modern to late holocene deposition in an anoxic fjord on...

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Marine Geology 219

Modern to Late Holocene deposition in an anoxic fjord on the west

coast of Canada: Implications for regional oceanography,

climate and paleoseismic history

Audrey Dallimorea,*, Richard E. Thomsonb, Miriam A. Bertramc

aGeological Survey of Canada-Pacific, Institute of Ocean Sciences, Sidney, B.C., Canada, V8L 4B2bDepartment of Fisheries and Oceans, Canada, Institute of Ocean Sciences, Sidney, B.C., Canada, V8L 4B2

cPacific Marine Environmental Laboratory, Seattle, Washington, USA

Received 9 October 2003; received in revised form 21 April 2005; accepted 12 May 2005

Abstract

Laminated sediments preserved in the anoxic inner basin of Effingham Inlet on the Pacific coast of Vancouver Island, British

Columbia, Canada, yield a high-resolution sediment deposition record spanning about 6000 yr. The varying thickness of

diatom/terrigenous mud varves in sediment cores from the basin can be interpreted in terms of annual changes in surface

productivity and freshwater input within the inlet. Similarly, the occurrence of unlaminated mud units (homogenites)

intercalated amongst the laminated sediments can be interpreted in terms of oceanic and climatic changes. These units appear

to be associated with coastal upwelling events that result infrequently in highly oxygenated oceanic water penetrating to the

bottom of the inner and outer basins of the inlet. The sedimentary record also contains massive and graded mud units considered

to arise from debris flows and turbidity currents, some of which were probably initiated by seismic events, including a major

event about 4500 14C yr BP which may be earthquake related. A total of seventeen oceanographic surveys of the inlet beginning

in 1995 characterize the modern seasonal coastal upwelling regime and a unique bottom water oxygenation event which was

recorded in January 1999, following a rapid transition from the strong El Nino event of 1997–98 to the moderate La Nina event

of 1998–99. Circum-Pacific evidence suggests that a bregime shiftQ from warm to cold conditions occurred in the central

northeast Pacific in the late 1990s, indicating that the coastal ocean processes influencing Effingham Inlet sedimentation are

likely modified by climate-scale ocean variability.

D 2005 Elsevier B.V. All rights reserved.

Keywords: laminated sediments; anoxic fjords; Pacific climate; regime shift

1. Introduction

0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.margeo.2005.05.003

* Corresponding author. Fax: +1 250 363 6565.

E-mail addresses: [email protected] (A. Dallimore),

[email protected] (R.E. Thomson),

[email protected] (M.A. Bertram).

Annually laminated marine sediment records can

extend our knowledge of atmospheric and oceano-

graphic conditions beyond historic and instrumental

(2005) 47–69

A. Dallimore et al. / Marine Geology 219 (2005) 47–6948

records, once sediment proxies for climatic and ocean-

ographic conditions are identified (Kennett and

Ingram, 1995; Pike and Kemp, 1996a; Sancetta,

1996; Kemp, 1996, 2003; Ware and Thomson,

100

50

50

50

50

50

100

200

100

BARKLEYSOUND

INNER BASIN

EFFINGHAM

INLET

OUTER BASIN

Sill: 65 m

Sill: 40 m

100 m isobaths

Marsh

0 1 2 km

N

49 01'N

Va

PA

Oceanographystations

Mooring and sediment trap site

x EF07

125125° 10'W 10'W125 10'W

x EF01

x EF02

x EF03

x EF04

x EF05

x EF06

EF08 x

x EF09

EF10 x

x EF11

x EF12

EF01

Inner basin core site

Outer basin core site

Fig. 1. Location map of Effingham Inlet study area showing coring sites a

Aerial photograph inset shows drainage courses and recent logging activi

2000). In the last decade studies of laminated marine

sediment records from around the world increasingly

indicate that the variability in the preservation and

deposition of the annual winter laminae, which

n c o u v e r

I s l a n d

CANADAU. S. A.

S t r a i to f

G e o r g i a

SaanichInlet

yelkraB

dnuoS

CIFICOCEAN

VANCOUVERVANCOUVERVANCOUVER

VICTORIAVICTORIAVICTORIA

Stra i t of

Juan de Fuca

Enlarged area

location

CANADAJervisInlet

Effingham River floodplain

Logged areas

Barkley Sound

nd location of moorings, sediment traps and oceanography stations.

ty.

A. Dallimore et al. / Marine Geology 219 (2005) 47–69 49

records variations in precipitation, and the spring/

summer diatomaceous laminae, which are indicative

of annual productivity in the coastal ocean, archive

the abrupt crossing of depositional thresholds that are

related to global climate changes (Kennett et al., 1995;

Bull and Kemp, 1996; Hughen et al., 1996; Pike and

Kemp, 1996a; Schimmelmann and Lange, 1996;

Schimmelmann et al., 1998; Schultz et al., 1998;

Luckge et al., 2001; Kemp, 2003). Studies of lami-

nated marine sediments in Saanich Inlet, an anoxic

British Columbia fjord located on the southwest coast

of Vancouver Island (Fig. 1) (Gross et al., 1963;

Gucluer and Gross, 1964) have revealed a high-reso-

lution sediment record of the entire Holocene as a

result of ODP Leg 169S (Bornhold et al., 1998; Blais-

Stevens et al., 1997, 2001; Blais-Stevens and Clague,

2001). However, Saanich Inlet is restricted from full

open ocean processes due to its location along the

inner coast of Vancouver Island.

This study is located in anoxic Effingham Inlet,

British Columbia which is directly connected to the

open ocean (Fig. 1) and contains an annually laminat-

ed sediment record spanning about 6000 yr (Patterson

et al., 2000; Dallimore, 2001; Hay et al., 2003; Chang

et al., 2003). We have monitored an oceanographic

transect in Effingham Inlet for the past 9 yr to estab-

lish the modern oceanographic and climatic regime,

enabling us to infer sedimentological proxies for cli-

Sill

0 km 4

Barkley Sound

CoolIntermedi

Warm

Cool/Salty

840840Dissolved Oxygen (ml/l)

3.5 kHz Sub-bottom Survey

Distance

SOUTH

Fig. 2. 3.5 kHZ sub-bottom survey profile of the surficial sediments of Effin

over glacially-carved bedrock substrate. Vertical exaggeration of profil

characteristic estuarine stratified water column and the anoxic character o

mate-scale variability throughout the late Holocene.

Paleoseismic events are also recorded in the Effing-

ham Inlet sediment record (Dallimore et al., 2001)

since periodic tectonic processes affect sediment sta-

bility in the inlet due to close proximity to the zone of

subduction of the oceanic Juan de Fuca Plate beneath

the continental North America plate (Rogers, 1988;

Atwater et al., 1995a,b; Hyndman, 1995).

