIE, Kim W. CONWAY and Darcy MATHEWS; first, third and fourth
authors: Geological Survey of Canada (Pacific), 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada; second
author: Department of Geography, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada;
fifth author: Millenia Research Limited, 510 Alpha Street, Victoria, British Columbia V8Z 1B2, Canada and Department of
Anthropology, University of Victoria, P.O. Box 3050 STN CSC, Victoria, British Columbia V8W 3P5, Canada.
ABSTRACT Twenty-four new radiocarbon dates from isolation basin
cores, excavations and natural exposures, and an archeological site,
constrain relative sea-level change since the last glaciation in the northern Strait of Georgia, British Columbia. Relative sea level fell rapidly
from about 150 m elevation to 45 m elevation from 11 750 to 11 000 BP
(13 750 to 13 000 cal BP), then its rate of fall slowed. The initial rapid
emergence began soon after the transition from proximal to distal
glaciomarine sedimentation, when the glacial front retreated from the
Strait of Georgia and the Earth’s surface was unloaded. A sea-level
lowstand a few metres below present-day sea level may have occurred
in the early Holocene, but sea level was near its present level by
2000 BP. Sea-level change in the northern Strait of Georgia lagged
the mid Strait of Georgia, 80 km to the south, by a few hundred years
during initial emergence. The lowstand in the northern strait was later
and probably shallower than in the mid strait. Isostatic depression
inferred from the sea-level observations can be fit with two decaying
exponential terms with characteristic decay times of 500 and 2600
years. The faster decay time corresponds to a shallow mantle viscosity of about 1019 Pa s, consistent with previous glacio-isostatic modelling. The present-day crustal uplift rate from the residual isostatic effects
of the Cordilleran Ice Sheet is about 0.25 mm/a. Crustal uplift is not
expected to significantly ameliorate projected sea-level rise in the mid
and northern Strait of Georgia because present-day vertical crustal
movements are inferred to be small.
RÉSUMÉ Changement du niveau marin relatif de la partie nord du
détroit de Géorgie, Colombie-Britannique. Vingt-quatre nouvelles
dates au 14C provenant de carottes sédimentaires de bassins isolés,
d’excavations, de coupes naturellement exposées et archéologiques
permettent de cerner les changements du niveau marin relatif depuis
la dernière glaciation dans la partie nord du détroit de Géorgie,
en Colombie-Britannique. Le niveau marin relatif a rapidement
passé de 150 m à 45 m d’altitude entre 11 750 et 11 000 ans BP
(13 750-13 000 cal. BP), le taux d’abaissement ayant ralenti par la
suite. L’émersion initiale rapide correspond à la fin de la sédimentation
glaciomarine distale qui accompagnait le retrait du front glaciaire. Un
bas niveau de quelques mètres sous le niveau actuel a pu s’instaurer
durant l’Holocène inférieur, jusqu’en 2000 ans BP. Au début, le soulèvement isostatique a accusé un retard de quelques siècle sur celui
de la zone du détroit à 80 km plus au sud. Le bas niveau de la partie
nord du détroit est survenu plus tard et fut moins prononcé. Les données indiquent un enfoncement isostatique de type exponentiel avec
des constantes de désintégration de 500 et de 2600 ans 14C. Le taux
le plus rapide correspond à une viscosité du manteau terrestre d’environ 1019 Pa s, ce qui concorde avec les résultats de la modélisation
glacio-isostatique. Le taux de soulèvement glacio-isostatique actuel
résultant de l’inlandsis de la Cordillère s’établit à environ 0,25 mm/a.
Ce taux ne peut contrer les effets du rehaussement du niveau marin
prévu pour cette région en raison des faibles mouvements de la croûte
terrestre qui y sont envisagés.
Manuscrit reçu le 21 juillet 2005 ; manuscrit révisé accepté le 20 juin 2006 (publié le 1er trimestre 2007)
* Geological Survey of Canada contribution number 2005222
** E-mail address: tjames@nrcan.gc.ca
114
T. S. JAMES, I. HUTCHINSON, J. V. BARRIE, K. W. CONWAY and D. MATHEWS
INTRODUCTION
Sea-level change provides a fundamental control on the
paleogeography of coastal areas, and thus helps to determine
habitat distribution, including areas available for human settlement (Hetherington et al. , 2003). The development of
coastal landforms is intimately related to sea level (Hart and
Long, 1996), and the engineering properties of sediments are
affected by the history of subaerial exposure and marine inundation. Projections of future crustal movements and sea-level
change are aided by information on past sea-level change.
Knowledge of sea-level history consequently benefits coastal
and marine planning and engineering efforts.
Sea level in coastal British Columbia rapidly dropped from
a late-Pleistocene highstand immediately following deglaciation to a minimum level below the present-day ocean surface
in the early Holocene (Mathews et al., 1970; Clague et al.,
1982). Sea level then gradually recovered to the present elevation in concert with the rise in global, or eustatic, sea level.
The early sea-level fall is due to postglacial rebound, in which
the land rose in response to the removal of the ice load.
Sea-level histories developed recently for Victoria and the
eastern Strait of Juan de Fuca (James et al., 2002; Mosher
and Hewitt, 2004) and the mid-Strait of Georgia (Hutchinson et
al., 2004a) follow this pattern. Although less constrained, the
early sea-level history of the Queen Charlotte Islands and adjacent continental shelf areas also features sea-level fall (Barrie
and Conway, 2002a) to a pronounced sea-level low (Josenhans
et al., 1997), possibly related to a crustal forebulge generated
by the retreating ice sheet (Luternauer et al., 1989).
Computer modelling of sea-level change provides information on the rheology (flow properties) of the Earth’s mantle.
For southern British Columbia, the modelling indicates a lowviscosity mantle, but with limited available data, vertical and
horizontal variations in mantle viscosity were not discriminated
(James et al., 2000; Clague and James, 2002). Viscosity values inferred from modelling of sea-level observations have
been used in tectonic models to explain features of the tectonically active Cascadia subduction zone (Wang et al., 2001).
Postglacial rebound modelling also provides estimates of the
present-day residual crustal motion due to the melting of the
ice sheets. More sea-level observations would improve these
estimates of mantle viscosity and residual crustal motion.
To address these issues, systematic efforts have been
made to improve the observational record of sea-level change
in southern coastal British Columbia. The first results provided
new information for Victoria and Vancouver (James et al.,
2002) and have generated a well-constrained sea-level history for mid-Strait of Georgia (Hutchinson et al., 2004a). Here
we present a complementary study for the northern Strait of
Georgia and briefly discuss the implications for mantle flow
properties and projections of future sea-level change.
GLACIAL HISTORY AND TECTONIC SETTING
GLACIAL HISTORY
During the last ice age, the Cordilleran ice sheet nucleated
at high altitudes in the Coast Mountains. Ice accumulated,
flowed down, and spilled out onto coastal lowlands (Clague,
1989). The ice front advanced south along the Strait of
Georgia, reaching its maximum extent around 14 000 BP in
southern Puget Sound (Fig. 1). Peak ice thicknesses in the
Strait of Georgia were about 2 km (James et al. , 2000).
Vancouver Island was covered with ice, and ice flowed out the
Strait of Juan de Fuca. Retreat was rapid, with the southern
Strait of Georgia deglaciating earlier than 12 000 BP. Presentday ice cover was established by about 10 000 BP (Clague
and James, 2002).
