Modern to Late Holocene Deposition in an Anoxic Fjord on the West Coast of Canada: Implications for Regional Oceanography, Climate and Paleoseismic History

Marine Geology 219 (2005) 47–69 www.elsevier.com/locate/margeo

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 4B2 bDepartment 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 Nin˜o event of 1997–98 to the moderate La Nin˜a 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

* Corresponding author. Fax: +1 250 363 6565. E-mail addresses: [email protected] (A. Dallimore), Annually laminated marine sediment records can [email protected] (R.E. Thomson), extend our knowledge of atmospheric and oceano- [email protected] (M.A. Bertram). graphic conditions beyond historic and instrumental

0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.05.003 48 A. Dallimore et al. / Marine Geology 219 (2005) 47–69 records, once sediment proxies for climatic and ocean- 2000). In the last decade studies of laminated marine ographic conditions are identified (Kennett and sediment records from around the world increasingly Ingram, 1995; Pike and Kemp, 1996a; Sancetta, indicate that the variability in the preservation and 1996; Kemp, 1996, 2003; Ware and Thomson, deposition of the annual winter laminae, which

EFFINGHAM

x EF01

x EF02 INLET

Effingham River floodplain 100 m isobaths 100 INNER Inner basin 50 x EF03 BASIN core site Outer basin Logged areas core site Sill: 40 m Mooring and sediment 50 trap site

50 x EF04 EF01 Oceanography

stations 100 Marsh 200 x EF05

OUTER BASIN Barkley Sound EF06 N x

0 1 2 km x EF07

49 01'N 100

EF08 x Jervis S CANADA t Inlet r a i t V 50 a n o f x EF09 c o u v G location e r e o r g Sill: 65 m i a EF10 x Enlarged area VANCOUVERVANCOUVERVANCOUVER I s l a n d y e l d rk n a u o x EF11 B S Saanich CANA Inlet DA VVICTORIAICTORIA 50 U. S. J A. uan de F PACIFIC St uca rait BARKLEY OCEAN of x EF12 SOUND 125125° 10'W10'W10'W

Fig. 1. Location map of Effingham Inlet study area showing coring sites and location of moorings, sediment traps and oceanography stations. Aerial photograph inset shows drainage courses and recent logging activity. A. Dallimore et al. / Marine Geology 219 (2005) 47–69 49 records variations in precipitation, and the spring/ mate-scale variability throughout the late Holocene. summer diatomaceous laminae, which are indicative Paleoseismic events are also recorded in the Effing- of annual productivity in the coastal ocean, archive ham Inlet sediment record (Dallimore et al., 2001) the abrupt crossing of depositional thresholds that are since periodic tectonic processes affect sediment sta- related to global climate changes (Kennett et al., 1995; bility in the inlet due to close proximity to the zone of Bull and Kemp, 1996; Hughen et al., 1996; Pike and subduction of the oceanic Juan de Fuca Plate beneath Kemp, 1996a; Schimmelmann and Lange, 1996; the continental North America plate (Rogers, 1988; Schimmelmann et al., 1998; Schultz et al., 1998; Atwater et al., 1995a,b; Hyndman, 1995). Lu¨ckge et al., 2001; Kemp, 2003). Studies of laminated marine sediments in Saanich Inlet, an anoxic British Columbia fjord located on the southwest coast 2. Study area of Vancouver Island (Fig. 1)(Gross et al., 1963; Gucluer and Gross, 1964) have revealed a high-reso- Effingham Inlet is 17 km long multiple-silled gla- lution sediment record of the entire Holocene as a cial fjord located on the southwestern coast of Van- result of ODP Leg 169S (Bornhold et al., 1998; Blais- couver Island, which opens to the Pacific Ocean Stevens et al., 1997, 2001; Blais-Stevens and Clague, through Barkley Sound. The fjord has an inner and 2001). However, Saanich Inlet is restricted from full outer basin, each more progressively restricted from open ocean processes due to its location along the the open ocean waters by shallow bedrock sills (Fig. inner coast of Vancouver Island. 1). The Effingham River, with a drainage basin of This study is located in anoxic Effingham Inlet, about 79 km2, discharges into the northern end of the British Columbia which is directly connected to the inner basin. Run-off also enters the inlet directly from open ocean (Fig. 1) and contains an annually laminat- the steep bedrock basin walls during the frequent ed sediment record spanning about 6000 yr (Patterson heavy rainfalls of fall and winter, depositing signifi- et al., 2000; Dallimore, 2001; Hay et al., 2003; Chang cant amounts of terrigenous detritus into the inlet et al., 2003). We have monitored an oceanographic (Dallimore, 2001). Water property distributions in transect in Effingham Inlet for the past 9 yr to estab- the inlet (Fig. 2) are characteristic of a weakly lish the modern oceanographic and climatic regime, mixed estuary (Thomson, 1981; Patterson et al., enabling us to infer sedimentological proxies for cli- 2000) with small (b2 m) tidal variation and negligible

0km 4 Outer Basin Inner Basin Distance Dissolved Oxygen (ml/l) 8 0 4 8 0 4 8 0 4 8 0 4

Cool/Fresh (winter) - Warm/Less Fresh (summer) Intermediate 0 Warm/Salty Barkley Sound Sill Sill 50 Cool/Salty 100

SOUTH 3.5 kHz Sub-bottom Survey NORTH 200 Approximate Depth (m)

