Marine and limnic radiocarbon reservoir corrections for studies of late- and postglacial...

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Quaternary Research 61 (2004) 193–203

Marine and limnic radiocarbon reservoir corrections for studies of

late- and postglacial environments in Georgia Basin and Puget Lowland,

British Columbia, Canada and Washington, USA$

Ian Hutchinson,a,* Thomas S. James,b Paula J. Reimer,c Brian D. Bornhold,d and John J. Claguee

aDepartment of Geography, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6bGeological Survey of Canada, Sidney, B.C., Canada V8L 4B2

cCenter for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550, USAdCentre for Earth and Ocean Sciences, University of Victoria, Victoria, B.C., Canada V8W 2Y2

eDepartment of Earth Sciences, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6

Received 7 July 2003

Abstract

Models of late-glacial environmental change in coastal areas are commonly based on radiocarbon ages on marine shell and basal lake

sediments, both of which may be compromised by reservoir effects. The magnitude of the oceanic reservoir age in the inland waters of the

Georgia Basin and Puget Lowland of northwestern North America is inferred from radiocarbon ages on shell-wood pairs in Saanich Inlet and

previously published estimates. The weighted mean oceanic reservoir correction in the early and mid Holocene is �720F90 yr, slightly

smaller than, but not significantly different from, the modern value. The correction in late-glacial time is �950F50 yr. Valley-head sites yield

higher reservoir values (�1200F130 yr) immediately after deglaciation. The magnitude of the gyttja reservoir effect is inferred from pairs of

bulk gyttja and plant macrofossil ages from four lakes in the region. Incorporation of old carbon into basal gyttja yields ages from bulk

samples that are initially about 600 yr too old. The reservoir age declines to less than 100 yr after the first millennium of lake development.

When these corrections are accounted for, dates of deglaciation and late-glacial sea-level change in the study area are pushed forward in time

by more than 500 yr.

D 2004 University of Washington. All rights reserved.

Keywords: Radiocarbon; Reservoir effects; Marine shell; Gyttja; Pacific Northwest; Late-glacial environments

Introduction marine shells and provides information on glacial and

Radiocarbon ages on marine shells have provided

important information on ice margin retreat and sea-level

history in areas previously occupied by tidewater glaciers.

Following deglaciation of these areas, topographic depres-

sions above the marine limit fill with fresh water and

become sites of gyttja or peat deposition. Below the

marine limit, depressions are initially occupied by salt

water, but when sea level falls below the elevation of

the sill, the basin is isolated from the ocean and fresh-

water deposition then commences. Radiocarbon dating of

basal gyttja or peat complements radiocarbon ages of

0033-5894/$ - see front matter D 2004 University of Washington. All rights rese

doi:10.1016/j.yqres.2003.10.004

$ Supplementary data for Fig. 3 are available on IDEAL (http://

www.idealibrary.com).

* Corresponding author. Fax: (604) 291-5841.

E-mail address: [email protected] (I. Hutchinson).

sea-level history.

Both marine carbonates and gyttja, however, are subject

to various effects that require the laboratory-reported radio-

carbon age to be corrected or, at a minimum, interpreted

with care. Marine shells are subject to the well-known

marine reservoir effect arising from the delay in incorpora-

tion of atmospheric radiocarbon into oceanic deep water.

This delay, combined with the variable strength in deep-

water upwelling at coastal zones, results in spatial and

temporal variation in the marine reservoir effect (Bard,

1988).

Bulk terrestrial organic materials are also subject to a

variety of effects that can produce inaccurate radiocarbon

ages (Olsson, 1979; Sutherland, 1980). It is desirable to date

terrestrial macrofossils such as twigs, cones, and bark, but

owing to their paucity in some organic deposits, it is

sometimes necessary to date bulk material.

rved.

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I. Hutchinson et al. / Quaternary Research 61 (2004) 193–203194

In this paper we examine the origins, magnitude, and

uncertainty of the corrections associated with marine shells

and gyttja. Dates from coastal areas of southwestern British

Columbia and adjacent parts of Washington State (Fig. 1)

are analyzed to determine reservoir values. The spatial and

temporal variations in the reservoir values are assessed,

compared to other regions in the northeast Pacific, and

linked to changes in the local and regional geomorphic

and oceanographic environments.

