The Juan de Fuca Ridge is a seafloor spreading center lying 300-400 km off the coast of North America (Vine and Wilson, 1995). Hydrothermal activity in the region has been observed (Baker et al., 1987; Davis and Fisher, 1994; Ginster et al., 1994). The eastern flank of the Juan de Fuca Ridge is covered by upper Pliocene and Quaternary sediments. These sediments seal and host hydrothermal fluid flow, confining fluids in the crust at high average temperatures and leading to hydrothermal alteration of minerals in sediments (Lister, 1970; Davis et al., 1992; Davis, Mottl, Fisher, et al., 1992; Davis and Currie, 1993; Davis and Fisher, 1994).
The objective of Leg 168 was to study several common hydrothermal regimes that occur on the eastern flank of the Juan de Fuca Ridge (Davis, Fisher, Firth, et al., 1997). During Leg 168, 12 holes at 10 sites were drilled along a transect spanning oceanic crustal ages from 0.6 to 3.6 Ma with sediment thicknesses varying from 50 to 620 m (Table 1; Figure1; Figure 2; Figure 3). These sediments contain calcareous nannofossils with a variety of abundance and preservation states.
Previous studies of calcareous nannofossils from the high latitudes of the northeast Pacific dealt with nannofossil zonations (Gartner, 1970; Bukry and Bramlette, 1970; Bukry, 1971a, 1971b, 1973, 1981; Haq and Lipps, 1971; Hay, 1971; Wise, 1973; Worsley, 1973). Mao and Wise (1994) studied calcareous nannofossils from the Middle Valley of the Juan de Fuca Ridge and determined two upper Quaternary biostratigraphic zonations for sediment sequences of Leg 139.
Very few previous studies dealt with diagenesis of nannofossils subjected to hydrothermal activity. Mao and Wise (1994) first observed that nannofossils were strongly dissolved due to hydrothermal activity when high temperatures ranged from about 65° to higher than 200°C. In the area of Leg 168, on the eastern flank of the Juan de Fuca Ridge, basement temperatures are low, varying from 15° to 62°C (Davis, Fisher, Firth, et al., 1997). Leg 168 materials allow us to investigate the effect of low-temperature hydrothermal alteration on diagenesis of nannofossils.
Sediments from the eastern flank of the Juan de Fuca Ridge are dominated by turbiditic depositions. The effects of turbidites on the postdepositional preservation of nannofossils should be considered and examined.
For the reasons above, the nominal objective of this study is to provide a detailed biochronology for Pliocene and Quaternary sediments of Leg 168. Another aim of this study is to investigate the effects of hydrothermal circulation and turbidite activity on the postdepositional variations of nannofossils.