Fig. 2. 3.5 kHZ sub-bottom survey profile of the surficial sediments of Effingham Inlet and Barkley Sound, showing Holocene sediments draped over glacially-carved bedrock substrate. Vertical exaggeration of profile about 50 X. Water property structure shows a weakly mixed characteristic estuarine stratified water column and the anoxic character of the inner basin bottom waters. 50 A. Dallimore et al. / Marine Geology 219 (2005) 47–69 currents, except in the southern oceanward portion of nutrient waters and reduced or altered populations of the inlet and in constricted narrows in the vicinity of phytoplankton and fish (Barber and Chavez, 1983; the shallow sills. Wilkerson et al., 1987; Lange et al., 2000; Mackas As part of the Coastal Upwelling Domain along the et al., 2001). The expressions of a strong El Ninõ west coast of North America, the ocean and climate event in B.C. coastal waters are warm nearshore dynamics of the study region are strongly seasonally waters, and a reduced upwelling tendency along the dependent (Thomson, 1981; Thomson and Gower, B.C. shelf (Ware and Thomson, 2000). At times, a 1998; Ware and Thomson, 2000), and influenced by marked btransitionQ to moderate to strong La Ninãs (or the relative positions of the Aleutian Low and the cold phases) of the ENSO cycle, follows a major El North Pacific High atmospheric pressure systems, Ninõ event. La Ninã conditions appear to be linked to the Jet Stream and El Ninõ/La Ninã events (Roden, colder than normal B.C. coastal waters and enhanced 1989; Patterson et al., 2000). Paleoceanographic sig- upwelling tendency (Chavez et al., 2002; Castro et al., nals recorded in the sediment record are therefore 2002). indicative of past seasonality and climate. Annual summer (May through August) upwelling of deep (N200 m) slope waters onto the Vancouver Island 3. Methods continental shelf deliver nutrient-rich waters to the upper ocean, with particularly strong upwelling-driv- Sediment cores were obtained in October 1999 en nutrient flux occurring through the numerous sub- from the inner and outer basins of Effingham Inlet marine canyons that cut across the continental margin using a piston coring apparatus deployed from the in this region (Allen et al., 2001). Increased nutrients CCGS John P. Tully. The piston cores are each 10 in the photic zone subsequently support a dramatic cm in diameter and approximately 11 m in length. To increase in surface primary productivity (Thomson, characterize the sediment–water interface, five freeze 1981), leading to the diatom blooms that are com- cores (Hughen et al., 1996; Kulbe and Niederreiter, monly observed in spring and summer in this region, 2003) of 0.5 to 1.5 m in length were also retrieved, and which are ultimately deposited as the diatoma- three from the inner basin and two from the outer ceous laminae of the Effingham Inlet sediments basin core sites (Fig. 1, Table 1). Sediment slabs (20 (Grimm et al., 1996, 1997; Kemp, 1996; Sancetta, cm length, 4 cm width, 1 cm thickness) of the entire 1996; Chang et al., 1998, 2003; Patterson et al., core from the center of one outer basin core half 2000; McQuoid and Hobson, 2001). Upwelling of (TUL99B11) and one representative inner basin core deep shelf waters also occurs in the winter months half (TUL99B03) were X-rayed using conventional but because of low light levels and cool water tem- medical X-ray equipment and Kodak min-R 2000 peratures, conditions are not conducive to diatom single screen mammography film (Pike and Kemp, blooms. 1996b; Axelsson, 2002). Total organic carbon and The normal pattern of seasonal upwelling off Van- grain-size analyses of the sediments were performed couver Island can be interrupted in some years by at the Geological Survey of Canada in Ottawa. Grain- climatic events, such as El Ninõ–Southern Oscillation size samples were dried, treated with H2O2 to remove (ENSO) events (Philander, 1990; Diaz and Markgraf, organic materials and then dry-sieved using a particle 1992). The effects of El Ninõ events are felt on analyzer (Klassen et al., 2000). average every 3 to 7 yr in the equatorial ocean. Twenty-three samples of wood and shell material However, it is only major El Ninõ events, that occur were recovered from the piston cores for conventional on average every 10 to 25 yr, that impact mid-latitude and Accelerator Mass Spectrometry (AMS) radiocar- oceanic regions, such as the B.C. shelf (Thomson, bon dating which was performed at the IsoTrace 1981; Subbotina et al., 2001). A particularly strong Radiocarbon Laboratory of the University of Toronto. El Ninõ event in the equatorial Pacific Ocean, which Results are reported in radiocarbon years before pres- characteristically weakens or halts upwelling along ent (14C yr BP) and also as calibrated calendar years the North American eastern boundary regions as far (cal yr BP), as calculated using the C14Cal program north as California, leads to anomalously warm, low- and the INTCAL98 database for terrestrial (wood) A. Dallimore et al. / Marine Geology 219 (2005) 47–69 51

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 Depth in core Material Sample 14C age Calibrated age and Ave. sed. rate Core top water depth (m) number (yr BP) age range at 2r (mm/yr) missing (8N, 8W) (cal yr BP) (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) 120 m water depth 286 Wood TO-8673 2050F70 1858 (1795–1920) 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) 121 m water depth 629 Wood TO-8679 2050F80 2060 (1910–2210) 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) 205 m water depth 939 Wood TO-8685 2570F100 2660 (2400–2920) 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 undertaken between October 1995 and August 2004, shell material (R =390F25 yr ) is based on the MA- using a multi-sensor oceanographic profiling package RINE98 database for this area (Robinson and Thomp- consisting of a Conductivity–Temperature–Depth son, 1981; Stuiver and Braziunas, 1993; Stuiver and (CTD) probe, a 25-cm pathlength transmissometer, a Reimer, 1986, 1993; Southen et al., 1990; Stuiver et flourometer, and a 24-Niskin bottle rosette sampler for al., 1998a,b)(Table 1). Additionally, one inner basin determining concentrations of oxygen, nutrients and freeze core (TUL99B04) was dated by cesium-137 other dissolved constituents. These shipboard profile and lead-210 dating methods (Sorgente et al., 1999) data provide along-channel sections of temperature, and the 1963 Cesium peak was located at 23 cm depth salinity, density, dissolved oxygen, light attenuation in the core (cf. Chang et al., 2003). (transmissivity) and chlorophyll. Mooring strings sup- A total of seventeen water properties surveys in porting current meters were deployed at the entrance Effingham Inlet and adjoining Barkley Sound were and within the inner basin of the inlet and were 52 .Dlioee l aieGooy29(05 47–69 (2005) 219 Geology Marine / al. et Dallimore A.