Dating glaciomarine and marine deposits

Basal glaciomarine and marine sediments in cores from

isolation basins, and in exposures, commonly contain whole

valves of molluscs in a growth position. The shells yield

radiocarbon ages that are subject to error as a result of

postdepositional diagenesis and an oceanic reservoir effect.

The first arises from mineral exchange between the outer

layers of the shell and circulating groundwater (Grant-

Taylor, 1972). Leaching of the outer shell layers and dating

of the inner fraction of the shell minimize this error. Some

early radiocarbon ages on shell are suspect because of lack

of attention to this problem.

The second error arises from the fact that oceanic surface

waters are not in isotopic equilibrium because oceanic

circulation mixes 14C-depleted deep water with modern

surface water. If global oceanic waters were uniformly

mixed, then a universal oceanic reservoir correction would

apply. This is not the case, however, as deep-water residence

Fig. 1. (a) Location of the study area and other sites discussed in the text.

times vary considerably across the globe. The apparent 14C

age of surface waters thus varies spatially, depending on the

strength of ocean currents and the intensity of upwelling.

The magnitude of regional differences in the oceanic radio-

carbon reservoir in coastal waters has been assessed by

measuring the apparent age of modern molluscs collected

prior to contamination of the atmosphere with radioactive

products in the nuclear bomb tests of the 1950s, http://

calib.org/marine/).

The standard approach in determining the magnitude of

temporal variability in the regional oceanic reservoir effect

is to sample shells and co-occurring terrestrial organic

fossils. In areas where forests became established shortly

after deglaciation, fragments of wood are common in the

late-glacial coastal environment. Estimates of the magnitude

of the regional oceanic reservoir are commonly based on

shell-wood ‘pairs’ from this environment. Because wood

fragments are allocthonous, however, the shell-wood age

difference is always a minimum estimate of the oceanic

reservoir value.

A further source of ambiguity associated with shell

ages is that molluscan taxa with variable feeding strategies

and behaviors are commonly indiscriminately mixed in

sampling glaciomarine or marine sediments. Epifaunal

species such as oysters or mussels may yield older ages

than infaunal species such as clams at the same strati-

graphic level, in part because the latter burrow into

deposits that may be 30 cm or more below the sediment

surface (Ingram and Southon, 1996). In addition, the

feeding strategies of these species vary. Most bivalves

(b) Location of field sites in the Georgia Basin and Puget Lowland.

http:\\www.calib.org\marine\

I. Hutchinson et al. / Quaternary Research 61 (2004) 193–203 195

are filter feeders, removing phytoplankton and other small

organic particles from the water column. Others (such as

gastropods and bivalves in the family Tellinidae) graze

and scavenge organic material from the substrate (Ingram

and Southon, 1996). Variations in the age of the ingested

carbon are reflected in the apparent age of the shell (Dyke

et al., 2003).

A further source of variation is the differential seasonal

growth patterns displayed by different molluscan taxa.

Those species growing in spring or summer, for example,

may display older ages than those whose growth is limited

to winter, because of variations in the strength and position

of upwelling water masses through the year (Ingram and

Southon, 1996). For all these reasons, living shells of

different species gathered in the same area at the same time

can show substantial differences in their apparent age. For

example, pairs of shells in the genera Macoma, Mytilus, and

Ostrea sampled from cores spanning the last 5000 yr in San

Francisco Bay had age offsets ranging from 150 to 900 14C

yr (Ingram and Southon, 1996).

Temporal variations of the marine reservoir effect in

southwestern British Columbia

After correcting for the dates of collection of museum

specimens of molluscs, and for effects associated with

industrial contamination of the atmosphere (the Suess ef-

fect), Robinson and Thompson (1981) estimated the modern

oceanic reservoir age in coastal waters of the northeast

Pacific (Oregon–British Columbia) to be about �800 yr,

and showed that this value also applied to shell from local

Table 1

Age differences (oceanic reservoir effect) of shell-wood pairs sampled from ODP

Depth (m) Material Lab. No. Radiocarbon age

dated (CAMS) (14C yr B.P., F1j)