TECTONIC SETTING
The region comprises the northern part of the Cascadia
subduction zone, where the oceanic Juan de Fuca plate
subducts beneath North America at about 4 cm/a. Farther
north, the Explorer microplate underthrusts northern Vancouver
Island, although at reduced rates compared to the Juan de
Fuca plate. Inland, the Cascades volcanic chain indicates dehydration of the subducting oceanic lithosphere and partial melting of the overlying mantle. The subducting oceanic lithosphere
is relatively young, aged 6 Ma at the trench, and consequently
the shallow mantle beneath the oceanic lithosphere is expected
to be relatively hot and to have a low viscosity. The Cascadia
subduction zone is tectonically active and is a region of
enhanced earthquake hazard. Geodetic observations measure the crustal strain leading to earthquakes, but must be corrected for postglacial rebound and non-tectonic processes.
The study area is located at the northern end of the Strait of
Georgia (Figs. 1-2). It lies in the Cascadia forearc, above and
near the northern limit of the subducted Juan de Fuca Plate.
Because heat flow values are relatively low in the forearc
(Hyndman and Lewis, 1995), crustal temperatures are also
expected to be low. Consequently, in the Strait of Georgia, it is
probable that the entire thickness of overriding continental lithosphere (60-70 km) responded elastically, and not viscously, to
the bending and flexure induced by the Cordilleran ice sheet.
METHODS
SAMPLE COLLECTION
As noted by Hutchinson et al. (2004a), isolation-basin coring is a preferred method for obtaining high-resolution information on sea-level change. As sea level changes, an isolation
basin undergoes a transformation in the character of sedimentation when it passes through the marine-freshwater transition. In coastal British Columbia, a typical isolation basin is
a lake, marsh, or bog located below the limit of glaciomarine
inundation. An ideal basin has a rocky sill that has not experienced significant erosion; in early deglacial times, when sea
level was high, it underwent glaciomarine and marine deposition, then, as sea-level dropped, it emerged from the ocean
and freshwater sedimentation commenced.
Shallow marine basins can also record the time of sea-level
lowstands, when sea level dropped below present. In exceptional circumstances, a core may recover evidence of the initial
marine deposition when sea level was high, the transition from
marine to freshwater conditions when sea level dropped below
Géographie physique et Quaternaire, 59(2-3), 2005
RELATIVE SEA-LEVEL CHANGE IN THE NORTHERN STRAIT OF GEORGIA
115
QCI
VI
Fig. 2
L
SG
30
50
40
Victoria
20
km
10
N
W
FIGURE 1. Location map for the northern Cascadia subduction zone,
where the Juan de Fuca plate subducts beneath western North America
(after James et al., 2000). Dashed contour lines show the depth to the
top of the subducting oceanic lithosphere (Fluck et al., 1997). Thick
solid line shows the maximum extent of the Cordilleran Ice Sheet around
14 000 BP (Clague, 1983). The study region is located at the northern
boundary of the Strait of Georgia (SG), and detail is given in Figure 2.
L is Lasqueti Island, and the quadrilateral shows the region for which a
sea-level curve was developed for mid-Strait of Georgia (Hutchinson
et al., 2004a). Filled triangles are volcanic centers. (inset) Location map
of western North America. QCI is Queen Charlotte Islands and VI is
Vancouver Island.
Carte de localisation de la zone de subduction de Cascadia-nord, où
la plaque Juan de Fuca plonge sous la plaque nord-américaine occidentale (d’après James et al., 2000). Les lignes pointillées montrent la
profondeur à laquelle le haut de la lithosphère océanique entre en
subduction (Fluck et al., 1997). La ligne pleine délimite l’étendue maximale de l’inlandsis de la Cordillère il y a 14 000 ans (Clague, 1983). La
région à l’étude se situe dans la partie nord du détroit de Géorgie
(SG), d’autres détails se trouvent à la figure 2. L fait référence à l’île
Lasqueti et le quadrilatère montre la région pour laquelle une courbe
du niveau marin a été développée pour le détroit de Géorgie
(Hutchinson et al., 2004a). Les triangles représentent des centres volcaniques. (Insertion) Carte de localisation de l’ouest de l’Amérique du
Nord. QCI désigne les îles de la reine Charlotte et VI l’île de Vancouver.
the sill, and a second transition from freshwater back to marine
conditions when sea level rose again (e.g. Johnson Lagoon
on Lasqueti Island, Hutchinson et al., 2004a).
Elevations were determined by calibrating an altimeter to
the high-tide line. Sill depths for marine basins were determined from bathymetric charts, and were adjusted to reflect
the tidal range (difference between higher high water and
lower low water for a large tide) at Campbell River of 4.6 m
(Canadian Hydrographic Service, 2001).
A classified advertisement was placed in a local newspaper, seeking information on marine shells found at high elevations. Response from local residents was good, and samples for radiocarbon dating were collected from a recently dug
well, two dug ponds, a gravel pit, and a stream bed. The elevation and location of the samples were noted, and the samples were identified and radiocarbon dated.
Two radiocarbon samples were obtained from a recent
archeological excavation indicating human habitation near the
present-day shoreline.
Four freshwater basins and three marine basins were cored
in this study. Coring methods included percussion coring
(Reasoner, 1993) (one lake, one marine basin), vibracoring on
marshes and bogs (three sites), and piston coring (two marine
basins). Vibracores were extruded onto plastic gutters, logged,
and sampled for radiocarbon and diatom analyses in the field.
Piston and percussion cores were sectioned and transported
to cold storage for later splitting, logging, and sampling.
Samples were selected for radiocarbon dating to determine
the age of possible marine-freshwater transitions, or, if material was lacking near the inferred transition, to determine the
age and time span of lithostratigraphic units. When possible, the
species selected for radiocarbon dating was identified.
Géographie physique et Quaternaire, 59(2-3), 2005
116
T. S. JAMES, I. HUTCHINSON, J. V. BARRIE, K. W. CONWAY and D. MATHEWS
PALEOENVIRONMENTAL INTERPRETATION
Frequently, the paleonvironmental interpretation was straightforward, as the clastic marine sediments often contained marine
shells, and the overlying freshwater organic sediments (gyttja,
peat) exhibited strong colour and textural contrasts with the
marine sediments. For two cores, however, a diatom analysis
was done to test possible marine-freshwater transitions.
Organic matter in diatom samples was removed by H2O2
digestion, and the remaining material was washed in distilled
water, decanted to remove fines, and brought to a near-neutral pH. Aliquots of suspended material were dried on glass
slides and mounted in Hyrax. At least 100 specimens or, in
diatom-poor samples, the number of specimens counted on 10
random traverses, were identified for each sample.
Diatoms were identified from standard taxonomic sources,
principally Sims (1996) and Witkowski et al. (2000). Species
were assigned to habitat groups based primarily on information on salinity tolerance (van Dam et al., 1994; Sims, 1996).
RADIOCARBON ANALYSIS
In selecting samples for radiocarbon dating, articulated
shells were preferred over single valves, shell fragments, or
shell hash. For terrestrial samples, macroscopic material
(seeds, twigs, wood fragments) was preferred over bulk
organic material, such as gyttja or peat. Samples were sent to
IsoTrace Laboratory (University of Toronto) for radiocarbon
dating by the accelerator mass spectrometry (AMS) method.