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. Dallimore et al. / Marine Geology 219 (2005) 47–69 53 serviced every 6 mo. Two different sequentially sam- uniform temperature and salinity structure (Fig. 2) pling sediment traps were deployed on each mooring indicating that the bottom waters in the basins are between May 1999 and September 2000; a Baker-type normally highly stagnant and infrequently affected by trap (Baker and Milburn, 1983) at approximately 40 intrusive renewal (oxygenation) events from the outer meters above bottom (mab), and an OSU-type trap at portion of the inlet. The oxygen profiles in the inner approximately 30 mab. Water depths at the mooring basin show a progressive decrease in oxygen with sites were 116 m in the inner basin and 80 m at the depth, creating a water column that is uniformly low inlet mouth (Fig. 1). oxic (N100 Am/kg ) to dysoxic/suboxic (dissolved O2 b40 Am/kg) to anoxic (no oxygen) at the bottom. Anoxic and dysoxic conditions of the inner and outer 4. Results basins also appear to be exacerbated by the oxygen- consuming decomposition of the high fluxes of organic 4.1. Physical oceanography material into the basin during the diatom blooms in spring and summer, and also by the input of terrestrial The main anomalous features of the along-channel organic material in the fall and winter. oceanographic survey were the occurrences of well- oxygenated (low oxic) bottom waters in the inner basin 4.2. Conditions required for present day bottom water in January 1999, and again in July 1999 (Fig. 3). The oxygenation events typically anoxic/dysoxic conditions characteristic of the inlet were re-established sometime between the Oceanographic profile data collected just prior to October 1999 (dysoxic conditions) and March 2000 the oxygenation event of January 1999, highlight four (anoxic conditions) sampling dates. In the outer basin, bconditionsQ that appear to be necessary to allow higher oxygen levels were recorded periodically during oxygenated waters to penetrate to the bottom of the our time series pointing to enhanced oceanic influences inner basin (Fig. 4). These are: there which create dysoxic to oxic bottom water conditions. Throughout the rest of the time series waters of 1) Strong northwest coastal winds: the major up- the inner and outer basins below sill depth have near- welling events of January and July 1999 were

Fig. 4. Schematic representation of conditions required to allow oxygenated waters to enter the inner basin after a transition from a strong El Ninõ to a moderate La Ninã event; strong and persistent northwest coastal winds, a stratified water column more pronounced by heavy rainfall, weak (neap) tidal currents and bpre-conditioningQ of basin waters by vertical diffusion after a prolonged period of bottom water quiescence, i.e. after suppressed coastal upwelling associated with strong El Ninõ conditions. 54 A. Dallimore et al. / Marine Geology 219 (2005) 47–69

preceded by strong (N0.2 N/m2) persistent north- laminated sediments (Fig. 6a), massive mud units west winds along the coast. Relatively high den- (Figs. 6b,c, 7a,b), and graded mud units (Fig. 6b). sity oceanic water was transported inward toward The more rapidly deposited outer basin sediments the inlet. (Fig. 8) represent about 2600 yr of deposition under 2) A stratified near-surface water column in the inlet: oxic and dysoxic conditions since they consist of high precipitation and runoff increases the strati- indistinctly laminated, massive and graded mud units. fication of the upper layer of the water column reducing vertical mixing over the sills. 4.4.1. Geochronology 3) Weak tidal currents: shear-induced vertical mixing Radiocarbon dating of twenty-three samples of is minimal during weak (neap) tides, preserving the shell and wood material found in the piston cores density of the intruding bottom water and support- indicates that approximately 6000 yr of deposition ing oxygenation of the basins. were recovered in the inner basin, and about 2600 4) Pre-conditioning of the water column: an oceano- yr of deposition were recovered from the outer graphic condition whereby vertical diffusion, com- basin (Table 1). The average sedimentation rates bined with weak turbulent mixing (negligible tidal estimated from radiocarbon ages, are 2.6 mm/yr and wind-induced currents) within silled basins for the inner basin (average from 3 cores) and 6.4 gradually lowers the density of the deep and inter- mm/yr for the outer basin (Fig. 9a,b, Table 1). mediate waters, preparing (i.e. bpre-conditioningQ) However, for the inner basin, varve counting in the basin for flushing by more dense intrusive water well-laminated sections of the cores gives a variable from the open ocean (such as by condition 1 above). average sedimentation rate of between 2.1 and 2.8 mm/yr indicating some variability in sedimentation 4.3. Sediment flux rates downcore. There is roughly 0.3 to 5.0 m of sediment missing from the tops of the piston cores Total mass flux to the upper Baker sediment trap (Table 1, Figs. 9a,b, 11) due to the sediment distur- cylinders during the trap deployment between May bance inherent in piston coring soft sediments 1999 and September 2000 consisted of organic ma- (Bornhold et al., 1998; Blais-Stevens et al., 1997, terial, diatom hash and terrigenous fragments with 2001). This estimate is based on a combination of maximum flux values in the inner basin occurring in varve counting, average sedimentation rates and the spring months, from April to June of 2000 (Fig. radiocarbon ages. 5). A significant sediment flux occurred on the July Dating of the freeze core TUL99B04 by 137Cs and 14, 1999 opening date, coinciding with the oxygen- 210Pb methods confirms that the laminae couplets of ation event of the inner basin on July 8, 1999 Effingham Inlet, consisting of one diatom rich lami- (bottom water O2 =3.0 ml/L). A high sediment nae and one terrigenous-rich laminae, are annually flux peak (~2.5 g/m2 d) was recorded only in the deposited varves (cf. Chang et al., 2003). The most lower (OSU) sediment trap, while total fluxes to the recent laminae recovered in the freeze cores has been upper Baker trap, suspended 10 m above the lower dated and stratigraphically located at the calendar year trap on the mooring line, remained constant. A few 1993 (Fig. 10). weeks previous to this event on June 30, flux to both traps were equal, but on the July 14 opening 4.4.2. Laminated sediments date, the flux to the lower (OSU) sediment trap was The well-laminated sediments of the inner basin almost three times greater than average for the (Fig. 6a) consist of alternating annual laminae of deployment period (Fig. 5). olive to dark olive grey diatomaceous mud and dark olive grey to black mud including both clay and silt 4.4. Late Holocene sediments size particles (Chang et al., 2003), with an average TOC of 5%, indicating anoxic conditions when they The inner basin sediments represent about 6000 yr were deposited. Some indistinctly laminated sedi- of deposition under predominantly anoxic conditions, ments (Fig. 8) are present in the outer basin and consist of three distinct lithological facies: well- where the TOC is slightly lower at 4% indicating A. Dallimore et al. / Marine Geology 219 (2005) 47–69 55