ODP-1033

15.5 Wood 33768 1,770F50

15.8 Shell 33482 2,650F50

31.5 Shell 33486 4,450F50

35.0 Wood 33733 4,500F60

52.8 Wood 33775 8,890F60

56.2 Shell 33487 10,480F50

51.4 Shell 33483 8,810F50

52.8 Wood 33771 8,430F50

ODP-1034

55.4 Wood 33779 5,790F50

59.0 Shell 33361 7,160F50

59.7 Shell 30710 7,320F50

59.7 (Tephra) — 6,730F40b

65.1 Shell 33493 8,420F70

66.4 Wood 33784 7,910F60

81.6 Wood 33787 10,410F60

82.2 Shell 33497 11,450F60

a Reservoir ages (rounded to the nearest decade) are corrected for different times of

uncertainty is the root of the sum of squares of the errors of the constituent radiob Hallett et al. (1997).

archaeological sites dating from the last two millennia. This

reservoir age is also applicable to a late Holocene shell-

wood pair sampled from a core in Saanich Inlet (Bornhold et

al., 1998).

The 800-year oceanic reservoir correction applies explic-

itly only to suspension feeding molluscs (Dyke et al., 2003)

and to the late Holocene in this area. Researchers, however,

have used this value to establish chronologies of sea-level

change in the mid Holocene (e.g., Friele and Hutchinson,

1993) and in studies of ice-front fluctuations in late-glacial

time (Clague et al., 1997). The latter authors based their

decision on an oceanic reservoir estimate of � 820 yr from

wood and shell samples in a glaciomarine unit at a critical

site.

Other researchers have adopted a variety of expedients to

circumvent the problem of a poorly constrained oceanic

reservoir age. Porter and Swanson (1998) based their

analysis of rates of advance and retreat of the Cordilleran

ice sheet entirely on a small inventory of radiocarbon ages

on terrestrial plant and gyttja samples from the Puget

Lowland, shunning the larger database of shell ages because

of the uncertainty of the oceanic reservoir age correction.

Other researchers have deliberately reported uncorrected

shell ages (Dethier et al., 1995), or have adopted local

corrections for the period of interest. For example, Anund-

sen et al. (1994) used a marine reservoir correction of � 760

years to establish a chronology of sea-level change at

Carpenter Lake in Washington State but postulated a smaller

correction for late-glacial time based on a shell-gyttja pair,

assuming no reservoir correction for the gyttja. Swanson

and Caffee (2002) assumed a value of � 400 years for this

cores from Saanich Inlet

Predicted age difference Reservoir age

Varve count

(yr)

Sedimentation

rate (yr)

(F weighted uncertainity)a

82 — �800F70

— �670F80

720

— �950F80

639

— �830F70

397

543 — �770F70

— 0 �590F60

— 220 �730F90

— 110 �930F90

deposition based on varve counts or local sedimentation rate. The weighted

carbon ages.


same lake as a calibration for 36Cl production rates in the

Puget Lowland.

To test whether the late Holocene reservoir correction

can be applied to shell ages throughout the postglacial

period, we developed a precise marine reservoir correction

estimate based on shell-wood pairs from two cores (ODP-

1033 and ODP-1034) that span the last 14,000 years in

Saanich Inlet on southeastern Vancouver Island (Fig. 1). The

methodology and results of this coring program are pre-

sented in Bornhold et al. (1998) and Bornhold and Kemp

(2001). Dating control was provided by 27 radiocarbon ages

on wood, 40 ages on shell, and tephrochronology.

Seven closely spaced wood-shell pairs in the cores were

selected for calculating the marine reservoir correction.

Sedimentation in the middle and late Holocene was primarily

in the form of annual varves, so the difference in the time of

deposition of the paired wood and shell samples was

determined by counting the number of intervening varves

(Table 1). In the late-glacial and early Holocene, however,

varve boundaries are generally less well-defined, and com-

pletely lacking in some intervals, so age discrepancies were

calculated from the inferred sedimentation rate in each core

from a power function model (Fig. 2). Uncalibrated 14C ages

were used to determine the sedimentation rates since we are

seeking a radiocarbon age difference in the samples. The

oceanic reservoir correction is thus the laboratory-reported

wood age minus the laboratory-reported shell age plus the

disparity in depositional age determined from varve counts

or sedimentation rates (Table 1). The difference in age of the

Mazama eruption (6730F40 14C yr B.P.; Hallett et al., 1997)

and the age of a shell lying just above the resultant tephra in

ODP 1034 yields an additional data point (Table 1).

Fig. 2. Age-depth relations in ODP cores 1033 and 1034 from Saanich Inlet bas

Stevens and Clague (2001).