The marine reservoir correction has varied in the Strait of
Georgia since late-glacial time (Hutchinson et al., 2004b).
Radiocarbon ages on marine shells with uncorrected ages
older than 10 000 years were corrected by -950 ± 50 years.
Ages on younger marine shells were corrected by -720 ± 90
years. Radiocarbon ages on basal freshwater bulk organic
material are systematically older than macrofossils recovered
from the same level (Hutchinson et al., 2004b). Consequently,
ages on bulk organic material (gyttja) retrieved from immediately above the marine-freshwater transition were corrected by
-625 ± 60 years (Hutchinson et al., 2004b).
RESULTS
The sample sites range east to west from Cortes Island,
across Quadra Island, to near Campbell lake (Fig. 2). They
encompass a distance of about 40 km in an east-west direction and about 20 km south to north. The following site descriptions are ordered from highest to lowest elevation. Core stratigraphy is given in Figure 3 for sites above sea level
and Figure 4 for sites below sea level. Radiocarbon results
are given in Table I. In the following discussion, ages are in
corrected radiocarbon years.
LOVELAND MARSH
An undrained, open marsh north of Loveland Bay (Campbell
Lake) at 190 m elevation was vibracored. The core features
coarse clastic sediments at the base that fine upwards from
gravel to fine to medium sand overlain by gyttja and peat
(Fig. 3). Diatom analyses of three samples near the transition
from sand to gyttja indicate a freshwater environment. Two
diatom samples below the transition in a silty, very fine sand,
FIGURE 2. Sea-level sites in the
northern Strait of Georgia.
Sites de la partie nord du détroit
de Géorgie.
Quadra
L5
50° 12'
Island
Vancouver
L8
L2
L3
50° 06'
Island
L12
L11
L10
L4
L1
Campbell
Lake
Cortes
Island
L9
L6
L7
Strait of Georgia
50° 00' N
125° 24'
L1. Loveland Marsh
L2. Beaver Lake
L3. Belansky’s Well
L4. Whittington’s Bog
L5. Saxon Creek Pond
L6. Piggott’s Pond
125° 12'
125° 00' W
L7. Graham’s Gravel Pit
L8. Ballard’s Bog
L9. Cortes Bay Archeological Dig
L10. April Point Bay
L11. Gowlland Harbour
L12. Gorge Harbour
Géographie physique et Quaternaire, 59(2-3), 2005
RELATIVE SEA-LEVEL CHANGE IN THE NORTHERN STRAIT OF GEORGIA
L4. Whittington's Bog
L2. Beaver Lake
L1. Loveland Marsh
0
0
117
L8. Ballard's Bog
0
0
peat
peat
1
peat
1
1
1
gyttja
peat
S
Depth (m)
2
2
3
muddy peat
10 830 ± 90
peat
gyttja
S
fine/medium sand
G
coarse sand
G
G medium sand
coarse sand
G
4
mud/very fine
sandy mud
3
2
4
S
gyttja
3
5
G
G
11 160 ± 90
very fine sandy mud
8390 ± 130?
medium sand
S
3
G
4
7
coarse sand
11 480 ± 90
10 590 ± 70
2
mud
6
gravel
sub-rounded to
sub-angular pebbles
angular pebbles
sedge
peat
4
10 295 ± 117
8
G sandy mud
G
coarse sand/fine gravel
G
9
12 030 ± 112
sandy mud
10
shell
FIGURE 3. Sediment cores from isolation basin sites above sea level.
Ages are in corrected radiocarbon years. The letters S and G indicate sharp and gradual contacts, respectively.
Carottes de sédiments en provenance de sites isolés situés audessus du niveau marin. Les âges au radiocarbone sont en années
étalonnées. Les lettres S et G montrent respectivement les zones de
contact brusques et graduelles.
feature a few, commonly fragmented diatoms. The diatom
species present are typical of a cool, oligotrophic lake. A sample from gyttja above the transition shows an extremely rich
and diverse freshwater diatom assemblage.
is given by an age on a scallop valve (Chlamys rubida) of
11 670 ± 121 BP.
The apparent absence of a marine phase at Loveland
Marsh is consistent with the mapped marine limit of 175 m in
the Campbell River area (McCammon, 1977; Clague, 1981).
Loveland Marsh is located in a shallow, elongated depression.
The depression may be a former meltwater channel, in which
case the fining-upward clastic sediments indicate the decreasing energy levels of waning meltwater flow as the glacial front
retreated from the area. A radiocarbon age on seeds of the
yellow pond-lily (Nuphar lutea) from gyttja 10-20 cm above the
gyttja-sand contact is 10 830 ± 90 BP, giving a minimum age for
establishment of pond vegetation following deglaciation.
BEAVER LAKE
A percussion core from Beaver Lake, at 145 m elevation,
produced a sandy mud containing a coarse sand to fine gravel
layer which is overlain by a thick sequence of gyttja (>8 m)
(Fig. 3). A shell age from the coarse unit is 12 030 ± 112 BP.
The age of the basal gyttja is 10 295 ± 117 BP.
BELANSKY’S WELL
A well dug at 107 m elevation yielded marine shells in a bluegrey clay at depths of 4.3 to 5.5 m. The time of marine conditions
WHITTINGTON’S BOG
A vibracore from a drained bog at 75 m elevation features
mud and sand overlain by nearly 3 m of peat (Fig. 3). The clastic sequence fines upwards from a coarse to medium sand to
a very fine sandy mud. A wood fragment from the base of the
peat yielded an age of 11 160 ± 90 BP. In the coarse sand
near the bottom of the core, a poplar bud scale yielded an
age of 11 480 ± 90 BP. Apparently, sea level dropped below
75 m between 11 160 and 11 480 BP.
A small sample of plant material (12 mg) from the top of the
clastic sediments yielded an anomalous age of 8390 ± 130 BP.
The sample was formed by combining two smaller samples
and was near the minimum weight recommended by IsoTrace
for radiocarbon dating (10 mg). The anomalous age is probably due to contamination.
SAXON CREEK POND, PIGGOTT’S POND
AND GRAHAM’S GRAVEL PIT
Three ages on marine shells give the time of marine conditions for elevations ranging from 45-50 m. A Clinocardium nuttallii valve recovered from the bed of Saxon Creek on Quadra
Island at the outlet of a small pond at 50 m elevation has an age
Géographie physique et Quaternaire, 59(2-3), 2005
118
T. S. JAMES, I. HUTCHINSON, J. V. BARRIE, K. W. CONWAY and D. MATHEWS
L10. April Point Bay
Quadra Island
L12. Gorge Harbour,
Cortes Island
VEC 03-05
0
Depth (m)
0
Depth (m)
1
very fine to
fine sand
S
S
8720 ± 127
1
2
3
7760 ± 120
G
VEC 03-03
muddy very fine
to fine sand
VEC 03-04
0
medium sand
4
9840 ± 103
shell hash in
fine muddy sand
0.6
1.0
1.4
fine/med sand
2
2
silty clay
11 680 ± 130
sandy mud with
11 790 ± 112
med/coarse sand
11 780 ± 112 m burrows
coarse sand
11 700 ± 112
grey silty clay
12 150 ± 112
grey clay or silty clay
with sand and granules
and silt laminae
3
3
4
bioturbated
Depth (m)
Depth (m)
S
silt laminae
11 680 ± 130
Depth (m)
0
1
1
L11. Gowlland Harbour
Quadra Island
VEC 03-02
12 360 ± 121
4
pebbles/granules
silt laminae
subangular
2 cm pebble
massive
clay
5
shells
FIGURE 4. Sediment cores from sites below sea level. Ages are in corrected radiocarbon years. The letters S and G indicate sharp and
gradual contacts, respectively.
