MATERIALS AND METHODS

We obtained a high-resolution suite of samples (~25-cm intervals; T = 2.5 k.y.) from the upper 85 meters composite depth (mcd) of sediment at Site 1020 (41°N, 126°W), with overlap between Holes 1020C and 1020D to ensure a complete record (Fig. 3). All sediment samples were treated and analyzed for alkenones according to the methods described in Herbert et al. (1998). We routinely obtained a precision of better than 0.005 units (~0.15°C) as determined by replicate analyses of selected sediment samples. Comparison of core-top and LGM -SSTs from Prahl et al. (1995), Doose et al. (1997), Herbert et al. (1998), and this study show excellent agreement between the two labs.

Some workers disagree about whether to include the tetra-unsaturated ketone (C_37:4) in temperature estimations (Prahl et al., 1988, 1993; Sikes et al., 1997; Brassell, 1993; Conte and Eglinton, 1993; Rosell-Melé et al., 1993). The   calibration neglects the C_37:4 ketone from SST estimates. At Site 1020, this particular molecule occurs in greatest abundance during glacial phases, but it often co-elutes with unknown molecules, making it difficult to reproduce analytically. Consequently, many calibration studies favor the use of indices along the California margin (Prahl et al. 1988; Doose et al., 1997; Herbert et al., 1998) and in colder regions such as the Southern Ocean (Sikes et al., 1997), and we report only the index in Table 1.

All of our glacial-age samples contain di-, tri-, and tetra-unsaturated alkenones (C_37:2, C_37:3, C_37:4), whereas interglacial-age samples contain only di- and tri-unsaturated alkenones. This reflects the greater importance of C_37:4 molecules at cooler SSTs (Brassell et al., 1986; Rosell-Melé et al., 1993). We calculate alkenone concentrations (SC₃₇; nmol/g dry sediment) as the sum of the C_37:2, C_37:3, and C_37:4 molecules, and we assume that SC₃₇ represents a proxy for Haptophyte algal productivity (Villanueva et al., 1997; Rosell-Melé et al., 1997). These concentrations reflect the productivity and preservation of one important component of marine phytoplankton, subject to dilution from other sources.

Numerous laboratory culture and core-top experiments (Prahl and Wakeham, 1987; Brassell, 1993; Volkman et al., 1995) have shown that the index faithfully records modern SST (Fig. 4, inset). We believe that these results validate our choice to present our SST reconstruction using the index for this region.