Plotting these results, together with previously published

ages (Dyck et al., 1965; Kovanen and Easterbrook, 2002),

provides an estimate of the temporal variations in the

regional oceanic reservoir correction (Fig. 3a). The pattern

of reservoir variations since the end of the Wisconsinan

glaciation can be divided into two separate segments. The

oceanic reservoir correction for late-glacial time (>1000014C yr B.P.) ranges from about �900 to �1300 yr. These

variable results may reflect short-term changes in global

atmospheric CO2 or North Pacific-wide changes in ocean

climate (Kienast and McKay, 2001). There appears, howev-

er, to be a distinctive spatial pattern in the array of reservoir

ages for this time. Sites in sheltered locations, such as Furry

Creek, near the head of Howe Sound (Fig. 1), have a much

higher average oceanic reservoir correction in late-glacial

time (�1200F130 yr) than sites such as Saanich Inlet in

more open locations. For this latter group, the average

reservoir correction at this time is about �950F50 yr.

The correction for the late-glacial segment contrasts with

results from the early to mid Holocene (8000 to 3000 14C yr

B.P.). During that period the weighted mean oceanic reser-

voir correction, based on five shell-wood pairs from Saanich

Inlet, is �720F90 yr, which is slightly smaller than, but not

significantly different from, the modern value.

Temporal variations in marine reservoir ages in the

northeast Pacific

The results presented above indicate low variability in

reservoir ages in the Holocene, presumably reflecting quasi-

stable patterns of oceanic circulation in the northeast Pacific

during this period. This conclusion is buttressed by results

ed on radiocarbon ages of charcoal and wood fragments. Data from Blais-

Fig. 3. Temporal variations in marine reservoir ages in the late-glacial period and Holocene for: (a) Georgia Basin and Puget Lowland; (b) Queen Charlotte

Islands; and (c) Channel Islands, California. Data for Georgia Basin and Puget Lowland based on shell-wood pairs cited in this study (Table 1), Robinson and

Thompson (1980), James et al. (2002), and Kovanen and Easterbrook (2002).


reported by Kienast and McKay (2001), who show that

there has been little variation (F1jC) in sea surface temper-

atures in the waters off the west coast of Vancouver Island

over the last 8000 years.

Similar results have been obtained from studies of

reservoir ages elsewhere in the northeast Pacific. Southon

et al. (1990) assessed variations in the magnitude of the

oceanic reservoir effect in the vicinity of the Queen Char-

lotte Islands (Haida Gwaii). They dated 24 shell-wood pairs

spanning the Holocene, mostly from exposures on the

northern coast of Graham Island (Fig. 1). The samples

yielded marine reservoir ages that on average are slightly

lower than the mean prebomb value for the Oregon–British

Columbia coast.

A large number of paired radiocarbon ages have been

obtained from the Queen Charlotte Islands and neighboring

coastal waters in the last decade, extending the record of

reservoir variations back to about 12,400 14C yr B.P.

(Josenhans et al., 1995, 1997; Fedje et al., 1996; Fedje

and Christensen, 1999; Barrie and Conway, 2002). When

plotted (Fig. 3b), these data suggest that there have been

only small-scale variations in the intensity of upwelling in

the vicinity of the Queen Charlotte Islands during the

Holocene, with a mean reservoir age during this interval

of about �600 to �700 yr. An exception is a mean reservoir

age of about �1200 yr for four paired samples dating from

about 6400 14C yr B.P. This group of ages, however, is

derived from a high-energy beach deposit and may reflect

the sampling of reworked shell in a detrital melange. Dates

from the late-glacial period are sparse, but they suggest that

the mean reservoir age at this time was somewhat less than

in the Holocene (Fig. 3b).

Kennett et al. (1997) analyzed 10 paired shell-charcoal

samples dating back to about 9200 14C yr B.P. from

archaeological sites on the Channel Islands, off the coast

of southern California (Fig. 1). Their results show slightly

more variation during the mid to late Holocene than is

recorded in British Columbia, but this may in part be due to

sampling outliers (Fig. 3c).

Explaining the spatial variations

If the changes in marine reservoir ages in the late-glacial

period at the three sites in the northeast Pacific were in the

same direction and in phase, it might be possible to link

them to global phenomena such as variations in atmospheric14C associated with late-glacial climate fluctuations, or

variable intensity of deep-water formation in the North

Atlantic source area. These excursions are known to have

affected the age of surface waters elsewhere in the Pacific

(Sikes et al., 2000). Alternatively, as Kennett et al. (1997)

and Kovanen and Easterbrook (2002) suggest, they may be

a product of variations in the intensity of upwelling in the

Alaska–California Current system or the position and


strength of counter currents along the coast. If that were the

case, however, these changes should be roughly in phase

along the entire coast of the northeast Pacific, and should

not display the regional disparities evident in Figure 3.