Carottes de sédiments en provenance de sites situés sous le niveau
marin. Les âges au radiocarbone sont en années étalonnées. Les
lettres S et G montrent respectivement les zones de contact brusques
et graduelles.
of 11 880 ± 103 BP. A Chlamys rubida fragment from Piggott’s
Pond dug at 47 m elevation has an age of 11 660 ± 112 BP. An
age on a marine shell valve and fragments from the steeply-dipping foreset beds of a glaciofluvial sand and gravel delta
(Graham’s gravel pit) is 11 140 ± 103 BP. This age probably
closely dates the time that sea level dropped below 45 m, which
is the approximate elevation of the contact between topset and
foreset beds.
The upper 1.2 m of the clastic sediments fines upwards from
very fine sandy mud to mud. Wood or bark fragments 80 cm
below the mud-peat transition are dated to 10 590 ± 70 BP.
Diatom analysis of sediments surrounding the transition
from mud to overlying peat and gyttja indicate that the top of
the mud was deposited in a brackish to freshwater pond or
marsh (Fig. 5). Above the mud the basal peat freshens
upwards from a freshwater to brackish marsh or pond to a
freshwater marsh or pond.
A pond dug on the edge of Ballard’s bog yielded marine
shells at 5 to 6 m depth in a blue-grey mud. The age of a
Tectura persona valve is 11 740 ± 112 BP. Apparently, this site
BALLARD’S BOG
A vibracore from a drained bog by 23 m elevation features
mud to very fine sandy mud overlain about 1 m of peat (Fig. 3).
Géographie physique et Quaternaire, 59(2-3), 2005
Radiocarbon Ages
Site Namea
Latitude
Altitude or Depth in Material datedb
Longitude
(deg. min. N) (deg. min. W) sill elevation
core
(m)
(cm)
Weight
used
(mg)
Lab No.
Radiocarbon Corrected agec Calibrated age
(1 S.D.)
age
1
Loveland Marsh
50 03.39
125 27.30
190
215-220 Nuphar lutea seeds1
15
TO-10818 10 830 ± 90
10 830 ± 90 12 795-12 891
2
Beaver Lake
50 09.35
125 14.89
145
815-817 Gyttja3
925
TO-9910
10 920 ± 100
10 295 ± 117 11 826-12 380
2
Beaver Lake
50 09.35
125 14.89
145
869-874 Mya truncata valve
and sh. fragments2
242
TO-9911
12 980 ± 100
12 030 ± 112 13 774-13 996
3
Belansky's Well
50 07.07
125 1.67
107
735
TO-9897
12 620 ± 110
11 670 ± 121 13 392-13 656
11 160 ± 90 12 959-13 134
Géographie physique et Quaternaire, 59(2-3), 2005
4
Whittington's Bog
50 01.62
125 10.41
75
4
Whittington's Bog
50 01.62
125 10.41
75
4
Whittington's Bog
50 01.62
125 10.41
75
5
Saxon Creek Pond
50 12.40
125 15.93
50
n/a
Chlamys rubida valve fragments2
3
31
TO-9912
11 160 ± 90
287-291 Wood and plant fragments3
12
TO-9913
8390 ± 130
412
Poplar bud scale1
36
TO-10817 11 480 ± 90
11 480 ± 90 13 249-13 408
n/a
Clinocardium nuttallii valve2
416
TO-9895
12 830 ± 90
11 880 ± 103 13 618-13 849
2
265
Wood fragment
8390 ± 130
9259-9530
6
Piggott's Pond
50 02.68
124 59.68
47
n/a
Chlamys rubida fragments
205
TO-9915
12 610 ± 100
11 660 ± 112 13 389-13 641
7
Graham's Gravel Pit
50 02.07
124 59.22
45
n/a
Saxidomus giganteus valve and
Humilaria kennerleyi fragments2
308
TO-9916
12 090 ± 90
11 140 ± 103 12 939-13 120
8
Ballard's Bog
50 10.70
125 09.72
23
179
Wood or bark fragments3
216
TO-9914
10 590 ± 70
10 590 ± 70 12 406-12 777
8
Ballard's Bog
50 10.72
125 09.65
23
n/a
Tectura persona valve2
312
TO-9896
12 690 ± 100
11 740 ± 112 13 458-13 707
9
Cortes Bay1
50 03.92
124 55.44
1.5
n/a
Charcoal4
378
TO-11643
1950 ± 60
1950 ± 60
1825-1985
9
Cortes Bay2
50 03.92
124 55.44
1.3
n/a
Clam shell fragments4
998
TO-11644
2980 ± 60
2260 ± 108
2190-2510
2
10
April Point Marina
50 03.78
125 13.67
-8
427
Macoma sp. valve
411
TO-9909
10 790 ± 90
9840 ± 103 11 123-11 361
11
Gowlland Harbour
50 04.44
125 13.43
-17
66
Pelecypod valves5
478
TO-11632 12 630 ± 120
11 680 ± 130 13 396-13 673
11
Gowlland Harbour
50 04.44
125 13.43
-17
106
Macoma nasuta5
579
TO-11633 12 740 ± 100
11 790 ± 112 13 501-13 756
11
Gowlland Harbour
50 04.44
125 13.43
-17
137
Pelecypod valve5
494
TO-11634 12 730 ± 100
11 780 ± 112 13 492-13 745
5
11
Gowlland Harbour
50 04.44
125 13.43
-17
140
Pelecypod valve fragment
504
TO-11635 12 650 ± 100
11 700 ± 112 13 425-13 673
12
Gorge H.-V0303
50 05.51
125 00.69
-22
133
Pelecypod valves, shell fragments5
202
TO-11636 12 910 ± 100
11 960 ± 112 13 713-13 946
12
Gorge H.-V0303
50 05.51
125 00.69
-22
415
Paired pelecypod valves5
32
TO-11637 13 310 ± 110
12 360 ± 121 14 102-14 611
12
Gorge H.-V0304
50 05.54
125 00.82
-22
188
Shell fragments5
120
TO-11638 13 100 ± 100
12 150 ± 112 13 859-14 122
5
12
Gorge H.-V0305
50 05.51
125 00.46
-22
31
Shell fragments
434
TO-11639
8480 ± 80
7760 ± 120
8510-8850
12
Gorge H.-V0305
50 05.51
125 00.46
-22
85
Shell fragments5
341
TO-11640
9440 ± 90
8720 ± 127
9702-10 088
RELATIVE SEA-LEVEL CHANGE IN THE NORTHERN STRAIT OF GEORGIA
TABLE I
a. Archeological designations for Cortes Bay (Mathews, 2003) are (1) EaSf-36, Unit 5, Stratum 6, (2) EaSf-36, Unit 2, Level 8, Stratum 8.
b. Sample identification by (1) Hebda, (2) Hetherington, (3) James, (4) Mathews, (5) Conway
c. Marine reservoir corrections are -950 ± 50 and -720 ± 90 years for laboratory radiocarbon ages older and younger than 10 000 BP, respectively. The reservoir correction for basal gyttja is -625 ± 60 years.