6.0 Inner basin sediment traps 5.0 May 1999 to September 2000 July 14 opening date -more sediment in lower (OSU) trap than

d-1 2 4.0 upper (Baker trap)

Baker trap, 40 m above basin floor

x, g.m 3.0 OSU trap, 30 m above basin floor

2.0

Total mass flu 1.0

0.0

19/05/9902/06/9916/06/9930/06/9914/07/9928/07/9911/08/9925/08/998/09/9922/09/996/10/9920/10/993/11/9917/11/991/12/9915/12/9929/12/9912/01/0026/01/009/02/0023/02/008/03/0022/03/005/04/0019/04/003/05/0017/05/0031/05/0014/06/0028/06/0012/07/0026/07/009/08/0023/08/006/09/00 Open date 6.0 Baker trap, 40 m above basin floor Inlet mouth sediment traps 5.0 OSU trap, 30 m above basin floor May 2000 to September 2000

d-1 2 4.0

3.0

2.0

Total mass flux, g.m 1.0

0.0

9 9 9 0 0 0 0 0 0 0 0 0 0 0

19/05/902/06/9916/06/9930/06/9914/07/9928/07/9911/08/9925/08/998/09/9922/09/9 6/10/9920/10/993/11/9917/11/991/12/9915/12/9929/12/912/01/026/01/0 9/02/0023/02/008/03/0022/03/0 5/04/0019/04/03/05/0017/05/031/05/014/06/028/06/012/07/026/07/0 9/08/0023/08/0 6/09/00 Open date

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 The terrigenous laminae are mostly made up of dysoxic conditions. In well-laminated sediments, microscopic organic debris including robust diatoms the diatomaceous laminae can be further divided and silicoflagellate tests, and silt and clay minerals. into near mono-specific laminae, mixed species lam- The lower contact of the terrigenous laminae is inae and silty diatomaceous laminae, indicating that gradational, indicating increasing precipitation in the varves are recording on a microscopic level the autumn, whereas the upper contact is sharp, changes in diatom-limiting factors such as nutrient indicating an abrupt end to winter precipitation availability and light levels (cf. Chang et al., 2003). and run-off, concurrent with increasing biological 56 A. Dallimore et al. / Marine Geology 219 (2005) 47–69

Tiny "rip-up clasts" (a) beneath massive unit 873 454 Subtly disturbed laminations within massive unit

456

104 years of continuously strong diatom blooms abruptly give way to years of higher precipitation and lower light levels 880

(b) 167

Resuspension of laminated sediments beneath small massive unit 170 885

Depth in core (cm) Abrupt return to 172 anoxic conditions

Sharp basal contact 890 of graded unit 176 177

893

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 the inner basin. The units consist of a silty diatom spring and summer (Chang et al., 2003; Hay et al., bhashQ containing wood fragments, organic debris, 2003). oceanic silicoflagellates, benthic foraminifera from oxygenated regions of the inlet, fecal pellets, and 4.4.3. Debris flow deposits pollen and diatom bblebsQ which indicate a distur- Thin (b10 cm) dark olive grey to black massive bance and re-deposition of previously laminated sedi- mud units with no sedimentary structure are common- ments (Fig. 6c) (Chang et al., 1998, 2003). Bottom ly intercalated within the well-laminated sediments of contacts are gradational, can exhibit disturbed laminae A. Dallimore et al. / Marine Geology 219 (2005) 47–69 57

(a) (b) 469 936

Heavy precipitation winter season 940 475 Dark colored sub-laminae indicate summer precipitation events

Brief oxygenation event disrupts previously laminated sediments below about 2,500 y BP Depth in core (cm) Part of 50 cm thick debris flow unit Depth in core (cm) 480 945 containing many wood fragments deposited about 4,500 y BP

