Sediments of Saanich Inlet, British Columbia, contain a virtually continuous record of Holocene climatic and oceanographic change, with seasonal resolution, together with a possible record of paleoseismicity associated with the Cascadia convergent margin. Situated in a fjord near Victoria, British Columbia, the proposed sites record both terrestrial floral change since deglaciation (9000 to 11,000 m.y. ago) as well as marine biological productivity variations in a temperate latitude coastal setting. The Saanich Inlet sites will provide an important complement to the high-resolution record obtained at Leg 146 Site 893 in the Santa Barbara Basin.
Study Area
Saanich Inlet, a fjord in southeastern Vancouver Island, lies at 48°35'N and 123°30'W (Fig. 1). Its width varies from 0.4 to 7.6 km over its 26-km length. The average depth is 120 m and the maximum depth is 236 m. Unlike most other fjords in British Columbia and Alaska, the drainage basin for the Saanich Inlet is relatively small and contains no significant rivers. While a small amount of sediment enters from the south, at the head of the inlet, through the Goldstream River, most of the 9 x 104 tons of terrigenous sediment deposited annually in the fjord comes from the sediment plume of the Cowichan River, which enters Satellite Channel to the northwest of the head of the inlet (Gross et al., 1963); a minor amount comes from the Fraser River plume which, on rare occasions, penetrates into the inlet.
A bedrock sill at the north end of Saanich Inlet rises to within 70 m of the surface and restricts normal water circulation. As a result, the lower part of the water column in the inlet is anoxic; the boundary between oxygenated and anoxic waters lies between 70 m water depth in October and 150 m in December (Gross et al., 1963). The sill also prevents any axial input of coarse sediments into the fjord as turbidity currents from the north (Gucluer and Gross, 1964).
Sediments
Holocene sediments in Saanich Inlet consist primarily of silt and clay deposited during fall and spring freshets and diatoms deposited during spring and summer blooms. Sediments are rhythmically laminated; individual couplets have been shown to be annual deposits and are thus termed varves (Gross et al., 1963; Sancetta and Calvert, 1988; Bobrowsky and Clague, 1990; Blais, 1992). The varved sequences are interbedded with sporadic massive beds from a few centimeters to tens of centimeters thick (Buddemeier, 1969; Powys, 1987; Bobrowsky and Clague, 1990; Bobrowsky et al., 1993; Blais, 1992) (Figs. 2,3,4). These beds are coarser than the laminated sequence and have been interpreted as the products of sediment gravity flows (Bobrowsky and Clague, 1990; Blais, 1992). Because of the prevailing anoxic conditions there is an absence of epifauna and infauna, accounting for the excellent preservation of strata observed in piston and gravity cores from the fjord.
There is a trend from more organic-rich basinal sediments in the southern part of the inlet to less in the north, reflecting the greater influence of terrigenous sediment coming from the Cowichan River to the northwest of the mouth of Saanich Inlet and possibly from the Fraser River. Between latitudes 48°34'N to 48°38'N, there is a decrease in the average organic carbon value in the upper 2.5 m of sediment from 4.57% to 2.56% (Brown et al., 1972). The greater contribution of terrigenous sediment in the northern part of the inlet is reflected in the 20% to 25% greater thickness of Holocene sediments compared with the southern part (Figs. 3,4).
Surficial sediments are extremely unconsolidated such that precise determination of the sediment/water interface is often difficult. As a result of the high water contents of surficial sediments, approximately 25 to 50 yr of the sediment record are commonly missed during conventional piston coring (A. Blais, person. comm., 1994).
Based on a single sample obtained in coring from a barge in the 1960s, it is anticipated that the Holocene sediments described here in are immediately underlain by stiff gray glaciomarine muds. It is expected that beneath, and possibly interstratified with, these glaciomarine muds are glacial outwash sands and gravels related to regional deglaciation 10,000 to 13,000 m.y. ago The possibility that interstadial and even interglacial deposits are preserved in this section also exists, though this is unlikely. Because of the high strength of the glaciomarine sediments and the likely abundance of sand, gravels, and boulders of ice-rafted and ice-contact origin, it is unlikely that piston coring will be possible much below the base of the Holocene muds.
Accumulation Rates
Sedimentation rates for the varved sediments have been estimated, based primarily on relatively short cores representing at most 4,600 yr, to have been between 4 and 6 mm/yr (Gucluer and Gross, 1964). These authors estimate that between 0.24 and 0.36 g of dry sediment are deposited annually per square centimeter. Bobrowsky and Clague (1990) reported an average couplet (varve) thicknesses over the past 1400 yr of 4 mm; overall sedimentation rates during this period, including the intercalated more massive silty units, averaged about 8.84 m/1000 yr. Gucluer and Gross (1964) presented radiocarbon data for the past 3100 yr that yield sedimentation rates of 4.1 to 6.3 m/1000 yr. Buddemeier (1969), from a core located slightly closer to the Goldstream River delta at the southern end of the inlet, obtained average varve thicknesses (i.e., annual accumulation) of 5.7 mm at about 18 m depth in the core to about 14.2 mm at 3 m depth; this difference was found to be in general accord with sediment compaction. He determined average accumulation rates based on radiocarbon dating over the past 3860 years to be 4.89 m/1000 yr. Blais (1995) showed that uncompacted varves range from about 4.5 mm in the southern inlet to 12.7 mm in the north, whereas compacted varves range from 4.4 to about 9.0 mm. Between the two proposed sites there is approximately a twofold increase in varve thickness, most of which is attributable to a greater input of terrigenous sediment from the Cowichan River.
Sediment Thickness
Buddemeier (1969), reporting on a site in the narrowest part of the inlet, estimated a thickness of 30 to 50 m based on geophysical results and in one composite core obtained from a drilling barge, encountered indurated gray muds (presumably glaciomarine) at about 38 m with overlying sediments dated at 9000 m.y. ago. Although the thickness of sediments in the deeper and broader regions of the inlet are somewhat greater than this (Figs. 2,3,4), it is unlikely that the total Holocene thickness will be much greater than 100 to 120 m.