An alternative hypothesis is that the higher reservoir

correction in late-glacial time in the Strait of Georgia and

Puget Sound is a consequence of the influx of 14C-depleted

waters from the melting Cordilleran ice sheet. Direct dis-

charge of cold, 14C-depleted freshwater from melting tide-

water glaciers and icebergs into the surface waters of the

Strait would lead to an increase in the reservoir age, as was

the case in the late-glacial Champlain Sea of eastern Canada

(Rodrigues, 1988).

The discharge of large amounts of meltwater into these

enclosed seas created a low-salinity surface layer that likely

froze each winter. In southern Alaska at the present time sea

ice is confined to areas near stream mouths at the heads of the

deeper fjords (Dumond, 1987). By analogy, sea ice cover in

the study area likely persisted longest in fjord-head locations

in late-glacial time. Sea ice inhibits atmospheric–ocean

exchanges of CO2 (Gordon and Harkness, 1992), augment-

ing the oceanic reservoir effect in fjord-head locations. The

decline in reservoir ages during late-glacial time in these

locations in the study area may thus reflect a reduction in

meltwater production and a gradual loss of sea ice cover.

Another factor that may help explain the regional varia-

tions in oceanic reservoir age is relative sea-level position.

Relative sea level stood at or below �150 m in the Queen

Charlotte Islands area 12,400 14C yr B.P. (Josenhans et al.,

1997), and, based on published eustatic curves (Lambeck

and Chappell, 2001), it was likely at or below �100 m in

southern California at the same time. Was this sufficient to

reduce incursions of 14C-depleted deep-water in neighbor-

ing coastal areas and produce a decrease in the reservoir age

of the surface mixed layer? In contrast, relative sea level in

the Georgia Basin and the Puget Lowland was up to 180 m

higher than at present in late-glacial time (Clague et al.,

1982; James et al., 2002). Higher relative sea levels would

undoubtedly have enhanced the flow of deep water through

the narrow straits that separate these interior waterways

from the open Pacific. If this hypothesis is correct, then

the lowest oceanic reservoir ages in the study area should

coincide with the early Holocene sea-level lowstand, but

more data are needed to explore this.

Dating limnic deposits

Limnic deposits lying above till and basal marine and

glaciomarine deposits consist primarily of the detrital resi-

dues of phytoplankton and aquatic and terrestrial plants.

Processing of these organic residues by benthic fauna

produces gelatinous, fine-textured ooze. Further sedimenta-

tion and progressive dewatering compress this material into

compacted gyttja. The gyttja may also contain a mineral

component derived from erosion of the lake catchment and

shoreface. Larger organic fragments and more refractory

materials (e.g., wood fragments, twigs, cones, conifer nee-

dles, seeds) that escape faunal processing are preserved in

the gyttja as plant ‘macrofossils’. Screening of limnic

deposits from just above the freshwater contact may yield

samples of macrofossils that can return a precise radiocar-

bon age on basin emergence. In their absence, however, the

researcher may be forced to date bulk samples of gyttja.

It has long been recognized that bulk sediment samples

from ‘hardwater’ lakes (pH >7) return radiocarbon ages that

are too old. Quaternary geoscientists are well aware of the

problems posed by dating bulk gyttja samples from hard-

water lakes, but are generally not cognizant of the fact that

bulk samples from softwater lakes in glaciated areas may

also display ageing effects. For example, whereas ageing

effects associated with influxes of 14C-depleted carbon into

lakes in the semi-arid interior of the Pacific Northwest are

widely acknowledged, and assessments of the magnitude of

this effect have been made (Mack et al., 1978; Mathewes

and Westgate, 1980), along the coast, and in the adjacent

Coast Mountains and North Cascade Range, little attention

has been given to the problem. Radiocarbon ages on bulk

samples of basal limnic sediments from these areas are

therefore usually reported without correction (Hebda,

1983; Cwynar, 1987; Anundsen et al., 1994; Heine,

1998a, 1998b; James et al., 2002; Brown and Hebda,

2002, 2003). Is this justified, or should ages on bulk

sediment from softwater lakes in deglaciated areas be

corrected for a potential old carbon effect?