Anoxic conditions, productive summers create thick diatom laminae Wood fragment

485

950

489 953

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 has a distinct gravel layer at the base, is traceable contacts are sharp and well-defined indicating a rapid across the inner basin and therefore serves as a return to anoxic conditions after deposition of the bed. datum for a stratigraphic cross-section of inner A thin layer of concentrated diatoms is often found at basin sediments recovered in the piston cores (Fig. the top of the unit. These features are also common to 11). Two thick (N50 cm) black debris flow deposits thin massive mud units in the laminated sediments of containing many large pieces of wood and terrige- Saanich Inlet (Blais-Stevens et al., 1997, 2001) which nous organic debris (Figs. 7b and 10) were found in were interpreted as debris flow deposits (Middleton the Effingham Inlet cores and can, unlike the thin and Hampton, 1973, 1976) originating from localized debris flow units, be correlated across the inlet. The failures of the build-up of loose water-saturated mud black color and the many terrigenous fragments on the walls of the fjord and near the mouths of contained in the thick debris flow units suggest a ephemeral streams. We use the same terminology for sudden sediment transport from shallower oxic areas these units in Effingham Inlet and similarly conclude of the inlet. A thick debris flow unit that occurs that they are probably the result of localized slope near the base of the inner basin piston cores (920– failures. 970 cm, TUL99B03; Fig. 7b) was deposited about The thin debris flow units cannot be easily cor- 4500 14C yr BP, and is underlain by up to 120 cm related across the inlet. However one of these units of disrupted and de-watered previously laminated 58 A. Dallimore et al. / Marine Geology 219 (2005) 47–69

729 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 Massive mud unit representing estuaries (McCave and Jones, 1988). Further, we frequent deposition from oceanic source and possible suggest below that these homogenites are the result bioturbation of re-suspension of previously laminated sediments by bottom currents and are not the result of depo- 735 sition following sidewall failures from oxygenated parts of the inlet.

4.4.5. Turbidites Ten thin (b10 cm) graded mud units with clay Coarse grained winter laminae about 1,800 y BP and silt size particles, dark olive-grey to black in 740 color, with sharp basal contacts, are intercalated Depth in core (cm) within the laminated sediments at irregular intervals, and are identified from the X-rays of inner basin Poorly defined laminations reflect greater core sediments (TUL99B03) (Fig. 6b). One distinct sediment load from oceanic thick (18 cm) graded unit occurs at the base of sources than in the inner some of the inner basin piston cores and is dated basin at ~5250 14C yr BP (1110 cm core TUL99B03; Fig. 745 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 749 emplacement by turbidity flows following sediment disturbance (Bouma, 1962; Middleton and Hampton, Fig. 8. Typical outer basin sediments of ~1800 yr in age from the 1973, 1976; Middleton and Southard, 1984; Collin- outer basin core TUL99B11. son and Thompson, 1989; Einsele, 1991), possibly from shallower oxic, or terrestrial areas of the inlet, sediments (Fig. 12). The other thick debris flow unit and we therefore refer to them as turbidites. found in Effingham Inlet occurs in all of the freeze cores of the inner and outer basins and was depos- 4.4.6. Changes in depositional conditions over the ited in 1946, an age determined by varve counting, late Holocene 137Cs and 210Pb dating (Fig. 10). A distinct change in the character of sedimentation is evident in sediments older than about 4000 4.4.4. Homogenites 14C yr BP in age (820–1137 cm at base of core, Many of the thin massive mud units in Effing- TUL99B03). Uninterrupted laminated sediments ham Inlet, unlike the debris flow deposits, do not from ~4000 to 4300 14C yr BP, (820–920 cm, show evidence of shearing or erosion at the base, TUL99B03) represent three centuries of persistent contain no varve intraclasts and appear to consist anoxia and undisturbed annual sedimentation in the simply of re-worked laminated sediments (Figs. 6b inner basin (Fig. 6a) since notably, no debris flow, and 7a). These units are particularly concentrated in homogenite or turbidite units are found in these the well-laminated sediments of the inner basin sediments. This persistent anoxia may have lasted cores of less than about 4000 14CyrBPinage for almost a millennia since, beneath these uninter- across the inner basin. We refer to these featureless rupted laminated sediments, the record has been deposits as homogenites, following terminology pro- disrupted by a thick (50 cm) debris flow unit con- posed by Stow and Piper (1984) and Einsele (1991), taining wood fragments (Fig. 7b) which is underlain to describe deposits from highly concentrated sus- by another 120 cm (975–1096 cm, TUL99B03) of (a) (b) Age (cal YBP) Age (cal YBP) ~ 100 cm missing 1,000 ~ 515 cm missing 2,000 3,000 4,000 5,000 6,000 1,000 2,000 3,000 4,000 0 0

100 (a) 2.8 mm/yr 1 cm (a) 2.7 mm/yr Inner basin core 100 TUL99B03 200 200 .Dlioee l aieGooy29(05 47–69 (2005) 219 Geology Marine / al. et Dallimore A.

