MATERIALS AND METHODS

Site 1085 was drilled in a water depth of 1713.2 m at 2922S, 1359E (Wefer, Berger, Richter, et al., 1998) (Fig. F1). Drilling at Hole 1085A penetrated a total of 604 meters below seafloor (mbsf) and recovered a relatively continuous section of calcareous nannofossil ooze, extending to the middle Miocene (Wefer, Berger, Richter, et al., 1998). Hole 1085B (2922S, 1359E) was drilled to 321.2 mbsf (Wefer, Berger, Richter, et al., 1998) and was used to create a composite record.

Shipboard calcareous nannofossil and planktonic foraminifer datums (see Table T1) were used to constrain the initial age model using core magnetic susceptibility (MS) records for the Pliocene section of Site 1085. Since the biostratigraphic (BIO) age model was based on shipboard datums determined from core-catcher material, the initial shipboard stratigraphic position is approximate and datum positions reflect at least a 9.5-m possible range of stratigraphic position. The MS was tuned to the eccentricity curve for the interval (Laskar, 1990) by matching the high-resolution MS record with increased insolation using Analyseries software (Paillard et al., 1996) to create the tuned core magnetic susceptibility (TCMS) age model (see Table T2 for the control points used to create the TCMS model). Both the BIO and the TCMS models were evaluated for Milankovitch periodicities using Analyseries software (Paillard et al., 1996) using both spectral and the cross-spectral methods. The Blackman-Tukey method of spectral analysis was used to identify the dominant periodicity in the BIO and TCMS age models. The analysis of the linear detrended age models was performed using a 2-k.y. step, bandwidth of 0.00139, and 2149 lags. Similar results were returned at both 80% and 95% confidence levels. Cross-spectral analysis between the eccentricity and TCMS records was also performed using Analyseries software (Paillard et al., 1996). The TCMS vs. eccentricity analysis was performed at 95% confidence level using a step of 0.5 k.y., 2100 lags, and a bandwidth of 0.00142 on linear detrended data.

The age models were developed using a composite MS record (Fig. F2) of Holes 1085A and 1085B to ensure analysis of a complete section. The composite was created by splicing multisensor track (MST) MS core records from Hole 1085A and Hole 1085B and was based on the splice that was developed shipboard (see Wefer, Berger, Richter, et al., 1998, for details on the construction of the shipboard composite record). A nine-point smoothing function was applied to the records. The composite section compensates for gaps due to drilling by overlapping records from adjacent holes and splicing them together to create what is ideally a complete record of deposition. The standard composite section is generated from core recovered from three holes and compensates for local variations in deposition by removing sections that are unlike the other two holes at that site. Because only two holes were drilled at Site 1085, there is potential for some error in the composite record. Berger et al. (1998) document an association between gas expansion and offset of MS data between holes in the Congo Fan region. Similarly, expansion is anticipated in the Cape Basin because of the presence of gases (notably methane). This expansion should add some noise, but smoothing should remove much of it. However, core growth, as measured by core offsets generated in the composite depth analysis, indicates that there is only an expansion of ~5% in this interval (Wefer, Berger, Richter, et al., 1998). Even if the core is disturbed by gas expansion, the time represented is probably minimal. Assuming an 8-cm/k.y. average sedimentation rate, even a very long interval of expanded sediments (e.g., 40 cm) would only represent 5 k.y., significantly less than the sampling required to identify 41- and 100-k.y. cyclicity.

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