Tills may contain substantial reserves of carbonate, even

in areas not underlain by limestone or coal beds. Graphite

schists (Sutherland, 1980) and pre-glacial organic-rich sedi-

ments such as peat, forest beds, and marine or glaciomarine

deposits may also contribute to the reserves of old carbon.

Tills containing organic or inorganic carbon from these

sources can supply 14C-depleted carbon to local lakes in

particulate or dissolved form as soon as the surface is

exposed to weathering. Uptake of carbon from dissolved

bicarbonate by aquatic plants results in the production of

organic detritus that is diluted in 14C. Elevated levels of old

carbon can be incorporated into limnic sediments in glaci-

ated areas as a result of processes operating in the nascent

phase of lake development, or during later phases as a result

of ecological disturbances in the catchment.

Engstrom et al. (2000) developed a hydrological model

of the chemical evolution of lakes in recently deglaciated

terrain from studies undertaken in the vicinity of Glacier

Bay in southeastern Alaska (Fig. 1), an area that is climat-

ically and ecologically similar to the Georgia Basin and

Puget Lowland. They demonstrate that leaching of weath-

ered products from the catchment to the lake basin is rapid

for about the first century after deglaciation, but thereafter

declines dramatically. They ascribe this progressive decline

to rapid loss of readily leachable carbonates and forest

development. Tree litter promotes soil acidification and

the development of clay and iron-cemented layers in deeper

Fig. 4. (a) Age-depth relations in limnic sediments on Pleasant Island, southeast Alaska, from radiocarbon ages on plant macrofossils and gyttja, and (b)

temporal variations in gyttja reservoir age determined from these macrofossil-gyttja age differences. Data from Hansen and Engstrom (1996).


soil horizons. These hardpans effectively reduce subsurface

weathering and groundwater transport of soluble products to

the lake. The glacier foreland investigated by Engstrom et

al. (2000) is in carbonate terrain, but even so, the supply of

Ca2+ ions to the lake diminishes by an order-of-magnitude

after the first three centuries of subaerial exposure.

Temporal changes in the release and retention of inor-

ganic carbon in a lake catchment are recorded in the limnic

sediments. The weathering of carbonate-rich tills introduces

large amounts of 14C-depleted carbon into the gyttja, either

directly as particulate matter or indirectly as bicarbonates

that are processed through the limnic food web. For exam-

ple, Engstrom et al. (1990) and Hansen and Engstrom

(1996) noted that radiocarbon ages on late-glacial bulk

gyttja samples from a lake near Glacier Bay in southeast

Table 2

Age differences of plant macrofossil and gyttja pairs from basal sediments of lak

Site

(Source)

Material

dated

y13C Labora

numbe

Mike Lake Pollen �28.8b RIDD

(Brown et al., 1989) Gyttja �27.9 RIDD

Marion Lake Pine needlesd n/a I-6857

(Mathewes, 1973) Gyttja n/a I-5960

Heal Lake Pine cone CAMS

(Reimer, 1998) Gyttja QL-46

232nd St. bog Plant detritus n/a TO-92

(James et al., 2002) Gyttja n/a TO-92

a Radiocarbon ages are from immediately above the glacial- or glaciomarine-lacub Mean of two determinations.c Age difference adjusted for an assumed +250F72 yr. error in the ages derived fromd Pine needles are from glacial sediments 0.0–0.3 m below contact with overlyine Age difference is adjusted for the difference in depth of the midpoints of the tw

Alaska were consistently about 850 yr older than ages on

plant macrofossils at equivalent depth. A plot of their data

(Fig. 4) suggests that the gyttja reservoir age may be almost

double the cited value in the initial phase of deglaciation (at

about 12,000 14C yr B.P.), and decreases thereafter for about

5000 years.