2. Study area

Effingham Inlet is 17 km long multiple-silled gla-

cial fjord located on the southwestern coast of Van-

couver Island, which opens to the Pacific Ocean

through Barkley Sound. The fjord has an inner and

outer basin, each more progressively restricted from

the open ocean waters by shallow bedrock sills (Fig.

1). The Effingham River, with a drainage basin of

about 79 km2, discharges into the northern end of the

inner basin. Run-off also enters the inlet directly from

the steep bedrock basin walls during the frequent

heavy rainfalls of fall and winter, depositing signifi-

cant amounts of terrigenous detritus into the inlet

(Dallimore, 2001). Water property distributions in

the inlet (Fig. 2) are characteristic of a weakly

mixed estuary (Thomson, 1981; Patterson et al.,

2000) with small (b2 m) tidal variation and negligible

App

roxi

mat

e D

epth

(m

)

50

0

100

200

Inner BasinOuter Basin

Sill

/Fresh (winter) - Warm/Less Fresh (summer)ate

/Salty

840840

NORTH

gham Inlet and Barkley Sound, showing Holocene sediments draped

e about 50 X. Water property structure shows a weakly mixed

f the inner basin bottom waters.


currents, except in the southern oceanward portion of

the inlet and in constricted narrows in the vicinity of

the shallow sills.

As part of the Coastal Upwelling Domain along the

west coast of North America, the ocean and climate

dynamics of the study region are strongly seasonally

dependent (Thomson, 1981; Thomson and Gower,

1998; Ware and Thomson, 2000), and influenced by

the relative positions of the Aleutian Low and the

North Pacific High atmospheric pressure systems,

the Jet Stream and El Nino/La Nina events (Roden,

1989; Patterson et al., 2000). Paleoceanographic sig-

nals recorded in the sediment record are therefore

indicative of past seasonality and climate. Annual

summer (May through August) upwelling of deep

(N200 m) slope waters onto the Vancouver Island

continental shelf deliver nutrient-rich waters to the

upper ocean, with particularly strong upwelling-driv-

en nutrient flux occurring through the numerous sub-

marine canyons that cut across the continental margin

in this region (Allen et al., 2001). Increased nutrients

in the photic zone subsequently support a dramatic

increase in surface primary productivity (Thomson,

1981), leading to the diatom blooms that are com-

monly observed in spring and summer in this region,

and which are ultimately deposited as the diatoma-

ceous laminae of the Effingham Inlet sediments

(Grimm et al., 1996, 1997; Kemp, 1996; Sancetta,

1996; Chang et al., 1998, 2003; Patterson et al.,

2000; McQuoid and Hobson, 2001). Upwelling of

deep shelf waters also occurs in the winter months

but because of low light levels and cool water tem-

peratures, conditions are not conducive to diatom

blooms.

The normal pattern of seasonal upwelling off Van-

couver Island can be interrupted in some years by

climatic events, such as El Nino–Southern Oscillation

(ENSO) events (Philander, 1990; Diaz and Markgraf,

1992). The effects of El Nino events are felt on

average every 3 to 7 yr in the equatorial ocean.

However, it is only major El Nino events, that occur

on average every 10 to 25 yr, that impact mid-latitude

oceanic regions, such as the B.C. shelf (Thomson,

1981; Subbotina et al., 2001). A particularly strong

El Nino event in the equatorial Pacific Ocean, which

characteristically weakens or halts upwelling along

the North American eastern boundary regions as far

north as California, leads to anomalously warm, low-

nutrient waters and reduced or altered populations of

phytoplankton and fish (Barber and Chavez, 1983;

Wilkerson et al., 1987; Lange et al., 2000; Mackas

et al., 2001). The expressions of a strong El Nino

event in B.C. coastal waters are warm nearshore

waters, and a reduced upwelling tendency along the

B.C. shelf (Ware and Thomson, 2000). At times, a

marked btransitionQ to moderate to strong La Ninas (or

cold phases) of the ENSO cycle, follows a major El

Nino event. La Nina conditions appear to be linked to

colder than normal B.C. coastal waters and enhanced

upwelling tendency (Chavez et al., 2002; Castro et al.,

2002).

3. Methods

Sediment cores were obtained in October 1999

from the inner and outer basins of Effingham Inlet

using a piston coring apparatus deployed from the

CCGS John P. Tully. The piston cores are each 10

cm in diameter and approximately 11 m in length. To

characterize the sediment–water interface, five freeze

cores (Hughen et al., 1996; Kulbe and Niederreiter,

2003) of 0.5 to 1.5 m in length were also retrieved,

three from the inner basin and two from the outer

basin core sites (Fig. 1, Table 1). Sediment slabs (20

cm length, 4 cm width, 1 cm thickness) of the entire

core from the center of one outer basin core half

(TUL99B11) and one representative inner basin core

half (TUL99B03) were X-rayed using conventional

medical X-ray equipment and Kodak min-R 2000

single screen mammography film (Pike and Kemp,

1996b; Axelsson, 2002). Total organic carbon and

grain-size analyses of the sediments were performed

at the Geological Survey of Canada in Ottawa. Grain-

size samples were dried, treated with H2O2 to remove

organic materials and then dry-sieved using a particle

analyzer (Klassen et al., 2000).

Twenty-three samples of wood and shell material

were recovered from the piston cores for conventional

and Accelerator Mass Spectrometry (AMS) radiocar-

bon dating which was performed at the IsoTrace

Radiocarbon Laboratory of the University of Toronto.

Results are reported in radiocarbon years before pres-

ent (14C yr BP) and also as calibrated calendar years

(cal yr BP), as calculated using the C14Cal program

and the INTCAL98 database for terrestrial (wood)

Table 1

Results of radiocarbon dating performed at Isotrace Radiocarbon Laboratory of the University of Toronto, as calculated using the C14Cal

program and the INTCAL98 database for terrestrial (wood) and the MARINE98 database for marine (shell) material (Stuiver et al., 1998a,b;

Stuiver and Reimer, 1986, 1993)

Core location and

water depth

(8N, 8W)

Depth in core

(m)

Material Sample

number

14C age

(yr BP)

Calibrated age and

age range at 2r(cal yr BP)

Ave. sed. rate

(mm/yr)

Core top

missing

(cm)

Inner basin

TUL97A02 14 Wood TO-8130* 940F50 894 (784–1004) ?* ?

498 04.360 1258 09.540 285 Fish bones To-8131y 3410F50 3689 (3599–3779)

100 m water depth 551 Wood TO-8132 4050F50 4582 (4464–4699)

TUL99B03 97 Wood TO-8671 160F40 195 (45–345) 2.3 102 cm

498 04.275 1258 09.359 169 Shell TO-8672 1770F60 970 (835–1105) r2= .95 (~437 yr)


553 Wood TO-8674 2830F60 2980 (2830–3130)

822 Shell TO-8675 3890F80 3435 (3250–3620)

937 Wood TO-8676 4190F80 4745 (4570–4920)

TUL99B06 252 Wood TO-8677 1080F50 1048 (975–1120) 2.8 33 cm

498 04.188 1258 09.337 276 Wood TO-8678 1340F50 1295 (1225–1365) r2= .96 (~118 yr)


1006 Wood TO-8680 3590F50 3950 (3860–4040)

TUL99B09 340 Wood TO-8681 1890F50 1870 (1760–1980) 2.6 156 cm

498 04.173 125809.279 821 Shell TO-8682 4100F60 3683(3505–3860) r2=1 (600 yr)

122 m water depth

TUL99B13 377 Wood TO-8687 1080F60 1060 (970–1150) ?** ?