Corrected uncertainties are determined by adding in quadrature the laboratory radiocarbon uncertainties and reservoir correction uncertainties.
119
120
T. S. JAMES, I. HUTCHINSON, J. V. BARRIE, K. W. CONWAY and D. MATHEWS
Ballard’s Bog
Depth
in core
(cm)
70
80
Gyttja
(black)
Peat
(grey-brow
grading
brown)
Inferred
environment
Dominant species
Pinnularia viridis, Gomphonema angustatus,
Eunotia pectinalis
Freshwater
marsh/bog
Freshwater
marsh/pond
Diploneis elliptica, Meridion circulare,
Rhoicosphenia abbreviata, Aulacoseira spp.
Pinnularia viridis, Gomphonema angustatus,
Aulocoseira subarctica
Cocconeis placentula, Rhopalodia gibba, Epithemia turgida,
Staurosira construens, Pseudostaurosira brevistriata,
Nitzschia frustulum
90
Epithemia turgida, Staurosira construens, Rhopalodia gibba,
Eunotia formica, Nitzschia frustulum, Sellaphora pupula
Mud
(yellow-grey)
100
Cocconeis placentula, Navicula peregrina,
Gomphonema acuminatum, Epithemia turgida, E. sorex,
Navicula radiosa, Rhopalodia gibba, occ. fragments
of Tryblionella
FIGURE 5. Dominant diatom
species and inferred paleoenvironment, for a selected interval of a
vibracore from Ballard’s Bog.
Diatomées dominantes et les
conditions paléo-environnementales estimées d’une partie spécifique de la carotte de Ballard’s Bog.
Freshwater
marsh/pond
Fresh/
brackish
marsh/
pond
Brackish/
fresh
pond/
marsh
experienced marine and brackish conditions from earlier than
11 740 BP to later than 10 590 BP. The site probably fully
emerged from saltwater soon after 10 590 BP.
CORTES BAY ARCHEOLOGICAL DIG
An archeological excavation (EaSf-36) on Cortes Island
revealed cultural deposits, generally about 1 m thick, overlying
a former beach (Mathews, 2003). The earliest occupation was
probably during the Marpole Period (2500-1000 cal BP),
although earlier occupation is not ruled out. The site was
occupied sporadically up to the early Historic Period (Mathews,
2003).
Two samples from this excavation were radiocarbon dated.
A mixed sand-shell horizon at the base of the cultural deposits
is probably littoral. Slightly wave-worn shell fragments at 1.3 m
elevation (Unit 2, Level 8, Stratum 8; Mathews, 2003), taken
from near the landward margin of this horizon, are
2260 ± 108 BP old. The stratum contains a burnt clamshell
and wave-rolled mammal bone fragments, indicating probable
human occupation at slightly higher elevations when the stratum was deposited. Charcoal (Unit 5, Stratum 6; Mathews,
2003) at 1.5 m elevation may have been water-deposited, and
is 1950 ± 60 BP old. The ages are consistent with earliest
occupation occurring during the Marpole Period.
If the littoral deposits are high-tide deposits, then this suggests that sea level was at about 1.5 m elevation at 2000 BP.
Alternatively, if the site is an area of cultural accumulation on
a former storm beach, then sea level may have stood lower at
2000 years ago, perhaps near its present level. In either case,
at 2000 BP sea level probably stood less than 1.5 m higher
than its present level.
APRIL POINT BAY
A percussion core from April Point Bay (sill at about -8 m)
has about 3 m of very fine to fine sand overlying about 1.5 m
of muddy very fine to fine sand (Fig. 4). Shells are present
from about 1.75 m depth to the base of the core. An age on a
Macoma sp. valve from the base of the core is 9840 ± 103 BP.
The relatively homogeneous character of the core suggests a
uniform depositional environment, and may indicate that sea
level did not drop below -8 m during the Holocene.
GOWLLAND HARBOUR
Four radiocarbon dates on marine shells from a 1.4 m long
piston core (sill at about -17 m) range from 11 680 to
11 790 BP (Fig. 4). One age is from an articulated shell. The
core fines upwards from a coarse sand to a sandy mud (with
medium to coarse sand in clam burrows) to silty clay. The texture and radiocarbon ages are consistent with the distal
glaciomarine unit described by Barrie and Conway (2002b).
The core is capped with fine to medium sand, which is probably a Holocene lag deposit.
GORGE HARBOUR
Three piston cores were taken in Gorge Harbour (sill is at
approximately -22 m) in 30-40 m of water (Fig. 4). The two
longer cores (VEC03-03 and VEC03-04) grade upwards from
a silty clay with silt laminae, sand, granules, and pebbles, to a
shell-rich mud. Three radiocarbon ages determined from
marine shells range from 11 680 to 12 360 BP. The textures
and radiocarbon ages are consistent with the cores grading
upwards from proximal to distal glaciomarine sedimentation
Géographie physique et Quaternaire, 59(2-3), 2005
RELATIVE SEA-LEVEL CHANGE IN THE NORTHERN STRAIT OF GEORGIA
(Barrie and Conway, 2002b). Both cores are capped with a
thin layer of medium sand.
The third core (VEC03-05) records a period of early
Holocene sedimentation. It contains a stiff silty clay overlain by
an olive-coloured clay. Shell fragments from the base of the
olive-grey clay have an age of 8720 ± 127 BP. An olive-grey
medium sand at the top of the core yielded shell fragments
with an age of 7760 ± 120 BP.
SEA-LEVEL CURVE
A sea-level curve for the northern Strait of Georgia, based
on the foregoing descriptions and corrected radiocarbon ages,
is given in Figure 6. Sea level fell rapidly from above 145 m to
below 50 m between about 11 800 and 11 000 BP. The rate of
sea-level fall then slowed substantially, reaching 15 or 20 m
elevation by 10 000 BP. Sea level is relatively unconstrained in
the early and mid-Holocene, but probably did not drop below
8 m depth, or rise above 20 m elevation. A shallow lowstand
phase in the early Holocene is possible but, if so, sea level
121
recovered in the mid or late Holocene. Sea level stood at, or
slightly below, 1.5 m elevation by 2000 BP. In the last 2000 BP,
sea level dropped to its present level.
The laboratory radiocarbon ages were calibrated using
Calib 5.01 (Stuiver and Reimer, 1993; www.calib.org) to determine the corresponding calendar ages. The ages of marine and
terrestrial samples were calibrated with the Marine04 (Hughen
et al., 2004) and IntCal04 (Reimer et al., 2004) calibration
datasets, respectively. The calibration program assumes a
global marine reservoir value of about 400 years. Consequently,
a regional reservoir correction DR = 550 ± 50 years was used
for marine ages older than 10 000 BP; for younger ages,
DR = 320 ± 90 years. The age of the basal gyttja sample from
Beaver Lake was corrected for a 625 ± 60 year reservoir effect
before calibration (Hutchinson et al., 2004b).
