Composite Section Construction

The purpose of composite section construction is to reconstruct a complete sedimentary sequence at each site, an objective crucial to the paleoceanographic objectives of this leg and to orbital time scale development. To recover complete sequences, two or three holes were drilled at each site, with deliberate hole-to-hole offsets in depth. This drilling strategy increases the likelihood that intervals missing from one APC hole as a result of gaps between successive cores are recovered in an adjacent hole. The need for drilling multiple holes and for composite section development is illustrated in Figure F11. Previous ODP legs with paleoceanographic objectives have successfully used this strategy to recover complete sedimentary sequences. On Leg 181, we used methods similar to those employed on Legs 138 (Hagelberg et al., 1992), 154 (Curry, Shackleton, Richter, et al., 1995), 162 (Jansen, Raymo, Blum, et al., 1996), 167 (Lyle, Koizumi, Richter, et al., 1997), and 177 (Gersonde, Hodell, Blum, et al., 1999).

High-resolution (2- to 16-cm intervals), nondestructive measurements of gamma-ray attenuation porosity evaluator (GRAPE), bulk density, magnetic susceptibility, natural gamma-ray emissions, and P-wave velocity were made by the MST on each whole-round core section after the cores had equilibrated to room temperature (~2-3 hr after retrieval). After splitting, reflectance spectra in the visible range (400-700 nm) were measured on each archive-core half using the MST. For composite section development, we used magnetic susceptibility, natural gamma-ray emissions, and spectral reflectance at 550 nm as the primary data sets for hole-to-hole correlations. Voids, identified by anomalously low GRAPE density values, and disturbed intervals were omitted from the composite sections. Use of more than one data type provided a cross-check on correlations between holes and was helpful in intervals where one data set produced ambiguous correlations. Biostratigraphic data from multiple holes provided an additional check on correlations among holes.

To construct composite sections, we used an interactive software package ("Splicer" v. 2.0) developed specifically for this purpose by the Borehole Research Group at Lamont-Doherty Earth Observatory. This program is based on correlation software developed for Leg 138 (Hagelberg et al., 1992). The software allows the operator to align corresponding features in data from adjacent holes, based on visual graphics and the correlation coefficient for a user-defined depth interval (typically 2 m long). Features were aligned by linearly adjusting the ODP coring depths (mbsf), measured from the length of drill string advanced, on a core-by-core basis. For each core in each hole, we used a single tie to one other core from a different hole; no depth adjustments (stretching or squeezing) were made within a core. The resulting adjusted depth scale is the meters composite depth scale (mcd) and the section produced by the aligned cores is termed a composite depth section (Fig. F11B).

Offsets between the mbsf and mcd scales arise both from random uncertainty in mbsf depths resulting from ship motion and heave, and from differential distortion of cores during the coring and retrieval process, for example, core expansion caused by rebound. The composite section is thus longer than the in situ section, typically by about 10%. Because the depth of each core is adjusted by a single constant, and distortion of any specific interval is different in different holes, this procedure does not align all features between holes. Depth adjustments/tie points were chosen to both optimize the correlation among multiple holes and to facilitate construction of a single representative "spliced record" from aligned cores. Where an appropriate depth adjustment for a single core was uncertain, or overlapping data from other holes were unavailable, the depth adjustment applied was the cumulative offset from the overlying aligned cores.

A table in each site chapter summarizes core offsets for conversion from the mbsf to the mcd scales. For each section of each core, the depth adjustment required to convert from the mbsf depth scale to the mcd scale is given. The last two columns in each table give the cumulative depth offset added to the ODP curatorial sub-bottom depth (mbsf) and the composite depth (in mcd), respectively. The mcd depth for any sample equals the mbsf depth plus the offset.

Splice Construction

After composite section construction, we assembled a single spliced record representative of the complete recovered sedimentary sequence at each site. Conceptually, building the "splice" consists of patching the intervals missing in a single hole with data from adjacent holes. The advantage of the spliced record is that it provides a single representative record of any lithologic parameter (e.g., magnetic susceptibility, spectral reflectance, or GRAPE), and it is ideally suited to guide sampling plans for high-resolution paleoceanographic studies. The output of this procedure is called the "spliced record."

Splice tie points ideally were made between adjacent holes at the level of visually obvious features that are strongly correlated between holes and where the parameter measured differs by less than 10% between holes. Because there is some stretching and/or compression within cores, the precise length of the splice depends on which intervals of core were selected to build it. Each splice was constructed by beginning at the mud line at the top of the composite section and working downwards. Intervals were chosen for the splice so that section continuity was maintained while disturbed intervals were avoided. An example is given in Figure F12. Splice tie points always connect features with exactly the same composite depths. As a result, the final alignment of the adjacent holes may be slightly different (Fig. F12) from the one giving the best overall visual or quantitative hole-to-hole correlation. Further adjustments to the composite depth section by expanding and compressing the depth scale within individual core intervals is required to align all features exactly (e.g., Hagelberg et al., 1995). Tables that give the tie points for construction of the spliced records are presented in each site chapter.