498 04.173 1258 09.401 620 Wood TO-8688 2140F60 2205 (2035–2375)

122 m depth 779 Wood TO-8689 3170F50 3595 (3370–3520)

842 Wood TO-8690 3640F50 4010 (3880–4140)

Outer basin

TUL99B11 531 Shell TO-8683 2460F90 1695 (1470–1920) 6.4 ~515 cm

498 02.632 125809.23 898 Shell TO-8684 2830F60 1818 (1665–1970) r2= .45 (~1180 yr)


969 Shell TO-8686 1820F60 1048 (920–1175)

! *The least squares method does not give reliable estimates for sedimentation rate or core top loss for TUL97A02. In particular, the youngest

date, TO-8130 appears anomalously young and TO-8131y is from fish bones which have an uncertain reservoir correction, leaving only one

remaining reliable date for a least squares linear regression.

! **For these dates, the least squares method gives a negative age at the core top, therefore it is difficult to estimate an average sedimentation

rate and sediment loss at the core top for core TUL99B13.


material. The reservoir correction used for marine

shell material (R =390F25 yr ) is based on the MA-

RINE98 database for this area (Robinson and Thomp-

son, 1981; Stuiver and Braziunas, 1993; Stuiver and

Reimer, 1986, 1993; Southen et al., 1990; Stuiver et

al., 1998a,b) (Table 1). Additionally, one inner basin

freeze core (TUL99B04) was dated by cesium-137

and lead-210 dating methods (Sorgente et al., 1999)

and the 1963 Cesium peak was located at 23 cm depth

in the core (cf. Chang et al., 2003).

A total of seventeen water properties surveys in

Effingham Inlet and adjoining Barkley Sound were

undertaken between October 1995 and August 2004,

using a multi-sensor oceanographic profiling package

consisting of a Conductivity–Temperature–Depth

(CTD) probe, a 25-cm pathlength transmissometer, a

flourometer, and a 24-Niskin bottle rosette sampler for

determining concentrations of oxygen, nutrients and

other dissolved constituents. These shipboard profile

data provide along-channel sections of temperature,

salinity, density, dissolved oxygen, light attenuation

(transmissivity) and chlorophyll. Mooring strings sup-

porting current meters were deployed at the entrance

and within the inner basin of the inlet and were

Fig. 3. Oxygen and density profiles superimposed on inlet bathymetry illustrating the strong oxygenation event of January 1999 and the weak oxygenation event of July 1999, with

characteristic anoxic conditions in the inner basin returning within months. Although the oxygen profiles record these events, the stability of the density profiles indicate that density

disturbance of the bottom waters probably occurred between sampling dates.

A.Dallim

ore

etal./Marin

eGeology219(2005)47–69

52


serviced every 6 mo. Two different sequentially sam-

pling sediment traps were deployed on each mooring

between May 1999 and September 2000; a Baker-type

trap (Baker and Milburn, 1983) at approximately 40

meters above bottom (mab), and an OSU-type trap at

approximately 30 mab. Water depths at the mooring

sites were 116 m in the inner basin and 80 m at the

inlet mouth (Fig. 1).

4. Results

4.1. Physical oceanography

The main anomalous features of the along-channel

oceanographic survey were the occurrences of well-

oxygenated (low oxic) bottom waters in the inner basin

in January 1999, and again in July 1999 (Fig. 3). The

typically anoxic/dysoxic conditions characteristic of

the inlet were re-established sometime between the

October 1999 (dysoxic conditions) and March 2000

(anoxic conditions) sampling dates. In the outer basin,

higher oxygen levels were recorded periodically during

our time series pointing to enhanced oceanic influences

there which create dysoxic to oxic bottom water con-

ditions. Throughout the rest of the time series waters of

the inner and outer basins below sill depth have near-

Fig. 4. Schematic representation of conditions required to allow oxygenat

Nino to a moderate La Nina event; strong and persistent northwest coastal

weak (neap) tidal currents and bpre-conditioningQ of basin waters by vertic

after suppressed coastal upwelling associated with strong El Nino conditi

uniform temperature and salinity structure (Fig. 2)

indicating that the bottom waters in the basins are

normally highly stagnant and infrequently affected by

intrusive renewal (oxygenation) events from the outer

portion of the inlet. The oxygen profiles in the inner

basin show a progressive decrease in oxygen with

depth, creating a water column that is uniformly low

oxic (N100 Am/kg ) to dysoxic/suboxic (dissolved

O2b40 Am/kg) to anoxic (no oxygen) at the bottom.

Anoxic and dysoxic conditions of the inner and outer

basins also appear to be exacerbated by the oxygen-

consuming decomposition of the high fluxes of organic

material into the basin during the diatom blooms in

spring and summer, and also by the input of terrestrial

organic material in the fall and winter.

4.2. Conditions required for present day bottom water

oxygenation events

Oceanographic profile data collected just prior to

the oxygenation event of January 1999, highlight four

bconditionsQ that appear to be necessary to allow

oxygenated waters to penetrate to the bottom of the

inner basin (Fig. 4). These are:

1) Strong northwest coastal winds: the major up-

welling events of January and July 1999 were

ed waters to enter the inner basin after a transition from a strong El

winds, a stratified water column more pronounced by heavy rainfall,

al diffusion after a prolonged period of bottom water quiescence, i.e.

ons.


preceded by strong (N0.2 N/m2) persistent north-

west winds along the coast. Relatively high den-

sity oceanic water was transported inward toward

the inlet.

2) A stratified near-surface water column in the inlet:

high precipitation and runoff increases the strati-

fication of the upper layer of the water column

reducing vertical mixing over the sills.

3) Weak tidal currents: shear-induced vertical mixing

is minimal during weak (neap) tides, preserving the

density of the intruding bottom water and support-

ing oxygenation of the basins.

4) Pre-conditioning of the water column: an oceano-

graphic condition whereby vertical diffusion, com-

bined with weak turbulent mixing (negligible tidal

and wind-induced currents) within silled basins

gradually lowers the density of the deep and inter-

mediate waters, preparing (i.e. bpre-conditioningQ)the basin for flushing by more dense intrusive water

from the open ocean (such as by condition 1 above).

