METHODS

A total of 116 samples, with a resolution of ~50–100 k.y., were analyzed for long-chain alkenones using gas chromatography (see Huang et al. [1997a] and Pelejero et al. [1999b] for methodology) (Table T1). Sample ages were determined by nannofossil, foraminifer, and paleomagnetic techniques for reference depths (as described in the ODP Leg 184 Preliminary Report) and then interpolated for all depths (Shipboard Scientific Party, 2000). Sediments were freeze-dried and then ground to a fine powder. An internal standard consisting of C₂₁-alcohol, 5-cholestane, and C₃₆ n-alkane was added to 1–6 g of sediment, and organic matter was extracted using a Benchmate II robotic station with a mixture of dichloromethane and methanol (3:1 ratio). Briefly, solvent was added by the Benchmate workstation. Samples were manually stirred, sonicated in a water bath for 10 min, and centrifuged at 3000 rpm for 10 min; then the supernatant was drawn off the sediment by the Benchmate workstation. The extraction method was repeated three times, and the supernatants for each sample were combined to ensure complete extraction of alkenones. The supernatant was dried down completely with nitrogen gas and derivatized with N,O-bis(trimethylsilyl)trifluoroacetamide. Long-chain alkenones were detected with a Varian 3400 gas chromatograph (GC)/flame ionization detector on an HP-1 crosslinked methylsilicone 50-m column. The initial column temperature was set at 80°C with an increase of 25°C/min to 200°C, then 4°C/min to 250°C, and then 2°C/min to 300°C, where temperature was held constant for 20 min. Alkenones eluted at 300°C (representative peaks are shown in Fig. F2). U^k´₃₇ values and SST were determined for samples containing both C_37:2 and C_37:3 with the following equations (Prahl et al., 1988):

U^k´₃₇ = C_37:2/(C_37:2 + C_37:3) and (1)
 

SST (°C) = (U^k´₃₇ – 0.039)/0.034 (2)

In addition, the SSTs from equation 2 were compared to temperatures determined with the following equation, calibrated by Pelejero and Grimalt (1997) for the modern SCS:

SST (°C) = (U^k´₃₇ – 0.092)/0.031. (3)

Comparison between equations 2 and 3 confirms Pelejero and Grimalt's (1997) conclusions that calibration of the SST equation near the upper boundary gives rise to SST values very close to or within the error limits (~0.5°C, from Villanueva and Gimalt, 1997) of equation 2 (Prahl et al., 1988) (Fig. F3). In this case, the younger samples (<1 Ma) fall within the error limits and the older samples (>1 Ma) fall just outside of the error limits. Here, because of the large temporal scale involved, we express all U^k´₃₇ values and temperatures using equation 2 (Prahl et al., 1988).