It might be argued that these results, whilest perhaps

typical of lakes in a humid climate and carbonate terrain, are

unlikely to apply to lake catchments in similar climates

developed on carbonate-poor substrates. A study of lake

development in a catchment on granitic bedrock in southern

Sweden, following recession of the Litorina Sea (at about

6000 14C yr B.P.), by Hedenstrom and Possnert (2001)

shows that this assumption is not valid. In the first few

decades following lake isolation, bulk samples of clay gyttja

es and bogs in the Georgia Basin and Puget Lowlanda

tory

r

Age

(14C yr B.P., F1j)Age difference

(Fweighted uncertainty)

L-1063/4 11,670F110b �680F165c

L-653 12,600F100

11,920F250 �565F465e

12,350F190

-23928 11,470F60

78 12,120F75 �650F96

08 11,395F110

06 11,920F80 �540F136

strine contact, except where noted.

pollen concentrates based on dating of the Mt. Mazama tephra in this core.

g limnic sediments. Gyttja sample is from 0.0–0.07 m above contact.

o samples based on the late-glacial sedimentation rate in Marion Lake.

Fig. 5. Temporal variations in gyttja reservoir age in Heal Lake (Table 3;

Reimer, 1998).


yielded radiocarbon ages that are about 500 years older than

plant macrofossils from the same depth. This gyttja reser-

voir age declines to about 60 yr within 200 to 300 years

after lake isolation.

Table 3

Age differences of gyttja and plant macrofossil pairs from Heal Lake, Vancouver

Stratigraphic or

isotopic horizon

Material

dated

y13C Laborato

number

Mt. Mazama tephra Tephraa — —

Gyttja �27.9 QL-4677

Midpoint of y13C rise Pondweed seeds �25b CAMS-2


Pine cones �24.9/�25b (QL-4740

Colluvial layer

Gyttja �32.4 CAMS-2

Pine cone �25.0 CAMS-2

Base of core


a Age cited by Hallett et al. (1997).b Estimated value.c Weighted mean.

Does this behavior also apply to softwater lakes in

deglaciated terrain in southwestern British Columbia and

adjacent areas of Washington State? Among the many

studies of late-glacial environments in this area based on

proxy indicators in lakes, there are several that report

radiocarbon ages based on both bulk samples of limnic

sediments and plant macrofossils. On occasion, radiocarbon

ages on gyttja-macrofossil pairs are cited. More often,

however, it is necessary to interpolate apparent gyttja-

macrofossil age differences from estimates of sedimentation

rates based on the macrofossil ages. Here we assess the

magnitude of the age differences in four isolation basins

where high-quality radiocarbon ages on basal sediments are

available. We also compare the results with those from other

lake basins in the study area. The four basins are Marion

Lake (Mathewes, 1973), Mike Lake (Brown et al., 1989),

Heal Lake (Reimer, 1998), and ‘‘232nd St bog’’ (James et

al., 2002) (Fig. 1b, Table 2). Despite differences in the

geology of the catchments, the basal sediments in each lake

yield gyttja age differences that are similar. The weighted

mean difference in the basal gyttja (F 1j) is �625F60 yr.

The source of the old carbon in the glacial deposits in

these catchments is not readily apparent. The 232nd St. bog

site lies below the postglacial marine limit and may have

been a sink for old carbon dissolved from shell in glacio-

marine deposits in the catchment. The other three basins,

however, lie above the marine limit. Old carbon in these

catchments may be derived from the scattered local outcrops

of limestone or coal that occur in the Georgia Basin,

although preglacial forest beds or glaciomarine deposits

may account for a substantial portion of the total, particu-

larly in the Fraser Lowland, where the surrounding moun-

tains are either plutonic or volcanic.

In the case of Heal Lake, matching gyttja-macrofossil

measurements at different levels in the core permit the

temporal variation in gyttja age errors to be assessed (Fig.

5). The reservoir ages were calculated from sample pairs

found either in the same 1-cm sediment interval or through

correlation of isotopic or stratigraphic markers between

Island, British Columbia

ry Age

(14C yr B.P., F1j)Age difference

(F weighted uncertainty)

6,730F40 �80F60

6,810F45

3057 9,960F50

9,990F30 �30F58

/CAMS-23063) 10,920F30c

�200F66

3062 11,120F60

3928 11,470F60

�650F96

12,120F75


cores and faces cut during excavation of the bed of the

drained lake. A thick layer of colluvium found in the

nearshore environments was dated to 10,920F 30 14C yr

B.P. using cones of lodgepole pine (Pinus contorta)

embedded in the top of the layer (Table 3). This colluvial

input could be detected as a visible mineral layer with low

organic carbon content in the deep-water sediments. A

rapid shift in organic carbon y13C at the beginning of the

Holocene served to correlate cores for one pair of samples.