4.3. Sediment flux

Total mass flux to the upper Baker sediment trap

cylinders during the trap deployment between May

1999 and September 2000 consisted of organic ma-

terial, diatom hash and terrigenous fragments with

maximum flux values in the inner basin occurring in

the spring months, from April to June of 2000 (Fig.

5). A significant sediment flux occurred on the July

14, 1999 opening date, coinciding with the oxygen-

ation event of the inner basin on July 8, 1999

(bottom water O2=3.0 ml/L). A high sediment

flux peak (~2.5 g/m2 d) was recorded only in the

lower (OSU) sediment trap, while total fluxes to the

upper Baker trap, suspended 10 m above the lower

trap on the mooring line, remained constant. A few

weeks previous to this event on June 30, flux to

both traps were equal, but on the July 14 opening

date, the flux to the lower (OSU) sediment trap was

almost three times greater than average for the

deployment period (Fig. 5).

4.4. Late Holocene sediments

The inner basin sediments represent about 6000 yr

of deposition under predominantly anoxic conditions,

and consist of three distinct lithological facies: well-

laminated sediments (Fig. 6a), massive mud units

(Figs. 6b,c, 7a,b), and graded mud units (Fig. 6b).

The more rapidly deposited outer basin sediments

(Fig. 8) represent about 2600 yr of deposition under

oxic and dysoxic conditions since they consist of

indistinctly laminated, massive and graded mud units.

4.4.1. Geochronology

Radiocarbon dating of twenty-three samples of

shell and wood material found in the piston cores

indicates that approximately 6000 yr of deposition

were recovered in the inner basin, and about 2600

yr of deposition were recovered from the outer

basin (Table 1). The average sedimentation rates

estimated from radiocarbon ages, are 2.6 mm/yr

for the inner basin (average from 3 cores) and 6.4

mm/yr for the outer basin (Fig. 9a,b, Table 1).

However, for the inner basin, varve counting in

well-laminated sections of the cores gives a variable

average sedimentation rate of between 2.1 and 2.8

mm/yr indicating some variability in sedimentation

rates downcore. There is roughly 0.3 to 5.0 m of

sediment missing from the tops of the piston cores

(Table 1, Figs. 9a,b, 11) due to the sediment distur-

bance inherent in piston coring soft sediments

(Bornhold et al., 1998; Blais-Stevens et al., 1997,

2001). This estimate is based on a combination of

varve counting, average sedimentation rates and

radiocarbon ages.

Dating of the freeze core TUL99B04 by 137Cs and210Pb methods confirms that the laminae couplets of

Effingham Inlet, consisting of one diatom rich lami-

nae and one terrigenous-rich laminae, are annually

deposited varves (cf. Chang et al., 2003). The most

recent laminae recovered in the freeze cores has been

dated and stratigraphically located at the calendar year

1993 (Fig. 10).

4.4.2. Laminated sediments

The well-laminated sediments of the inner basin

(Fig. 6a) consist of alternating annual laminae of

olive to dark olive grey diatomaceous mud and dark

olive grey to black mud including both clay and silt

size particles (Chang et al., 2003), with an average

TOC of 5%, indicating anoxic conditions when they

were deposited. Some indistinctly laminated sedi-

ments (Fig. 8) are present in the outer basin

where the TOC is slightly lower at 4% indicating

Inlet mouth sediment traps May 2000 to September 2000

Open date

6.0

5.0

4.0

3.0

2.0

1.0

0.0

6/09/00

23/08/00

9/08/00

26/07/00

12/07/00

28/06/00

14/06/00

31/05/00

17/05/00

3/05/00

19/04/00

5/04/00

22/03/00

8/03/00

23/02/00

9/02/00

26/01/00

12/01/00

15/12/99

29/12/99

1/12/99

17/11/99

3/11/99

20/10/99

6/10/99

22/09/99

8/09/99

25/08/99

11/08/99

28/07/99

14/07/99

30/06/99

16/06/99

02/06/99

19/05/99

Inner basin sediment trapsMay 1999 to September 2000

Open date

Tot

al m

ass

flux

, g.m

2 d-1

6.0

5.0

4.0

3.0

2.0

1.0

0.0

6/09/00

23/08/00

9/08/00

26/07/00

12/07/00

28/06/00

14/06/00

31/05/00

17/05/00

3/05/00

19/04/00

5/04/00

22/03/00

8/03/00

23/02/00

9/02/00

26/01/00

12/01/00

15/12/99

29/12/99

1/12/99

17/11/99

3/11/99

20/10/99

6/10/99

22/09/99

8/09/99

25/08/99

11/08/99

28/07/99

14/07/99

30/06/99

16/06/99

02/06/99

19/05/99

July 14 opening date -more sediment in lower (OSU) trap than upper (Baker trap)

OSU trap, 30 m above basin floor

Baker trap, 40 m above basin floor

Baker trap, 40 m above basin floor

OSU trap, 30 m above basin floor

Tot

al m

ass

flux

, g.m

2 d-1

Fig. 5. Total mass flux to Baker (upper) and OSU (lower) sediment traps deployed in the inner basin and the mouth of the basin, between May

1999 and September 2000. Total mass flux peaks are more pronounced at the mouth of the inlet showing the greater sedimentation rate and

oceanic influence there. A sediment flux peak to the lower OSU trap of the inner basin on the July 14, 1999 sampling date is attributed to

sediment-laden density currents traveling along the bottom of the inlet, associated with a weak oxygenation event of the inlet in July 1999.


less organic input than in the inner basin, and

dysoxic conditions. In well-laminated sediments,

the diatomaceous laminae can be further divided

into near mono-specific laminae, mixed species lam-

inae and silty diatomaceous laminae, indicating that

the varves are recording on a microscopic level

changes in diatom-limiting factors such as nutrient

availability and light levels (cf. Chang et al., 2003).

The terrigenous laminae are mostly made up of

microscopic organic debris including robust diatoms

and silicoflagellate tests, and silt and clay minerals.

The lower contact of the terrigenous laminae is

gradational, indicating increasing precipitation in

the autumn, whereas the upper contact is sharp,

indicating an abrupt end to winter precipitation

and run-off, concurrent with increasing biological

873

880

885

890

893

104 years of continuously strongdiatom blooms abruptly give way to years of higher precipitation and lowerlight levels

167

170

176

Resuspension of laminated sedimentsbeneath small massive unit

Abrupt return toanoxic conditions

Sharp basal contact of graded unit

177

172

(b)

452

454

456

Tiny "rip-up clasts"beneath massive unit

Subtly disturbedlaminations within massive unit

(c)

(a)

Dep

th in

cor

e (c

m)

Fig. 6. X-rays from inner basin core TUL99B03 showing (a) typically well-laminated sediments illustrating the variability in thickness of

diatom varves and the abrupt change to wetter conditions when summer diatom blooms failed at 880 cm (~4250 yr BP), (b) thin

homogenite and turbidite massive units separated by about 5 yr of anoxic conditions at 172 cm, (~1100 yr BP), and (c) debris flow unit at

454 cm, (~2500 yr BP).


production as a result of increased light levels in

spring and summer (Chang et al., 2003; Hay et al.,

2003).