The calibrated ages, expressed as probability density functions, determine sea-level change in calendar years (Fig. 7).
Sea level fell from above 145 m around 13 750-13 500 cal BP
to 75 m at about 13 250 cal BP. In the next 250 years sea level
dropped another 25 m to about 50 m elevation. Sea-level fall
Marine limit
Marine limit
?
?
150
Environment of deposition
Marine
Brackish
Freshwater
Environment of deposition
Marine
Brackish
Freshwater
150
Inferred sea level
Well constrained
Inferred sea level
Well constrained
Loosely constrained
100
Loosely constrained
Elevation (m)
Estimated
Elevation (m)
Estimated
100
50
?
?
0
50
0
?
12
10
8
?
6
4
2
0
16
14
Time (1000s BP)
12
10
8
6
4
2
0
Time (1000s cal BP)
FIGURE 6. Inferred relative sea-level curve, in corrected radiocarbon
years, for the northern Strait of Georgia.
Courbe du niveau marin relatif estimée, en années 14C étalonnées, de
la partie nord du détroit de Géorgie.
FIGURE 7. Inferred relative sea-level curve, in calendar years, for the
northern Strait of Georgia.
Courbe du niveau marin relatif estimée, en années calendrier, de la
partie nord du détroit de Géorgie.
Géographie physique et Quaternaire, 59(2-3), 2005
122
T. S. JAMES, I. HUTCHINSON, J. V. BARRIE, K. W. CONWAY and D. MATHEWS
then slowed substantially, and dropped to 15-20 m by
12 000 cal BP. Subsequent to 12 000 cal BP, sea level is
uncertain, but probably did not drop below 8 m depth or rise
above 20 m elevation. A shallow low-stand between 11 000 to
8000 cal BP is possible, but, if so, sea level recovered and
stood at, or slightly below, 1.5 m elevation by 2000 cal BP.
Sea level then dropped to its present position.
DISCUSSION
SEA LEVEL AND DEGLACIAL HISTORY
The sea-level curve presented here is consistent with the
deglacial sedimentary and paleoenvironmental history of the
Strait of Georgia (Barrie and Conway, 2002b; Guilbault et al.,
2003; radiocarbon ages given by Barrie and Conway (2002b)
are corrected by an additional 150 years to reflect the larger
marine reservoir correction assumed here). Following
deglaciation, the Strait of Georgia experienced proximal
glaciomarine sedimentation for a few hundred years centered
around 12 250 BP (14 100 cal BP). The proximal phase
occurred at the same time throughout the mid and northern
Strait of Georgia, suggesting regional downwasting and stagnation of ice. Extensive gravel platforms built to the west of
Campbell River that are graded to about 175 m elevation indicate the marine limit (McCammon, 1977; Clague, 1981), and
were probably built during the proximal glaciomarine phase. A
transition to distal glaciomarine sedimentation then occurred,
which persisted until about 11 100 BP (13 000 cal BP).
Proximal glaciomarine sediments were rapidly deposited
and feature abundant ice-rafted debris, well-sorted sand layers, and silt laminae deposited from turbid meltwater plumes.
They indicate nearby, actively calving, tidewater glaciers and
abundant meltwater input. The distal glaciomarine unit is bioturbated, indicating slower sedimentation rates. It generally
contains finer sediments (80% clay and silt) but also contains
minor ice-rafted gravel. The distal phase indicates that the
Strait of Georgia was still connected to tidewater glaciers that
generated icebergs. The absence of inferred meltwater plumes
shows, however, that the glacial front had retreated, probably
into fjord heads of the British Columbia mainland where they
were fed by remnant ice in the Coast Mountains.
Sea level may have been maintained near 175 m elevation for a few hundred years during the proximal phase around
12 250 BP (14 100 cal BP). Around the time of the onset of
distal glaciomarine sedimentation, or soon after, when tidewater glaciers retreated from the Strait of Georgia, sea level
started to drop rapidly. The thinning and retreating ice sheet
exerted a decreasing load on the lithosphere and caused the
land to start rising. By 11 800 BP (13 700 cal BP) sea level had
dropped 30 m to below 145 m. In the next 700 years, during
the remainder of the phase of distal glaciomarine deposition,
sea level dropped another 85 m to about 60 m elevation. The
rate of sea-level fall then slowed.
Sea-level fall began to slow around 11 000 BP (12 900 cal
BP), at the beginning of the Younger Dryas chronozone. The
climatic cooling associated with the Younger Dryas was less
severe in coastal British Columbia than in the North Atlantic
(Mathewes et al., 1993), but the slowing rate of sea-level fall
could indicate that nearby ice masses stopped thinning or
even thickened and advanced at this time in the northern Strait
of Georgia. The stillstand or readvance would, however, have
been short lived, as the Cordilleran ice sheet attained its present configuration shortly after 10 000 BP (11 450 cal BP)
(Clague, 1989). As well, the cessation of glaciomarine sedimentation just before the beginning of the Younger Dryas indicates that the climatic cooling did not cause a return to earlier
deglacial conditions. Instead, as discussed below, the slowing of sea-level fall may primarily be related to the Earth’s
response to the surface unloading.
CRUSTAL RESPONSE
AND PROJECTED SEA-LEVEL CHANGE
The northern Strait of Georgia sea-level curve provides
information on the amount of isostatic depression following
deglaciation. It also indicates how isostatic depression has
changed up to the present day. To a first order, postglacial
sea-level change is a combination of changes to the volume
of water in the oceans (eustatic sea level) and local changes
in the elevation of the Earth’s surface. Eustatic sea level can
be approximated by sea-level observations from sites located
far away from the continental ice sheets of the last ice age.
Although the sea-level histories of “far-field” sites differ in detail
(Milne et al., 2002), for present purposes the Barbados sealevel record (Fairbanks, 1989) adequately represents eustatic
sea-level change.
The isostatic depression was calculated by subtracting the
Barbados sea-level curve from the observed relative sea-level
change (Fig. 8). The resulting isostatic depression curve shows
that the surface of the Earth was depressed by about 230 m
at 13 750 cal BP at the northern Strait of Georgia. Since that
time, the lithosphere has rebounded and the Earth’s surface
has risen. The amount of subsidence has decreased, up to
the present, at an ever-slower rate.
The surface loading theory of a linear viscous or viscoelastic planet predicts that the Earth’s response to glacial
mass changes is composed of one or more decaying exponential terms (Peltier, 1985; Turcotte and Schubert, 2002). In
the simplest case a single exponential term suffices, and the
isostatic depression D decreases with time: D = D0exp(-t/τ),
where D0 is the initial isostatic depression after deglaciation,
and t is the decay time. The exponential decay constant
depends on the viscosity of the Earth’s mantle, showing why
postglacial rebound analysis is a key method for inferring mantle viscosity.
Isostatic depression is shown in Figure 9A on a semi-logarithmic plot. The slope of the straight line through the points
is the negative inverse of the decay time, -1/τ, and here gives
τ = 2500 years. A single decaying exponential curve fit to the
entire time series (Fig. 9B) under-predicts the observed early,
large subsidence and over-predicts subsidence later on. If,
however, only the post-13 000-year data is fit to the single
exponential term, a relatively good fit is obtained.