300 300

400 400 1 cm

500 500 (b) ? mm/yr 600 600 2.3 mm/yr (b) 2.1 mm/yr 700 r 2 = 0.94 700 6.4 mm/yr 1 cm Depth in core (cm)

Depth in core (cm) 2 800 800 r = 0.45

1,000 1,000 Outer basin core 1,100 1,100 1 cm TUL99B11 1,200 1,200 wood shell 1 cm 1,300 wood 1,300 shell 1 cm

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. 59 60 A. Dallimore et al. / Marine Geology 219 (2005) 47–69

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). A. Dallimore et al. / Marine Geology 219 (2005) 47–69 61

East West

Sediment-water TUL99B13 TUL99B06 TUL99B03 TUL97A02 TUL99B09 interface (~ 1 m missing) (~ 33 cm missing) (~ 1.5 m missing) (~4.5 m missing) (~1 m missing) 0 0 0 100 0 (45-345) 100 Fish skeleton 100 200 100 200 (975-1120) (835-1105) 200 (1225-1365) 300 (1795-1920) 200 300 300 (1760- 1980) 400 300 (970-1150) 400 (~2,000 y BP) 400 0 500 Dropstone 400 Fish skeleton Fish skeleton 500 500 Fish skeleton 600 100 500 Fish skeleton (784-1004) (1910-2210) 600 Fish skeleton 600 (2830-3130) 200 700 (2035-2375) Fish skeleton 600 Fish skeleton Dropstone (3599-3779) 700 700 800 300 700 Fish skeleton Fish skeleton (3370-3520) 800 800 900 400 (3505-3860) 800 Fish skeleton (3880-4140) (3250-3620) (~4,000 y BP) 900 900 1,000 500 (3860-4040) 900 (4464-4699) (4570-4920) 1,000 1,000 600 1,100 1,000 Dropstone 1,100 LEGEND 700 1,100 Disturbed laminated bed

Indistinctly to faintly laminated bed 800 Laminated bed Thin (<10 cm) massive mud bed Thick (>10 cm) massive and graded mud beds Wavy light grey mud lens (3880-4140) cal yBP

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. Laminated sediments of this age contain the thickest 12) representing another lengthy interval (~450 yr) diatomaceous laminations of the cores (Chang et al., of persistent anoxic conditions in the inner basin. 2003), and the highest sedimentation rate (~2.8 mm/ 62 A. Dallimore et al. / Marine Geology 219 (2005) 47–69

5. Discussion 974 5.1. Evidence for a Pacific regime change

The oceanographic conditions recorded prior to and during the oxygenation events of January and

Previously laminated sediments July 1999 (Section 4.2), followed a transition from loose consolidation and de-water the strong El Nin˜o event of 1997–98 to the moderate 980 when shaken, producing convoluted La Nin˜a event of 1998–99 (Castro et al., 2002). This and disturbed laminations 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

Depth in core (cm) basin, is an indication that coastal ocean dynamics 985 influencing Effingham Inlet are likely driven by oceanic 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 Nin˜o. 990 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, Fig. 12. X-ray of part of TUL99B03 showing disruption of previously further evidence that oxygenation of the bottom laminated sediments (980 cm, ~4750 yr BP), beneath a debris flow unit waters in 1999 was a unique event in the past half identified at the base of the inner basin cores that is attributed to a significant seismic event that occurred about 4500 yr BP. century. Increasingly, other oceanographic data collected along a monitoring transect in the northern Pacific by the Institute of Ocean Sciences is indicating yr) due to an increase in the average thickness of profound changes in ocean circulation since 1997, diatom laminae in sediments. further evidence that the sudden shift in coastal ocean- In contrast, anoxic conditions appear to have been ographic conditions we recorded in Effingham Inlet frequently interrupted in the inner basin from ~4000 may be part of a larger oceanic and climatic phenom- 14C yr BP to the present (from 820 cm, TUL99B03). ena within the North Pacific (Schwing et al., 2002). Inner basin core sediments of this age contain forty- Other circum-Pacific physical and biological evidence five thin (up to 10 cm) debris flow and homogenite (Minobe, 1999; Ware and Thomson, 2000; Bertram et units and ten of the eleven turbidite units found in the al., 2001; Chavez et al., 2003) also indicate that the inner basin cores (Fig. 11). These indicate frequent ENSO event of 1997–98 may be related to a possible oxygenation events and/or an increase in the incidence bregime shiftQ in the Pacific Ocean heralding a change of sediment disturbance and/or a change in deposi- from warm to cold climatic conditions, counter to the tional conditions affecting the consolidation and well-known 1976–77 bregime shiftQ from cold to hence stability, of the basin sediments during this warm conditions. time, perhaps due to higher precipitation. Significant- ly, only one well-laminated unit in this section of the 5.2. Deposition of homogenites during oxygenation cores is longer than 10 cm, representing a maximum events of about 50 yr of uninterrupted anoxic conditions during the period from about 4000 14CyrBPtothe We propose, based on the modern sediment trap present. evidence, that