4.4.3. Debris flow deposits

Thin (b10 cm) dark olive grey to black massive

mud units with no sedimentary structure are common-

ly intercalated within the well-laminated sediments of

the inner basin. The units consist of a silty diatom

bhashQ containing wood fragments, organic debris,

oceanic silicoflagellates, benthic foraminifera from

oxygenated regions of the inlet, fecal pellets, and

pollen and diatom bblebsQ which indicate a distur-

bance and re-deposition of previously laminated sedi-

ments (Fig. 6c) (Chang et al., 1998, 2003). Bottom

contacts are gradational, can exhibit disturbed laminae

469

475

480

485

489

Anoxic conditions,productive summers createthick diatom laminae

Brief oxygenationevent disrupts previouslylaminated sediments belowabout 2,500 y BP

Dark colored sub-laminaeindicate summer precipitation events

Heavy precipitation winter season

(a)

Dep

th in

cor

e (c

m)

936

940

945

950

953

Part of 50 cm thick debris flow unitcontaining many wood fragmentsdeposited about 4,500 y BP

Dep

th in

cor

e (c

m)

(b)

Wood fragment

Fig. 7. X-rays from inner basin core TUL99B03 showing (a) thin homogenite units intercalated within laminated sediments about every 10 to 15

yr in this section of the core (~2500 yr BP), indicating frequent oxygenation events of the inner basin during this time, and (b) part of large

massive unit at the base of TUL99B03 which is also found in the other inner basin piston cores, representing a possible significant seismic event

that occurred about 4500 yr BP.


at the base and can show evidence of erosion. Top

contacts are sharp and well-defined indicating a rapid

return to anoxic conditions after deposition of the bed.

A thin layer of concentrated diatoms is often found at

the top of the unit. These features are also common to

thin massive mud units in the laminated sediments of

Saanich Inlet (Blais-Stevens et al., 1997, 2001) which

were interpreted as debris flow deposits (Middleton

and Hampton, 1973, 1976) originating from localized

failures of the build-up of loose water-saturated mud

on the walls of the fjord and near the mouths of

ephemeral streams. We use the same terminology for

these units in Effingham Inlet and similarly conclude

that they are probably the result of localized slope

failures.

The thin debris flow units cannot be easily cor-

related across the inlet. However one of these units

has a distinct gravel layer at the base, is traceable

across the inner basin and therefore serves as a

datum for a stratigraphic cross-section of inner

basin sediments recovered in the piston cores (Fig.

11). Two thick (N50 cm) black debris flow deposits

containing many large pieces of wood and terrige-

nous organic debris (Figs. 7b and 10) were found in

the Effingham Inlet cores and can, unlike the thin

debris flow units, be correlated across the inlet. The

black color and the many terrigenous fragments

contained in the thick debris flow units suggest a

sudden sediment transport from shallower oxic areas

of the inlet. A thick debris flow unit that occurs

near the base of the inner basin piston cores (920–

970 cm, TUL99B03; Fig. 7b) was deposited about

4500 14C yr BP, and is underlain by up to 120 cm

of disrupted and de-watered previously laminated

729

735

740

745

749

Massive mud unit representingfrequent deposition from oceanic source and possible bioturbation

Coarse grained winterlaminae about 1,800 y BP

Poorly defined laminations reflect greatersediment load from oceanic sources than in the innerbasin

Dep

th in

cor

e (c

m)

Fig. 8. Typical outer basin sediments of ~1800 yr in age from the

outer basin core TUL99B11.


sediments (Fig. 12). The other thick debris flow unit

found in Effingham Inlet occurs in all of the freeze

cores of the inner and outer basins and was depos-

ited in 1946, an age determined by varve counting,137Cs and 210Pb dating (Fig. 10).

4.4.4. Homogenites

Many of the thin massive mud units in Effing-

ham Inlet, unlike the debris flow deposits, do not

show evidence of shearing or erosion at the base,

contain no varve intraclasts and appear to consist

simply of re-worked laminated sediments (Figs. 6b

and 7a). These units are particularly concentrated in

the well-laminated sediments of the inner basin

cores of less than about 4000 14C yr BP in age

across the inner basin. We refer to these featureless

deposits as homogenites, following terminology pro-

posed by Stow and Piper (1984) and Einsele (1991),

to describe deposits from highly concentrated sus-

pensions known as fluid mud as would be common

in energetic shallow tidal waters (Piper and Stow,

1991) and as has been described at the bottom of

estuaries (McCave and Jones, 1988). Further, we

suggest below that these homogenites are the result

of re-suspension of previously laminated sediments

by bottom currents and are not the result of depo-

sition following sidewall failures from oxygenated

parts of the inlet.

4.4.5. Turbidites

Ten thin (b10 cm) graded mud units with clay

and silt size particles, dark olive-grey to black in

color, with sharp basal contacts, are intercalated

within the laminated sediments at irregular intervals,

and are identified from the X-rays of inner basin

core sediments (TUL99B03) (Fig. 6b). One distinct

thick (18 cm) graded unit occurs at the base of

some of the inner basin piston cores and is dated

at ~5250 14C yr BP (1110 cm core TUL99B03; Fig.

11). These units can contain silt and mud, fine to

coarse sand grains, terrigenous organic and shell

fragments, and occasionally gravel at the base.

The characteristics of these graded units suggest

emplacement by turbidity flows following sediment

disturbance (Bouma, 1962; Middleton and Hampton,

1973, 1976; Middleton and Southard, 1984; Collin-

son and Thompson, 1989; Einsele, 1991), possibly

from shallower oxic, or terrestrial areas of the inlet,

and we therefore refer to them as turbidites.

4.4.6. Changes in depositional conditions over the

late Holocene

A distinct change in the character of sedimenta-

tion is evident in sediments older than about 400014C yr BP in age (820–1137 cm at base of core,

TUL99B03). Uninterrupted laminated sediments

from ~4000 to 4300 14C yr BP, (820–920 cm,

TUL99B03) represent three centuries of persistent

anoxia and undisturbed annual sedimentation in the

inner basin (Fig. 6a) since notably, no debris flow,

homogenite or turbidite units are found in these

sediments. This persistent anoxia may have lasted

for almost a millennia since, beneath these uninter-

rupted laminated sediments, the record has been

disrupted by a thick (50 cm) debris flow unit con-

taining wood fragments (Fig. 7b) which is underlain

by another 120 cm (975–1096 cm, TUL99B03) of

Age (cal YBP)

1,000 2,000 3,000 4,000 5,000 6,000

200

300

400

500

600

700

800

900

1,000

1,200

1,300

1,100

0

~ 100 cmmissing

1 cm

(a) 2.7 mm/yr

1 cm

(b) 2.1 mm/yr

1 cm

(c) 2.6 mm/yr

2.3 mm/yr

r 2 = 0.94

wood

shell

Dep

th in

cor

e (c

m)

100Inner basin core TUL99B03

Age (cal YBP)

1,000 2,000 3,000 4,000

200

300

400

500

600

700

800

900

1,000

1,200

1,300

1,100

0

~ 515 cm missing

1 cm

(c) 3.1 mm/yr

(b) ? mm/yr

(a) 2.8 mm/yr

6.4 mm/yr

r2 = 0.45

wood

shell

Dep

th in

cor

e (c

m)

1 cm

1 cm

100

Outer basin coreTUL99B11

(a) (b)

Fig. 9. Plot of radiocarbon dates using the least squares method. Average sedimentation rate for (a) inner basin core TUL99B03 is 2.3 mm/yr and (b) 6.4 mm/yr for the outer basin

core TUL99B11. However, inset X-rays show the variability in sedimentation rate as derived by varve counting in laminated sections of the cores.

