The sediment region on the eastern flank of the Juan de Fuca Ridge belongs to the Cascadia Basin off the coast of North America. Quaternary sediments were obtained from all Leg 168 sites. Lithostratigraphically, they are mainly composed of thin-bedded turbidites (silt to sandy silt), thick-bedded sandy turbidites, and/or hemipelagic mud. These turbidite sediments are supplied from Pleistocene glacial sources along the continental margin. Upper Pliocene sediments were recovered only at Site 1027 and are characterized by hemipelagic mud and carbonate-rich mud (Davis, Fisher, Firth, et al., 1997).
We examined 804 nannofossil samples from 12 holes at the 10 sites (Table 1). These samples span, commonly, 20-150 cm of depth in core and were usually carefully selected from hemipelagic mud layers where nannofossils are common to abundant. However, samples from long sequences of thin-bedded turbidites were also taken to obtain enough time resolution. In these samples, nannofossils are rare or barren.
Generally, nannofossils of Leg 168 are well preserved in most sediments. Nannofossil age assignments were based mainly on samples in which nannofossils are well preserved.
Smear slides were prepared using standard techniques. For species analyses, relative counts were made and nannofossils were examined by means of a light microscope (LM) at 1200x to 2000x magnifications. Most Quaternary nannofossil species are too small to be identified accurately using LM; thus a number of samples were selected to ensure determination by means of scanning electron microscope (SEM) at ~10,000x magnification.
The relative group abundance (RGA) of total nannofossils in sediments was estimated by using a semiquantitative technique (e.g., roughly determining relative counts of nannofossils and other sediment particles within view fields on smear slides).
Calcareous nannofossils are built by delicate CaCO3 elements. In deep-sea sediments, carbonate precipitation results in recrystallization of CaCO3 elements of nannofossils, and they become larger or thicker, resulting in overgrowth of nannofossils. Carbonate dissolution leads nannofossils to be partly etched or totally dissolved. Both overgrowth and dissolution of nannofossils can be seen by using a microscope. Stages or degrees of overgrowth and dissolution can be estimated by a number of semiquantitative methods (Roth and Thierstein, 1972; Roth et al., 1975). In this study, nannofossil preservation was examined by means of LM, and the qualitative and quantitative estimation of degrees of dissolution and overgrowth was made using the computer program FossiList, as shown in Table 2.
In the Quaternary, a few Gephyrocapsa species are used as biostratigraphic markers. The method for separating and identifying Gephyrocapsa species follows Su (1996). Nannofossil datum ages used for Leg 168 are based on Su (1996) and on Shipboard Scientific Party (1997, Leg 167, California Margin). These datums are calibrated to the Cande and Kent (1995) time scale.