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doi:10.2204/iodp.proc.303306.103.2006

Composite section

We constructed two spliced composite sections at Sites U1302 and U1303. The first includes only intervals from Site U1302, with the exception of a short segment of Core 303-U1302A-1H that was not recovered near the top of Site U1302. This segment, referred to as the “shipboard” splice, was used for shipboard reporting purposes. It was not possible to construct a complete composite record at Site U1303 because not all of the gaps in Hole U1303A could be filled by cores from Hole U1303B. Consequently, a second splice, referred to as the “sampling” splice, was constructed using the best sections from both locations regardless of whether they came from Site U1302 or Site U1303. One may question the validity of creating a spliced record using two sites located 5.68 km apart, but both sites are in the same depositional environment and the density and magnetic susceptibility signals can be easily correlated between sites. We recommend that the “sampling” splice and its composite depth scale be used for postcruise sampling and analysis because it optimizes the quality and correlation of the stratigraphic section. The “sampling” splice table has been implemented in the Janus database.

We initially depth-shifted cores on the basis of magnetic susceptibility data (magnetic susceptibility core logger [MSCL] aka “fast track”) and then refined correlations once GRA density data were available from the multisensor track (MST). The offsets and composite depths differ between the shipboard and sampling spliced sections (Tables T27, T28). In general, the magnetic susceptibility and GRA bulk density records from all holes at both sites are nearly identical (Figs. F24, F25). Differences among holes can often be attributed to drilling disturbance or the occurrence of dropstones that resulted in spikes in the density and magnetic susceptibility records. For the uppermost 30 m of section, the density and magnetic susceptibility signals at Sites U1302 and U1303 were compared to piston Core MD99-2237 to assess the completeness of the stratigraphic section (Fig. F26). Aside from some differences resulting from differential stretching of the MD core relative to the spliced Site U1302 and U1303 record, the density signals are virtually identical, permitting the identification of all Heinrich and detrital carbonate events during the last glacial period as identified in conventional piston cores from the same location (Hillaire-Marcel et al., 1994; Stoner et al., 1996).

For both splices, cores were correlated by aligning features in GRA density and magnetic susceptibility profiles. It is impossible to align every feature in cores across all holes because of stretching and compression of the core during the coring process. Because of this effect, we selected features in magnetic susceptibility or GRA density that were present in a majority of records to anchor the correlation among cores. An example is the density maximum at ~16 mcd that appears in six holes (Fig. F25). With this feature aligned, there is a 0.5 m mismatch of another density maximum at ~13.4 mcd in Hole U1303B. Such discrepancies are unavoidable when using the program Splicer but can be corrected postcruise using software that permits stretching and squeezing within cores.

The sections of core used for both the shipboard and sampling splices are identified in the spliced data tables (Tables T29, T30). We avoided using the tops and bottoms of each core in the splice, whenever possible, because of potential core disturbance, and were particularly wary of the first section of each core. The composite data indicate that the cores from Sites U1302 and U1303 provide a continuous stratigraphic sequence to ~107.0 mcd. Below this level, cores were appended to the base of the composite section. This level corresponds to the top of a thick debris flow. The uncorrelated cores from below the base of Core 303-U1302A-10H were given the offset of the deepest correlated core. Cores from other holes were correlated with Hole U1302A cores below the splice, but the correlation is weak.

The growth factor (GF) is the ratio of mcd to mbsf and is used to correct the mcd scale for core lengthening. An overall growth factor from both sites is calculated by linear regression of all of the data (GF = 1.13) (Fig. F27). We used this value of GF to calculate corrected meters composite depth (cmcd) presented in Table T27 to aid in the calculation of mass accumulation rates. Growth factors are somewhat different for Sites U1302 (GF = 1.15) and U1303 (GF = 1.03), and the difference may be significant at deeper depths.

Sedimentation rates were difficult to calculate because the expanded section was all deposited in a single polarity chron and there are few reliable biostratigraphic markers. Age is plotted versus depth in Figure F28 for both biostratigraphic and paleomagnetic datums. A single least-squares linear fit to the data results in an average sedimentation rate of 9.8 cm/k.y. over the interval where we have data. However, the least-squares fit line does not intercept the 0 depth point at 0 age, suggesting a higher sedimentation rate of 22.5 cm/k.y. between the seafloor and our first datum at 39 mcd.