A.Dallim

ore

etal./Marin

eGeology219(2005)47–69

59

Fig. 10. X-ray composite of inner basin freeze core TUL99B04, showing well-laminated sediments representing uninterrupted anoxic conditions

in the inner basin from 1993 to 1947, followed by the deposition of a debris flow unit associated with the 1946 Vancouver Island magnitude 7.3

earthquake. Core was dated using 137Cs and 210Pb methods with the 1963 Cesium peak occurring at 23 cm (cf. Chang et al., 2003).


TUL99B09TUL99B13 TUL99B06 TUL99B03 TUL97A02

(~2,000 y BP)

(~4.5 m missing) (~1 m missing)(~ 1.5 m missing)(~ 33 cm missing)(~ 1 m missing)

(970-1150)

(2035-2375)

(3370-3520)

(3880-4140)

(975-1120)

(1225-1365)

(1910-2210)

(3860-4040)

(45-345)

(835-1105)

(1795-1920)

(2830-3130)

(3250-3620)

(4570-4920)

(784-1004)

(3599-3779)

(4464-4699)

(1760- 1980)

(3505-3860)

100

200

300

400

500

600

700

800

900

1,000

Fish skeleton

Fish skeleton

Fish skeleton

Fish skeleton

Fish skeleton

Fish skeleton

Dropstone

Fish skeleton

Dropstone

0

0100

200

300

400

500

600

700

800

900

1,000

1,100

0

100

200

300

400

500

600

700

800

900

1,000

1,100

Fish skeleton

Fish skeleton

Dropstone

0

100

200

300

400

500

600

700

800

Fish skeleton

Fish skeleton

0

100

200

300

400

500

600

700

800

900

1,000

1,100

cal yBP(3880-4140)

Wavy light grey mud lens

Thin (<10 cm) massive mud bed

Thick (>10 cm) massive and gradedmud beds

Disturbed laminated bed

Indistinctly to faintly laminated bed

Laminated bed

LEGEND

Sediment-waterinterface

East West

(~4,000 y BP)

Fig. 11. Stratigraphic correlation of inner basin piston cores based on a datum of a small coarse-grained turbidite unit (350 cm, TUL99B03) and

a debris flow unit underlain by disturbed laminations at the base of the cores (820 cm, TUL99B03). The amount of sediment missing from the

top of the piston cores is estimated stratigraphically.


disrupted, yet previously laminated sediments (Fig.

12) representing another lengthy interval (~450 yr)

of persistent anoxic conditions in the inner basin.

Laminated sediments of this age contain the thickest

diatomaceous laminations of the cores (Chang et al.,

2003), and the highest sedimentation rate (~2.8 mm/

974

980

985

990

Previously laminated sedimentsloose consolidation and de-waterwhen shaken, producing convoluted and disturbed laminations

Dep

th in

cor

e (c

m)

Fig. 12. X-ray of part of TUL99B03 showing disruption of previously

laminated sediments (980 cm, ~4750 yr BP), beneath a debris flow unit

identified at the base of the inner basin cores that is attributed to a

significant seismic event that occurred about 4500 yr BP.


yr) due to an increase in the average thickness of

diatom laminae in sediments.

In contrast, anoxic conditions appear to have been

frequently interrupted in the inner basin from ~400014C yr BP to the present (from 820 cm, TUL99B03).

Inner basin core sediments of this age contain forty-

five thin (up to 10 cm) debris flow and homogenite

units and ten of the eleven turbidite units found in the

inner basin cores (Fig. 11). These indicate frequent

oxygenation events and/or an increase in the incidence

of sediment disturbance and/or a change in deposi-

tional conditions affecting the consolidation and

hence stability, of the basin sediments during this

time, perhaps due to higher precipitation. Significant-

ly, only one well-laminated unit in this section of the

cores is longer than 10 cm, representing a maximum

of about 50 yr of uninterrupted anoxic conditions

during the period from about 4000 14C yr BP to the

present.

5. Discussion

5.1. Evidence for a Pacific regime change

The oceanographic conditions recorded prior to

and during the oxygenation events of January and

July 1999 (Section 4.2), followed a transition from

the strong El Nino event of 1997–98 to the moderate

La Nina event of 1998–99 (Castro et al., 2002). This

disruption of the bnormalQ (as recorded in our 9-yr

water property survey) coastal oceanographic and

climatic conditions in January 1999 which facilitated

the intrusion of oxygenated waters into the inner

basin, is an indication that coastal ocean dynamics

influencing Effingham Inlet are likely driven by oce-

anic events which are affecting the regional climate

along the continental margin. Pre-conditioning of the

deep portion of the water column within the two

basins was also likely strongly facilitated by the in-

tense 1997–98 El Nino.

Significantly, the uninterrupted laminations of the

freeze core record of modern sedimentation in Effing-

ham Inlet (Fig. 10) also indicate that the inlet was

persistently anoxic between at least 1946 and 1993,

further evidence that oxygenation of the bottom

waters in 1999 was a unique event in the past half

century. Increasingly, other oceanographic data col-

lected along a monitoring transect in the northern

Pacific by the Institute of Ocean Sciences is indicating

profound changes in ocean circulation since 1997,

further evidence that the sudden shift in coastal ocean-

ographic conditions we recorded in Effingham Inlet

may be part of a larger oceanic and climatic phenom-

ena within the North Pacific (Schwing et al., 2002).

Other circum-Pacific physical and biological evidence

(Minobe, 1999; Ware and Thomson, 2000; Bertram et

al., 2001; Chavez et al., 2003) also indicate that the

ENSO event of 1997–98 may be related to a possible

bregime shiftQ in the Pacific Ocean heralding a change

from warm to cold climatic conditions, counter to the

well-known 1976–77 bregime shiftQ from cold to

warm conditions.