The early, large subsidence values generate a break in
slope on the semi-logarithmic plot (Fig. 9C). The apparent
decay time at earliest times is about 1200 years, compared to
Géographie physique et Quaternaire, 59(2-3), 2005
RELATIVE SEA-LEVEL CHANGE IN THE NORTHERN STRAIT OF GEORGIA
Relative sea level
200
Global eustatic sea level
Isostatic depression
150
Elevation
100
50
0
-50
14
12
10
8
6
4
2
0
Time (1000s cal BP)
FIGURE 8. Isostatic depression (long-dashed line) obtained by subtracting an estimate of global eustatic sea level (short-dashed line;
Barbados sea-level curve; Fairbanks, 1989) from the northern Strait
of Georgia sea-level curve (solid line).
Dépression isostatique (ligne en long pointillé) obtenue par la soustraction d’une estimation du niveau marin global (ligne en court pointillé; courbe du niveau marin de la Barbade; Fairbanks, 1989) à celle
de la courbe du niveau marin de la partie nord du détroit de Géorgie
(ligne pleine).
2600 years for the rest of the record. Fitting two exponential
terms to the isostatic depression, and varying the decay time
of the faster term, yields the best fit shown in Figure 9D. A
500-year decay time, combined with the slower 2600-year
term, fits the entire time series well. At early times, the
500-year and 2600-year decay times combine to produce the
apparent decay time of 1200 years shown in Figure 9C.
The fit with two exponential terms provides an estimate of
the crustal uplift rate (Fig. 10) due to glacio-isostatic adjustment following the deglaciation of the Cordilleran ice sheet.
The crustal uplift rate was at least 100 mm/a and may have
exceeded 200 mm/a during early emergence. The rate of
crustal uplift dropped below 10 mm/a by 10 000 cal BP and
below 1 mm/a by about 3000 cal BP. The present-day rate of
postglacial crustal uplift is about 0.25 mm/a.
123
Over the last 2000 years, postglacial uplift has produced
about 55 cm of crustal uplift, causing 55 cm of relative sealevel fall (Fig. 10). The contribution of other processes (tectonics, sedimentation, mountain glacier growth and retreat,
changes in oceanographic circulation) to relative sea-level
change is uncertain. The sparse late-Holocene data from
Cortes Bay, showing that sea-level has dropped by 1.5 m or
less in the past two thousand years, suggests, however, that
the net contribution from other processes is no more than
about twice that of postglacial rebound.
The inferred slow rate of present-day postglacial crustal
uplift and the small amount of observed late-Holocene sealevel change have implications for projections of future sealevel change in the mid and northern Strait of Georgia. The
Intergovernmental Panel on Climate Change (IPCC) projections of global sea-level rise from the year 1990 to 2100 range
from 9 to 88 cm, with the averages of the projections falling the
range of 30-50 cm (IPCC, 2001). Most of the projected sealevel rise is due to the warming of ocean water (steric effect)
and the melting of glaciers and ice caps. The actual sea-level
change in the Strait of Georgia will be affected by changes in
regional oceanographic conditions (circulation, temperature)
and may deviate from the global average.
In 110 years (the time elapsed from 1990 to 2100) the
amount of postglacial uplift is only 3 cm. This slightly reduces
the IPCC projected global sea-level rise to 6 to 85 cm. Crustal
motion from other processes may also contribute, but crustal
uplift is not expected to strongly ameliorate sea-level rise in the
mid and northern Strait of Georgia. This is in contrast to some
parts of Canada, where postglacial uplift occurs at much larger
rates. For example, the port of Churchill on Hudson Bay is rising at about 10 mm/a (Lambert et al., 2006). Here, local sea
level would be expected to continue to fall, even at the largest
projected values of global sea-level rise.
EARTH RHEOLOGY
The early, rapid crustal uplift and corresponding small
decay time of about 500 years could be related to transient
rheology or non-linear mantle flow (Ranalli, 1995), but could
also indicate a layered mantle viscosity. Commonly, geodynamic models of postglacial rebound and mantle convection
feature mantle viscosity increasing with depth. If this is the
case for the mantle underlying the Strait of Georgia, then the
500-year decay time is representative of a shallow, low-viscosity zone, and the 2600-year decay time represents deeper,
high-viscosity mantle.
Numerical techniques are required to rigorously compute
the spectra of decay times for layered viscous or viscoelastic
Earth models, and are beyond the scope of this paper.
However, an analytical formula gives the relation between
decay times and mantle viscosity for a uniform incompressible
viscous half-space (Turcotte and Schubert, 2002) and provides insight into Earth rheology. The decay time τ is given by
τ = 2ηk/(ρg), where η is mantle viscosity, k is the spatial
wavenumber (=2π/λ, where λ is wavelength), ρ is density of
the halfspace, here taken to be the density of shallow mantle
(3 300 kg/m 3), and g is the acceleration due to gravity
(9.82 m/s2). Assuming the shallow mantle responds to ice load
Géographie physique et Quaternaire, 59(2-3), 2005
124
T. S. JAMES, I. HUTCHINSON, J. V. BARRIE, K. W. CONWAY and D. MATHEWS
A
200
100
100
2500 years
50
Isostatic depression (m)
C
200
50
20
20
10
10
5
5
2
2
1
1
2600 years
1200 years
14
12
10
8
6
4
2
14
0
12
10
8
6
4
2
B
0
D
200
200
Decay time 2500 years
Isostatic depression (m)
Sum
150
150
all data
2600 years
100
100
50
50
post
13 ka
FIGURE 9. Analysis of isostatic
depression to determine exponential response times for the Earth’s
mantle. (A) Regression of isostatic
depression with a single decay
constant. (B) The resulting fit for a
2500 year decaying exponential to
all data (dashed line) and without
data before 13 000 cal BP (solid
line). (C) Regression of isostatic
depression, allowing a faster decay
time for the early emergence.
(D) The best fit to isostatic depression with two exponential decaying
terms. The slow decay time of
2600 years was assumed from (C),
and the fast decay time was varied
to obtain the best fit at 500 years.
Analyse de la dépression isostatique pour déterminer le temps de
réponse du manteau terrestre.
(A) Droite de régression avec une
constante. (B) Résultat de l’application d’une demi-vie de 2500 ans
sur l’ensemble des données (ligne
pointillée) et sans les données
d’avant 13 000 cal. BP (ligne
pleine). (C) Droite de régression de
la dépression isostatique permettant une émergence plus rapide.
(D) Meilleur modèle d’ajustement
de la dépression isostatique avec
deux constantes. La demi-vie lente
de 2600 ans est obtenue à partir
de (C) et la demi-vie rapide a été
sélectionnée en fonction du
meilleur ajustement à 500 ans.
500
years
0
0
14
12
10
8
6
4
Time (1000s cal BP)
2
0
14
12
10
8
6
4
2
0
Time (1000s cal BP)
changes on the scale of the Strait of Georgia, giving λ =
~200 km, the viscosity of the shallow mantle is about 1019 Pa s.
Deeper parts of the mantle have a viscosity at least five times
larger, and perhaps significantly more than five times, because
the deeper mantle responds to longer-wavelength components
of the surface load.
A viscosity of 1019 Pa s is consistent with mantle viscosity
inferred from the tilts of relict proglacial lake shorelines in
Puget Sound (James et al., 2000). Clague and James (2002)
showed that the lake shoreline tilts are only sensitive to the viscosity of the shallow mantle, above 670 km depth. In the
future, detailed numerical modelling could determine the thickness of the low-viscosity zone needed to satisfy both proglacial
lake shoreline tilts and the new relative sea-level data.