A portion of the difference between gyttja and plant

macrofossils ages could result from differential settling in

the basal sediment but is unlikely to have affected those

found on top of the thick colluvial layer.

As in the Glacier Bay foreland (Engstrom et al., 1990)

there is a rapid decline in the ageing effect as carbonates are

leached from the exposed glacial deposits in the catchment.

At Heal Lake the gyttja correction diminishes to about �200

yr some 500 yr after lake inception, and to �100 yr or less

at the time of the Mt. Mazama eruption, some 5000 yr after

deglaciation (Fig. 5). Mike, Mund, and Lost lakes (Fig. 1b)

show a similar response. Gyttja-plant macrofossil pairs from

these lakes have age differences of �190F210; �240F99

yr, and �290F113 yr within 500 to 1500 yr after emergence

(Brown et al., 1989; James et al., 2002).

Discussion

The adjusted radiocarbon age pairs from Saanich Inlet,

together with previously published ages, delimit the magni-

tude of the oceanic reservoir age in the inland waters of the

Georgia Basin and Puget Lowland during the late-glacial

period and Holocene. They indicate that the mean oceanic

reservoir correction during most of the Holocene is about

�720F90 yr, slightly below the modern value. In late-glacial

time, isostatic depression of the coastal area may have

promoted deep-water exchange between the Pacific Ocean

and these inland seas, leading to larger reservoir ages (about

�950F50 yr). Sites in sheltered locations yield much higher

mean oceanic reservoir corrections (�1200F130 yr) possi-

bly as a result of reduced ocean ventilation under seasonal

sea ice and local influxes of 14C-depleted glacial meltwaters.

Radiocarbon ages on bulk samples of gyttja from the

base of lake cores are also likely to be too old. A weighted

mean of the difference of four gyttja-macrofossil pairs from

basal limnic sediments from southwestern British Columbia

and adjacent Puget Lowland in Washington State indicates

that gyttja samples are biased to older ages by 625F60

years. While �625F60 yr may be the appropriate correction

for radiocarbon ages of basal gyttja in the study area, it is

important to recall that in carbonate-rich terranes, the age of

basal bulk limnic sediments may be as much as 1500 yr too

old (Figure 4). Similarly, in softwater lakes of eastern

Canada gyttja ages are reported to be as much as 1000 yr

too old (McNeely, 2002; see comments by R.J. Mott).

Where no plant macrofossils are recovered for radiocarbon

dating, we recommend subtracting 625F60 yr from reported

gyttja ages in the study area to offset the incorporation of

old carbon in the basal sediment, but note that caution is still

required in interpreting the corrected ages.

The majority of studies of late-glacial environments in

the study area are subject to these sources of error. Models

of late Pleistocene glacial retreat in the study area are

commonly constrained by uncorrected shell or gyttja ages

(Alley and Chatwin, 1979; Dethier et al., 1995; Porter and

Swanson, 1998). Studies of relative sea-level change may

take account of the oceanic reservoir effect, but ignore

potential errors associated with dating bulk gyttja (Anund-

sen et al., 1994; James et al., 2002). Calculating the oceanic

reservoir age on the basis of a shell-gyttja pair (Anundsen et

al., 1994) produces a floating chronology.

Incorporation of these corrections into studies of late-

glacial environmental change produces chronologies that are

pushed forward in time by >500 yr. This has implications

for the precise timing of late-glacial events and paleocli-

matic chronozones (e.g., Hebda, 1983; Brown and Hebda,

2002, 2003) and will aid further development of glacio-

isostatic models (James et al., 2000; Clague and James,

2002) that incorporate detailed observations of deglacial

history and that are tuned to fit rapidly changing sea-level

during late-glacial times.

Acknowledgments

This work was funded primarily through the Georgia

Basin Geohazards Initiative of the Geological Survey of

Canada. A portion of the research was performed under the

auspices of the U. S. Department of Energy by the

University of California, Lawrence Livermore National

Laboratory under Contract No. W-7405-Eng-48. The Heal

Lake study was funded by DoE 93-NIGEC-208. Discus-

sions with Art Dyke, Rolf Mathewes, Roger McNeely, Les

Cwynar, Vaughn Barrie, Roelf Beukens, and Kim Conway

are gratefully acknowledged. We thank Mebus Geyh, Irka

Hajdas and Vaughn Barrie for helpful reviews of this paper.

Geological Survey of Canada contribution 2003069.

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