5.2. Deposition of homogenites during oxygenation

events

We propose, based on the modern sediment trap

evidence, that bottom-hugging density currents asso-


ciated with oxygenation events may be capable of

carrying sediment loads close to the bottom of the

inlet into the outer basin, and infrequently to the inner

basin. These currents are then perhaps capable of

disturbing several years of recent sedimentation, cre-

ating the homogenites found in the sediment record.

We rule out bioturbation as an explanation for the

homogenites (Anderson et al., 1989; Anderson,

1996; Behl and Kennett, 1996; Kemp, 1996; Blais-

Stevens et al., 2001; Kemp, 2003) since distinct evi-

dence for bioturbation burrows (Behl and Kennett,

1996; Sageman et al., 1991; Kemp, 2003) was not

found in the X-rays of the inner basin sediments.

Furthermore, grab samples of bottom sediments of

the inner basin taken in May 1999, 4 mo after the

bottom water oxygenation event recorded in January

1999, were devoid of bioturbating benthic organisms.

The along-channel water property structure monitor-

ing program indicates that the bottom waters of the

inner basin return to their normally anoxic condition

after each intrusion within a matter of months, without

allowing sufficient time for a bioturbating benthos

community to become established in the soft sedi-

ments of the inner basin (cf. Savrda et al., 1991;

Behl and Kennett, 1996).

Although bottom currents are weak (maximum

bottom currents of Effingham Inlet are in the order

of 1 cm/s) the incoming density currents are perhaps

capable of disturbing poorly consolidated bsuspendedQsediments since unconsolidated mixed layer sedi-

ments with very high pore-fluid contents (up to 90%

near the sediment–water interface) typically have low

shear strengths (Savrda and Ozalas, 1993). The youn-

gest consolidated laminae sampled in the freeze cores

has been dated at 1993, and this indicates that sedi-

ments in the quiescent inner basin of Effingham Inlet

begin as a slurry of slowly consolidating and hence

high pore-fluid content bsuspendedQ sediments, a con-

sequence of several years of recent deposition essen-

tially still floating in the water column near the

sediment water interface. Observations from a remote-

ly operated submersible in another of our anoxic B.C.

study fjords (Mereworth Sound, B.C.) showed a

bsuspendedQ sediment layer of about 0.5 m thickness

at the basin floor.

An oceanic sediment source with incoming density

currents might also explain why in the outer basin,

where bottom waters are oxygenated more commonly

than the inner basin, the average sedimentation rate

calculated from radiocarbon dates (~6.4 mm/yr) is

more than twice that for the inner basin (~2.6 mm/

yr), despite the approximately equivalent catchment

areas of both basins. A similar phenomenon was

observed in Jervis Inlet, on the southern mainland

coast of B.C., where increased settling fluxes of sed-

iment coincided with deep-water renewal (oxygena-

tion) events (Timothy, 2001; Timothy and Soon,

2001).

5.3. Late Holocene climate interpretation

The well-laminated sediments of the inner basin

suggest that anoxic bottom waters were the norm

throughout the late Holocene. However, the clustered

occurrence of debris flow deposits, homogenites and

turbidite units in sediments younger than about 400014C yr BP, suggests variability in Holocene climate

since that time might have been associated with cli-

mate deterioration including increasing rainfall and

more frequent strong ENSO events in the southern

Pacific. This study shows that sediment disturbance in

the modern environment is associated with climatic

conditions, and other evidence also supports this con-

clusion. Microscopic analyses of the diatom laminae

of Effingham Inlet sediments shows that sediments

older than 4000 yr BP contain diatom laminae that

consistently exhibit a distinctive annual succession of

diatom species indicating conditions that were warmer

than those from 4000 yr BP to the present (Chang et

al., 2003). Absolute diatom production also appears to

have declined from 4000 yr BP to the present with a

significant decrease in the taxa generally associated

with the spring–summer bloom (M. Hay, oral com-

munication, 2004). A British Columbia Holocene

paleoclimate synthesis, based on morphological evi-

dence and pollen data from ten sites in B.C., including

one in nearby Barkley Sound (Hebda, 1995) also

indicates that a climate warmer and drier than today’s

existed from about 4000–10,500 14C yr BP although

the exact start and end times of paleoclimate states are

difficult to precisely determine from region to region.

However, the cooling at about 4000 14C yr BP from a

previous state of warmer and drier conditions, has

clear indicators along this coast with observed

changes in vegetation and neoglacial advances (R.

Hebda, oral communication).


An alternative explanation for the apparent persis-

tent anoxia of the inner basin prior to 4000 14C yr BP,

is that the inlet was isolated from the ocean at that

time during a lower sea-level stand. The inner basin

sill is only 40 m deep and post-glacial sea levels on

Vancouver Island have fluctuated rapidly, by as much

as 85 m in less than 1000 yr during isostatic response

to deglaciation in the early Holocene (Clague, 1989b).

Although the sea level history of the British Columbia

coast is notoriously complex (Clague, 1989a,b; Barrie

and Conway, 1999, 2001), the interpreted sea level

record for the nearby Tofino area just to the north of

Effingham Inlet indicates that sea level has been

dropping from a higher stand in that area since

about 4000 14C yr BP to present day sea levels

(Clague et al., 1982). A lake core record from our

on-going work in the Seymour and Belize Inlet com-

plex further to the north, also shows that sea level has

been dropping in that area (Dallimore et al., 2002).

We therefore tentatively conclude that the persistent

anoxia of the inner basin bottom waters recorded in

the Effingham cores prior to about 4000 14C yr BP is

probably representative of warmer and drier climatic

and associated oceanographic conditions and not due

to an increased physical isolation of the inner basin

from the ocean due to changes in sea level in the late

Holocene.

If the homogenites of Effingham Inlet are evi-

dence of incoming oxygenated waters advecting

over the sills, these lithological units, which are

particularly common in the sediments of less than

4000 yr of age, are a proxy for the regional ocean-

ographic and climatic conditions measured in our

oceanographic time series around the time of the

1999 oxygenation events. Similarly, transitions from

strong El Nino to moderate to strong La Nina

events were perhaps also required throughout the

late Holocene for oxygenation events of the inner

basin bottom waters, and deposition of the homo-

genite units to occur. Evidence from the Santa

Barbara Basin (Bull et al., 2000) shows that modern

interannual depositional variability within laminated

sediments in the anoxic Santa Barbara Basin asso-

ciated with ENSO effects on sedimentation, appears

to also have been common throughout the Quater-

nary (Kemp, 2003). Moy et al. (2002) show that the

frequency and strength of El Ninos in the equatorial

Pacific have varied over the past 12,000 yr with El

Nino activity occurring on a 2000 yr cycle through-

out the Holocene, with increasing ENSO event fre-

quency towards the present. We therefore conclude

that the modern oceanographic and depositional

processes in Effingham Inlet may be reliable

proxy indicators of the changes in climate and

ocean circulation associated with particularly strong

ENSO events since about 4000 yr ago in this area.