As discussed above, the slowing of sea-level fall at the
beginning of the Younger Dryas at 11 000 BP (~12 900 cal BP)
could indicate that remnant ice masses near the study area
slowed melting or even thickened for a few hundred years.
However, the good fit with two decaying exponential terms
suggests that this effect, if present, was not large.
COMPARISON WITH MID-STRAIT
OF GEORGIA SEA LEVEL
A comparison of the mid-Strait of Georgia sea-level curve
(Hutchinson et al., 2004a) to the northern strait shows a similar pattern of sea-level fall (Fig. 11). In the northern Strait of
Georgia, sea-level fall lagged the mid-Strait of Georgia by only
Géographie physique et Quaternaire, 59(2-3), 2005
RELATIVE SEA-LEVEL CHANGE IN THE NORTHERN STRAIT OF GEORGIA
300
300
200
200
100
100
20
10
10
5
5
Crustal uplift rate
2
2
1
1
0.5
0.5
0.3
0.3
0.2
0.2
10
8
6
Mid strait
4
2
0
Relative sea level (m)
20
12
Northern strait
50
Isostatic depression
14
150
Crustal uplift rate (mm a-1)
Isostatic depression (m)
50
125
Crustal tilt
100
50
0
14
Time (1000s cal BP)
10
12
Time (1000s cal BP)
8
6
FIGURE 10. Isostatic depression (m) and crustal uplift rate (mm/a)
from the best fit to the isostatic depression given in Figure 9D.
Dépression isostatique (m) et taux de soulèvement de la croûte terrestre (mm/a) calculés à partir du meilleur modèle d’ajustement de la
dépression isostatique présenté à la figure 9D.
FIGURE 11. Comparison of sea-level curves for the northern and midStrait of Georgia. The difference between the curves gives the amount
of crustal tilt (down to the north) between the two regions.
Comparaison des courbes du niveau marin relatif entre le nord et le
centre du détroit de Géorgie. La différence entre les courbes donne
l’inclinaison de la croûte terrestre (vers le nord) entre les deux régions.
200-300 years during early emergence. After 13 000 cal BP,
sea-level fall in the mid-strait continued at a faster rate than the
northern strait, and it underwent an earlier, and probably
larger-magnitude, lowstand than the northern strait.
CONCLUSIONS
Crustal tilt between the mid and northern Strait of Georgia
is given by the difference in sea-level curves. The largest difference occurred at the earliest time at about 13 500 cal BP,
and amounts to about 35 m, or only about 15% of the initial
isostatic depression of 230 m. The distance between the middle and northern strait is about 80 km, and this gives crustal
tilt of 35 m per 80 km = 0.45 m/km. This is less than one-half
of the peak crustal tilt of about 1.15 m/km observed in Puget
Sound soon after the Cordilleran ice sheet started retreating
from its maximum position (Thorson, 1989; James et al.,
2000). The smaller values of crustal tilt for the Strait of Georgia
compared to Puget Sound are consistent with rapid deglaciation of the Strait of Georgia (Barrie and Conway, 2002b;
Guilbault et al., 2003) and a thicker lithosphere (60-70 km
compared to 35-40 km for Puget Sound). A thicker lithosphere
has greater flexural rigidity and bends less readily. The mid
and northern Strait of Georgia responded to glacial unloading
nearly uniformly, with a relatively small amount of crustal tilting compared to the total vertical subsidence.
The sea-level observations described here are based on
isolation basin coring and other information. They provide constraints on deglacial and postglacial sea-level change for the
northern Strait of Georgia, a region previously lacking any information on past sea-level change. The observations indicate that:
1- Initial emergence was rapid, with sea level falling about
100 m between 13 750 and 13 000 cal BP. Sea level may
have dropped to a lowstand of a few metres depth between
about 11 000 and 8000 cal BP. It recovered and may have
stood slightly above its present level by 2000 cal BP. It then
dropped to its present level.
2- Sea-level fall in the northern Strait of Georgia closely paralleled, and slightly lagged, sea-level fall in the mid Strait of
Georgia (Hutchinson et al., 2004a). The time lag was only
200-300 years during early emergence. The mid Strait of
Georgia probably experienced an earlier, and slightly larger
magnitude, sea-level lowstand than the northern strait.
3- The initial rapid sea-level fall began at about the time that
sedimentation in the Strait of Georgia changed from proximal to distal glaciomarine, consistent with retreat of the ice
Géographie physique et Quaternaire, 59(2-3), 2005
126
T. S. JAMES, I. HUTCHINSON, J. V. BARRIE, K. W. CONWAY and D. MATHEWS
front from the Strait of Georgia and ensuing unloading of
the Earth’s surface.
4- The peak isostatic depression inferred from the sea-level
observations is about 230 m. Comparison of the mid and
northern Strait of Georgia sea-level curves shows that the
crust was tilted down to the north by about 35 m during initial emergence, about 15% of the peak isostatic depression. The Strait of Georgia responded to glacial unloading
relatively uniformly, with little crustal tilt compared to the
amount of isostatic depression.
5- The crust was tilted down to the north by about 35 m over
a distance of 80 km between the mid and northern Strait of
Georgia during initial emergence, giving a crustal tilt of
0.45 m/km. For comparison, crustal tilts from Puget Sound
were much larger (0.8 and 1.15 m/km), a consequence of
the thinner lithosphere underlying Puget Sound and the
larger nearby load.
6- The isostatic depression can be fit with two decaying exponential terms with decay times of about 500 and 2600
years. The response times may relate to a shallow, lowviscosity mantle layer, and deeper, high-viscosity mantle,
respectively. The mantle viscosity inferred from the faster
decay time is 1019 Pa s, consistent with earlier analyses of
tilted relict proglacial lake shorelines in Puget Sound
(James et al., 2000; Clague and James, 2002).
7- Initial rates of crustal uplift from the glacio-isostatic response
to the Cordilleran ice sheet exceeded 100 mm/a at the
northern Strait of Georgia. The rate of crustal uplift has
decayed since then to the present-day rate of about
0.25 mm/a. This value can be used to correct present-day
observations of sea-level change and crustal uplift for
glacio-isostatic adjustment to isolate the tectonic signal.
8- The potential of crustal uplift to ameliorate projected global
sea-level rise is limited in the mid and northern Strait of
Georgia because present-day crustal movements are
inferred to be small.
ACKNOWLEDGEMENTS
This work was initiated under the Georgia Basin
Geohazards Initiative of the Geological Survey of Canada,
and is an output of the Reducing Canada’s Vulnerability to
Climate Change Program of Natural Resources Canada. We
would like to thank the numerous property owners and residents of Quadra and Cortes Islands and Campbell River who
generously provided access to sites and responded to our
enquiries regarding marine shells. We thank Bill Hill, Jessica
Jorna, Paul Ferguson, and Morgan Soley for assistance in the
field. Karen Simon and Morgan Soley helped in the laboratory. We thank Renée Hetherington for marine shell identification and Richard Hebda for identification of terrestrial macrofossils. This is Geological Survey of Canada contribution
number 2005222.
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