bottom-hugging density currents asso- A. Dallimore et al. / Marine Geology 219 (2005) 47–69 63 ciated with oxygenation events may be capable of than the inner basin, the average sedimentation rate carrying sediment loads close to the bottom of the calculated from radiocarbon dates (~6.4 mm/yr) is inlet into the outer basin, and infrequently to the inner more than twice that for the inner basin (~2.6 mm/ basin. These currents are then perhaps capable of yr), despite the approximately equivalent catchment disturbing several years of recent sedimentation, cre- areas of both basins. A similar phenomenon was ating the homogenites found in the sediment record. observed in Jervis Inlet, on the southern mainland We rule out bioturbation as an explanation for the coast of B.C., where increased settling fluxes of sed- homogenites (Anderson et al., 1989; Anderson, iment coincided with deep-water renewal (oxygena- 1996; Behl and Kennett, 1996; Kemp, 1996; Blais- tion) events (Timothy, 2001; Timothy and Soon, Stevens et al., 2001; Kemp, 2003) since distinct evi- 2001). dence for bioturbation burrows (Behl and Kennett, 1996; Sageman et al., 1991; Kemp, 2003) was not 5.3. Late Holocene climate interpretation found in the X-rays of the inner basin sediments. Furthermore, grab samples of bottom sediments of The well-laminated sediments of the inner basin the inner basin taken in May 1999, 4 mo after the suggest that anoxic bottom waters were the norm bottom water oxygenation event recorded in January throughout the late Holocene. However, the clustered 1999, were devoid of bioturbating benthic organisms. occurrence of debris flow deposits, homogenites and The along-channel water property structure monitor- turbidite units in sediments younger than about 4000 ing program indicates that the bottom waters of the 14C yr BP, suggests variability in Holocene climate inner basin return to their normally anoxic condition since that time might have been associated with cli- after each intrusion within a matter of months, without mate deterioration including increasing rainfall and allowing sufficient time for a bioturbating benthos more frequent strong ENSO events in the southern community to become established in the soft sedi- Pacific. This study shows that sediment disturbance in ments of the inner basin (cf. Savrda et al., 1991; the modern environment is associated with climatic Behl and Kennett, 1996). conditions, and other evidence also supports this con- Although bottom currents are weak (maximum clusion. Microscopic analyses of the diatom laminae bottom currents of Effingham Inlet are in the order of Effingham Inlet sediments shows that sediments of 1 cm/s) the incoming density currents are perhaps older than 4000 yr BP contain diatom laminae that capable of disturbing poorly consolidated bsuspendedQ consistently exhibit a distinctive annual succession of sediments since unconsolidated mixed layer sedi- diatom species indicating conditions that were warmer ments with very high pore-fluid contents (up to 90% than those from 4000 yr BP to the present (Chang et near the sediment–water interface) typically have low al., 2003). Absolute diatom production also appears to shear strengths (Savrda and Ozalas, 1993). The youn- have declined from 4000 yr BP to the present with a gest consolidated laminae sampled in the freeze cores significant decrease in the taxa generally associated has been dated at 1993, and this indicates that sedi- with the spring–summer bloom (M. Hay, oral comments in the quiescent inner basin of Effingham Inlet munication, 2004). A British Columbia Holocene begin as a slurry of slowly consolidating and hence paleoclimate synthesis, based on morphological evi- high pore-fluid content bsuspendedQ sediments, a con- dence and pollen data from ten sites in B.C., including sequence of several years of recent deposition essen- one in nearby Barkley Sound (Hebda, 1995) also tially still floating in the water column near the indicates that a climate warmer and drier than today’s sediment water interface. Observations from a remote- existed from about 4000–10,500 14C yr BP although ly operated submersible in another of our anoxic B.C. the exact start and end times of paleoclimate states are study fjords (Mereworth Sound, B.C.) showed a difficult to precisely determine from region to region. bsuspendedQ sediment layer of about 0.5 m thickness However, the cooling at about 4000 14C yr BP from a at the basin floor. previous state of warmer and drier conditions, has An oceanic sediment source with incoming density clear indicators along this coast with observed currents might also explain why in the outer basin, changes in vegetation and neoglacial advances (R. where bottom waters are oxygenated more commonly Hebda, oral communication). 64 A. Dallimore et al. / Marine Geology 219 (2005) 47–69