5.4. Evidence of paleoseismic events

A thick (30 cm) debris flow unit containing wood

pieces and terrestrial organics occurs in all freeze

cores of the inner and outer basins (Fig. 10), and

has been dated by 137Cs and 210Pb methods as having

been emplaced in 1946. In that year on June 23, an

earthquake of M =7.3 with an epicenter in east-central

Vancouver Island occurred. Liquefaction of sedi-

ments, resulting in significant terrestrial and subma-

rine sediment slumps and slides, were initiated on

both coasts of Vancouver Island by the seismic shak-

ing associated with this earthquake (Rogers, 1980).

We conclude that the thick debris flow unit in the

freeze cores was deposited during this earthquake and

gives a rare modern analogue of the nature of sedi-

ment disturbance in Effingham Inlet resulting from a

large (M~7) earthquake.

In analogy with the 1946 debris flow unit, we

suggest that a ~4500 14C yr BP thick (50 cm) debris

flow unit of the inner basin cores (Fig. 7b), which in

places is underlain by disturbed laminated sediments

indicating a shaking of the sediment column beyond

the sediment consolidation threshold (Fig. 12) (cf.

Clague et al., 1992), may also be associated with a

large seismic event with an epicenter close to Effing-

ham Inlet. This event has not been previously reported

in other paleoseismic proxy records of the area (cf.

Atwater, 1987; Atwater et al., 1995a,b; Blais-Stevens

et al., 1997, 2001; Clague and Bobrowsky, 1999).

The thin debris flow and turbidite units of the inner

basin cores, if they have a paleoseismic origin, are

likely associated with events with a seismic impact

less than a M =7.3 earthquake at about 80 km dis-

tance. Particularly since they are difficult to correlate

positively across the inner basin, and cannot be cor-

related to the outer basin. These units are perhaps the

result of small debris flows and turbidity currents

initiated from localized and/or isolated sediment


slumps, triggered by minor seismic disturbances or

over-steepening of sediments on the side of the basin

(cf. Blais-Stevens et al., 1997, 2001).

The average recurrence interval for moderate to

large earthquakes on southernmost Vancouver Island

as inferred from laminated sediment records in nearby

Saanich Inlet is about 150 yr (Blais-Stevens et al.,

1997; Blais-Stevens and Clague, 2001). However, this

frequency of basin-wide sediment disturbance is not

evident in the Effingham Inlet sediment record. Al-

though a full discussion of this discrepancy is beyond

the scope of this paper, we propose two possible

explanations; differing physical properties in the sedi-

ments and difference in concentration of seismic ac-

tivity between the two study areas.

Several factors indicate that although similar, the

laminated sediments of Saanich Inlet may react quite

differently to strong seismic shaking as compared to

the laminated sediments of Effingham Inlet since the

reaction of any particular sediment column to seismic

shock depends on the physical properties and coher-

ence of the sediments (Cita and Lucchi, 1984). The

sedimentation rate in Saanich Inlet (7 to 11 mm/yr,

Blais-Stevens et al., 2001) is substantially greater than

that for Effingham Inlet inner basin sediments (2.1–

2.8 mm/yr), creating a more frequently unstable sed-

iment package in Saanich Inlet. Additionally, the sedi-

ments of northern Saanich Inlet contain a triplet

component in the varve sequence, of a clay layer

associated with spring run-off of the nearby Cowichan

River, therefore they have a higher clay component

than those of Effingham and consequently different

physical properties (Blais-Stevens and Clague, 2001).

Therefore the sediments in Saanich Inlet may be more

susceptible to seismic shaking than those in Effing-

ham Inlet due to their higher accumulation rate and

difference in physical properties. This could perhaps

explain the discrepancy in the apparent frequency of

paleoseismic events recorded in the sedimentary

records of the two inlets.

Perhaps more plausible is that the significant

difference in concentration of crustal seismicity in

the two study areas may explain the apparent lower

frequency of large earthquakes in the Effingham

Inlet area. Both areas are exposed to strong shaking

from infrequent giant Cascadian subduction earth-

quakes but Saanich Inlet will be exposed to a higher

rate of strong shaking from crustal earthquakes due

to its more proximal location to the concentration of

seismicity in Southern Georgia Strait and Puget

Sound (Hyndman et al., 2003).

6. Conclusions

Oceanographic, sedimentological and micropa-

leontological evidence from Effingham Inlet reveal

that oxygenation of the bottom waters in the partially

isolated inner and outer basins has occurred compar-

atively more frequently since about 4000 14C yr BP,

indicating that the late Holocene climate along the

B.C. coast has become progressively cooler and

wetter towards the present. The occurrences of

homogenite mud units, considered to be proxy indi-

cators of sedimentation during strong ENSO events,

are assumed to mark the onset of major changes in

the coastal ocean off British Columbia throughout

the late Holocene. Therefore, strong El Nino–La

Nina transitional events were perhaps more common

since about 4000 14C yr BP. Prior to about 4000 yr

ago, uninterrupted laminated sediments indicate that

centuries of sustained bottom water anoxia occurred

in the absence of strong ENSO events, during a

climate state warmer and drier than today’s. The

recent sedimentary record indicates that the rapid

basin-scale transition from the strong 1997/1998 El

Nino event to the ensuing moderate 1998–99 La

Nina event which was associated with enhanced

coastal upwelling is the first ENSO transitional

event to affect sedimentation in the inner basin of

Effingham since at least 1946, and may signal a

modern regime shift of the North Pacific coastal

climatic and oceanographic system. Although possi-

ble evidence for two major earthquakes were also

found in the sediments, (ca. 1946, M =7.3 and ap-

proximately 4500 14C yr BP) the apparent frequency

of major pre-historic earthquakes in this area inter-

preted from other studies, about one every 150 yr,

was not found in the Effingham Inlet record. This

may reflect that the lower frequency of seismic

activity in the Effingham area and/or the subtle

differences in physical properties of annually lami-

nated marine sediments may explain the lower fre-

quency of paleoseismic activity recorded in the

Effingham Inlet sediments as compared to that inter-

preted from more southern areas of the B.C. coast.


Acknowledgements

This work was funded by an NSERC Strategic

Grant to Professor R.T. Patterson of Carleton Uni-

versity, the Geological Survey of Canada-Pacific

and the Department of Fisheries and Oceans

Canada. Many thanks to Kim Conway, Bob Mac-

Donald, Dave Spear, Hugh Maclean and the Cap-

tains and crews of the CCGS John.P. Tully for their

expert help in the field; to Brenda Burd, Cindy

Wright, Alice Chang, Murray Hay, Trecia Schell,

Ralph Beukens and Janice Baker for their assistance

in the labwork, data analyses and manuscript con-

ception; and also our appreciation to J.Vaughn Bar-

rie and Ralph Currie of the Geological Survey of

Canada for their support throughout the research

and their helpful comments during the preparation

of the manuscript.

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