Stratigraphers have demonstrated that a continuous sedimentary section is rarely recovered from a single ODP borehole because core-recovery gaps occur between successive APC and XCB cores, despite 100% or more nominal recovery (e.g., Prell, 1982; Ruddiman et al., 1987; Farrell and Janecek, 1991). Construction of a complete section, referred to as a splice, requires combining stratigraphic intervals from two or more holes cored at the same site. To minimize alignment of core-recovery gaps across holes, the depths below the seafloor from which cores are recovered is offset among the holes. This practice ensures that most intercore intervals missing within a given hole are recovered in at least one of the adjacent holes.
Before a splice can be constructed, the cores from the various holes must be stratigraphically correlated with each other. Such correlation enables development of a depth scale referred to as an mcd scale. This differs from the traditional (hole specific) depth scale, called the mbsf scale. The latter is based on the length that the drill string is advanced, core by core. In contrast, the mcd scale is built by assuming that the uppermost sediment (commonly referred to as the mudline) in the first core from a given hole is the sediment/water interface. This core becomes the "anchor" in the composite depth scale and is the only one with depths that are the same on both the mbsf and mcd scales. From this anchor, core-logging data are correlated among holes downsection. The depth offset (a constant) required to best align stratigraphic structure is added to the mbsf depth of all cores below. Our mcd and splice construction methodology follows that successfully employed during a number of legs (e.g., Hagelberg et al., 1992; Curry, Shackleton, Richter, et al., 1995; Jansen, Raymo, Blum, et al., 1996; Lyle, Koizumi, Richter, et al., 1997; Gersonde, Hodell, Blum, et al., 1999).
The mcd scale and the splice are based on the stratigraphic correlation of whole-core MST and split-core color spectral reflectance (CSR) data (lightness, L*) usually collected at 2- to 8-cm intervals. From the MST, we used magnetic susceptibility (MS), gamma-ray attenuation (GRA) bulk density, and natural gamma radiation (NGR) data (see "Physical Properties"). Lithostratigraphic events (e.g., volcanic ash layers) and biostratigraphic and magnetostratigraphic datums proved useful as tie points in intervals where correlations were ambiguous.
The raw stratigraphic data were imported into the shipboard Splicer software program (version 2.0) and culled as necessary to avoid incorporating anomalous data influenced by edge effects at section boundaries. Splicer was used to assess the stratigraphic continuity of the recovered sedimentary sequences at each drill site and to construct the mcd scale and splice. Splicer enables the construction of a composite depth scale for each hole at a given site by depth shifting only individual cores to maximize the correlation of MST and CSR data. Because sediments can be distorted by the coring process (the tops of APC cores are generally stretched and the bottoms compressed, although this is lithology dependent) and because depth intervals within cores are not squeezed or stretched, all correlative features cannot be aligned. Correlations among cores from adjacent holes are evaluated visually and statistically. Depth-shifted data are denoted by mcd. A table is presented in each site chapter that summarizes the depth offsets for each core that are necessary to convert mbsf to mcd scales. The mcd for any point within a core equals the mbsf plus the cumulative offset. Correlation at finer resolution is not possible with Splicer since depth adjustments are applied linearly to individual cores; no adjustments, such as squeezing and stretching, are made within cores. Such finer scale adjustment is possible postcruise (e.g., Hagelberg et al., 1995). To assist the reader, all composite section tables and figures from each chapter are presented as either ASCII files, Microsoft Excel files, and/or Synergy Software KaleidaGraph plot files (see the "ASCII Tables" and "Supplementary Materials" contents lists). This provides the user with greater ability to view, manipulate, graph, and analyze data than is possible within Adobe PDF files, the standard presentation medium.
Ideally, the base of the continuous mcd scale is the bottom of the deepest core recovered from the deepest hole. In practice, however, the base often occurs where core recovery gaps align across all holes. Cores below this interval cannot be directly tied into the overlying and "continuous" mcd. However, below the base of the continuous mcd, cores from two or more holes can often be correlated with each other. At several sites, we constructed discontinuous or "floating" mcd scales for these deeper cores. They are appended to the base of the continuous mcd.
We also used Splicer software to patch together intervals from different holes (based on the mcd scale) to create the splice. The splice does not contain coring gaps, and an effort has been made to minimize inclusion of disturbed sections. The splice is ideally suited to guide core sampling for detailed paleoceanographic studies. A table is presented in each site chapter that summarizes the intervals from each hole used to construct the splice. Like the floating mcd, floating splices were also constructed in cases where sufficiently long sections could be generated but not tied directly to overlying continuous splices.
The length of the mcd scale at a given site is typically ~10% greater than the length of the cored section in any one hole as indicated by the mbsf scale. Although the exact reasons for this are unknown, it is commonly attributed to sediment expansion resulting from decreased overburden pressure in the deeper sections, stretching during the coring process, and other factors (MacKillop et al., 1995). For all sites, the shipboard mcd scale and splices should be considered preliminary and can be improved on with further shore-based work. This is especially true for the floating mcd scales and splice sections.
A certain degree of subjectivity comes into play when constructing a composite depth scale and a splice. If the mcd is built without simultaneously considering splice construction, the stratigrapher may focus on maximizing the stratigraphic fit between the entire cores instead of on the portions that eventually become splice segments. In this case, the splice tends to include a large number of small segments from all holes. Our approach was to construct an mcd scale with the goal of creating a splice from the longest possible segments of core and with the smallest number of interhole transitions. This approach has two practical advantages. First, postcruise sampling and sample processing are significantly less complex and therefore less prone to error. Second, this approach allows us to avoid the hole most heavily sampled at sea. This ensures greater availability of material for high-resolution time-series work.
Site 1144 contained intracore voids resulting from gas expansion. Shipboard technicians tried to eliminate or consolidate the voids on the catwalk by drilling gas-escape holes into the core liner and pushing the sediment toward one end of the core, ideally the top (because of the practice of splitting the cores from bottom to top). The voids not removed by this procedure were entered into the JANUS database. These remaining voids introduce a unique problem into the process of developing the mcd scale and splice because the core-splitting process, which involves pulling a cutting wire lengthwise along the section, can dislodge sediments and reposition voids, thus offsetting the split-core CSR measurements from those collected on the whole-core MST (Fig. F1).