An alternative explanation for the apparent persis- Ninõ activity occurring on a 2000 yr cycle through- tent anoxia of the inner basin prior to 4000 14CyrBP, out the Holocene, with increasing ENSO event fre- is that the inlet was isolated from the ocean at that quency towards the present. We therefore conclude time during a lower sea-level stand. The inner basin that the modern oceanographic and depositional sill is only 40 m deep and post-glacial sea levels on processes in Effingham Inlet may be reliable Vancouver Island have fluctuated rapidly, by as much proxy indicators of the changes in climate and as 85 m in less than 1000 yr during isostatic response ocean circulation associated with particularly strong to deglaciation in the early Holocene (Clague, 1989b). ENSO events since about 4000 yr ago in this area. Although the sea level history of the British Columbia coast is notoriously complex (Clague, 1989a,b; Barrie 5.4. Evidence of paleoseismic events and Conway, 1999, 2001), the interpreted sea level record for the nearby Tofino area just to the north of A thick (30 cm) debris flow unit containing wood Effingham Inlet indicates that sea level has been pieces and terrestrial organics occurs in all freeze dropping from a higher stand in that area since cores of the inner and outer basins (Fig. 10), and about 4000 14C yr BP to present day sea levels has been dated by 137Cs and 210Pb methods as having (Clague et al., 1982). A lake core record from our been emplaced in 1946. In that year on June 23, an on-going work in the Seymour and Belize Inlet com- earthquake of M =7.3 with an epicenter in east-central plex further to the north, also shows that sea level has Vancouver Island occurred. Liquefaction of sedi- been dropping in that area (Dallimore et al., 2002). ments, resulting in significant terrestrial and subma- We therefore tentatively conclude that the persistent rine sediment slumps and slides, were initiated on anoxia of the inner basin bottom waters recorded in both coasts of Vancouver Island by the seismic shak- the Effingham cores prior to about 4000 14CyrBPis ing associated with this earthquake (Rogers, 1980). probably representative of warmer and drier climatic We conclude that the thick debris flow unit in the and associated oceanographic conditions and not due freeze cores was deposited during this earthquake and to an increased physical isolation of the inner basin gives a rare modern analogue of the nature of sedi- from the ocean due to changes in sea level in the late ment disturbance in Effingham Inlet resulting from a Holocene. large (M~7) earthquake. If the homogenites of Effingham Inlet are evi- In analogy with the 1946 debris flow unit, we dence of incoming oxygenated waters advecting suggest that a ~4500 14C yr BP thick (50 cm) debris over the sills, these lithological units, which are flow unit of the inner basin cores (Fig. 7b), which in particularly common in the sediments of less than places is underlain by disturbed laminated sediments 4000 yr of age, are a proxy for the regional ocean- indicating a shaking of the sediment column beyond ographic and climatic conditions measured in our the sediment consolidation threshold (Fig. 12) (cf. oceanographic time series around the time of the Clague et al., 1992), may also be associated with a 1999 oxygenation events. Similarly, transitions from large seismic event with an epicenter close to Effing- strong El Ninõ to moderate to strong La Ninã ham Inlet. This event has not been previously reported events were perhaps also required throughout the in other paleoseismic proxy records of the area (cf. late Holocene for oxygenation events of the inner Atwater, 1987; Atwater et al., 1995a,b; Blais-Stevens basin bottom waters, and deposition of the homo- et al., 1997, 2001; Clague and Bobrowsky, 1999). genite units to occur. Evidence from the Santa The thin debris flow and turbidite units of the inner Barbara Basin (Bull et al., 2000) shows that modern basin cores, if they have a paleoseismic origin, are interannual depositional variability within laminated likely associated with events with a seismic impact sediments in the anoxic Santa Barbara Basin asso- less than a M =7.3 earthquake at about 80 km dis- ciated with ENSO effects on sedimentation, appears tance. Particularly since they are difficult to correlate to also have been common throughout the Quater- positively across the inner basin, and cannot be cor- nary (Kemp, 2003). Moy et al. (2002) show that the related to the outer basin. These units are perhaps the frequency and strength of El Ninõs in the equatorial result of small debris flows and turbidity currents Pacific have varied over the past 12,000 yr with El initiated from localized and/or isolated sediment A. Dallimore et al. / Marine Geology 219 (2005) 47–69 65 slumps, triggered by minor seismic disturbances or to its more proximal location to the concentration of over-steepening of sediments on the side of the basin seismicity in Southern Georgia Strait and Puget (cf. Blais-Stevens et al., 1997, 2001). Sound (Hyndman et al., 2003). The average recurrence interval for moderate to large earthquakes on southernmost Vancouver Island as inferred from laminated sediment records in nearby 6. Conclusions Saanich Inlet is about 150 yr (Blais-Stevens et al., 1997; Blais-Stevens and Clague, 2001). However, this Oceanographic, sedimentological and micropa- frequency of basin-wide sediment disturbance is not leontological evidence from Effingham Inlet reveal evident in the Effingham Inlet sediment record. Al- that oxygenation of the bottom waters in the partially though a full discussion of this discrepancy is beyond isolated inner and outer basins has occurred compar- the scope of this paper, we propose two possible atively more frequently since about 4000 14CyrBP, explanations; differing physical properties in the sedi- indicating that the late Holocene climate along the ments and difference in concentration of seismic ac- B.C. coast has become progressively cooler and tivity between the two study areas. wetter towards the present. The occurrences of Several factors indicate that although similar, the homogenite mud units, considered to be proxy indi- laminated sediments of Saanich Inlet may react quite cators of sedimentation during strong ENSO events, differently to strong seismic shaking as compared to are assumed to mark the onset of major changes in the laminated sediments of Effingham Inlet since the the coastal ocean off British Columbia throughout reaction of any particular sediment column to seismic the late Holocene. Therefore, strong El Ninõ–La shock depends on the physical properties and coher- Ninã transitional events were perhaps more common ence of the sediments (Cita and Lucchi, 1984). The since about 4000 14C yr BP. Prior to about 4000 yr sedimentation rate in Saanich Inlet (7 to 11 mm/yr, ago, uninterrupted laminated sediments indicate that Blais-Stevens et al., 2001) is substantially greater than centuries of sustained bottom water anoxia occurred that for Effingham Inlet inner basin sediments (2.1– in the absence of strong ENSO events, during a 2.8 mm/yr), creating a more frequently unstable sed- climate state warmer and drier than today’s. The iment package in Saanich Inlet. Additionally, the sedi- recent sedimentary record indicates that the rapid ments of northern Saanich Inlet contain a triplet basin-scale transition from the strong 1997/1998 El component in the varve sequence, of a clay layer Ninõ event to the ensuing moderate 1998–99 La associated with spring run-off of the nearby Cowichan Ninã event which was associated with enhanced River, therefore they have a higher clay component coastal upwelling is the first ENSO transitional than those of Effingham and consequently different event to affect sedimentation in the inner basin of physical properties (Blais-Stevens and Clague, 2001). Effingham since at least 1946, and may signal a Therefore the sediments in Saanich Inlet may be more modern regime shift of the North Pacific coastal susceptible to seismic shaking than those in Effing- climatic and oceanographic system. Although possi- ham Inlet due to their higher accumulation rate and ble evidence for two major earthquakes were also difference in physical properties. This could perhaps found in the sediments, (ca. 1946, M =7.3 and ap- explain the discrepancy in the apparent frequency of proximately 4500 14C yr BP) the apparent frequency paleoseismic events recorded in the sedimentary of major pre-historic earthquakes in this area inter- records of the two inlets. preted from other studies, about one every 150 yr, Perhaps more plausible is that the significant was not found in the Effingham Inlet record. This difference in concentration of crustal seismicity in may reflect that the lower frequency of seismic the two study areas may explain the apparent lower activity in the Effingham area and/or the subtle frequency of large earthquakes in the Effingham differences in physical properties of annually lami- Inlet area. Both areas are exposed to strong shaking nated marine sediments may explain the lower fre- from infrequent giant Cascadian subduction earth- quency of paleoseismic activity recorded in the quakes but Saanich Inlet will be exposed to a higher Effingham Inlet sediments as compared to that inter- rate of strong shaking from crustal earthquakes due preted from more southern areas of the B.C. coast. 66 A. Dallimore et al. / Marine Geology 219 (2005) 47–69

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