The scientific objectives of Leg 199 require the examination of the sedimentary record at high resolution as well as the recovery of a geological record that is as complete as possible. Core recovery from a single hole is insufficient to accomplish these goals because of recovery gaps between adjacent cores, even with a nominal 100% recovery. To obtain a complete sedimentary record, multiple adjacent holes are cored, with an offset in depth of typically 2-4 m between different holes to ensure that intervals missing within a single APC-cored hole can be recovered from an adjacent hole. The offset in depth required in subsequent holes must rapidly be determined before and during coring. The continuity of recovery is assessed by developing composite sections that align prominent features in physical properties data from adjacent holes. The procedure and rationale for this stratigraphic correlation is described in this section and follows the methodology pioneered during Legs 94 (Ruddiman et al., 1987) and 138 (Hagelberg et al., 1992). Similar methods were employed and developed further during Legs 154 (Curry, Shackleton, Richter, et al., 1995), 162 (Jansen, Raymo, Blum, et al., 1996), 167 (Lyle, Koizumi, Richter, et al., 1997), 171B (Norris, Kroon, Klaus, et al., 1998), and other more recent ODP legs such as 189 (Exon, Kennett, Malone, et al., 2001).
Onboard stratigraphic correlation requires closely spaced data that can be generated rapidly. After each whole-core section had equilibrated to room temperature (typically 4-5 hr), it was run through the MST assemblage (see "MST Measurements" in "Physical Properties"). The MST generates high-resolution (2-cm interval) measurements of MS, GRA wet bulk density, and P-wave velocity. The MST data set also comprises less closely spaced (20-cm interval) measurements of natural gamma ray (NGR) emissions, which are typically less useful for correlation work. In addition, measurements of color reflectance in the 400- to 700-nm band and calculated L*a*b* values were made at 2-cm resolution on the split archive half of the core using a Minolta spectrophotometer (see "Color Reflectance Spectrophotometry" in "Lithostratigraphy"). All measurements were entered into the shipboard database.
By convention, ODP sample and core depths are recorded as mbsf from the first core in each hole that shows a true mudline, and consecutive depth measurements are determined by the length of the drill string. Several factors can lead to a deviation of the relative distance of geological features in the core from their true in situ stratigraphic separation. For example, sediment inside the core can expand as a result of reduced environmental pressure following core recovery, leading to an expanded sedimentary sequence relative to its original length (Moran, 1997). Thus, sediment can be missing between cores, even if a nominal 100% recovery is achieved. In addition, variations in the ship motion, tides, and heave can result in stretching and/or squeezing of the recovered sediment in particular subintervals.
To allow the placement of geological data from different holes according to their stratigraphic position, a correlation of physical properties measurements is made to create a meters composite depth (mcd) scale. The generation of an mcd scale attempts to match coeval stratigraphic features, as recorded by the MST and color reflectance measurements, at the same level by generating a composite record from different holes. This process requires depth shifting of cores relative to each other. During this step, the total length on the mcd scale is typically expanded by 10% compared to the mbsf scale, although this factor can vary between ~5% and 15% (Hagelberg et al., 1992; Norris et al., 1998).
Hole-to-hole correlation was facilitated by using the graphical and interactive UNIX platform software, SPLICER, that was developed by Peter deMenocal and Ann Esmay of the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG). We used version 2.2 of this software (available on the World Wide Web at www.ldeo.columbia.edu/BRG/ODP). SPLICER allows data sets from adjacent holes to be correlated simultaneously, making use of an interactive, cross-correlation computation. After prominent features are aligned on the new mcd scale, a "spliced" record can be generated by switching holes to avoid core gaps or disturbed sediment, resulting in a continuous record that forms the basis for further sampling and analysis.
In addition to creating a high-resolution shipboard record for time-series analysis, one of the primary purposes of the composite depth scale is the generation of a template that forms the basis for sediment sampling along a complete section with a minimal waste of samples and analytical time. For this reason, it is highly desirable to apply a constant offset to an entire core, such that the depth increments along individual cores on the mcd scale are linearly related to the curated mbsf depth increments. The SPLICER software only allows the application of a constant offset for each core.
However, composite depth scales created by using the SPLICER method are usually not satisfactory to create a stacked record from different holes because not every individual feature present across different holes can be aligned. Hence, if the aim is to increase the signal-to-noise ratio of quasiperiodic cycles that might be recorded in the sedimentary record, it would be necessary to allow stretching and squeezing within individual sections to align data from different holes on the individual cycle level. The procedure for the generation of a common depth scale that allows stretching and squeezing on a fine level was pioneered by Hagelberg et al. (1995) and results in a revised mcd common depth scale. The generation of revised mcd scales is typically carried out as part of postcruise work.
In detail, the composite depth generation using SPLICER begins by assigning the core with the best record of the upper few meters of a hole as the top of the composite record. This core is assigned an mcd identical to its mbsf depth. A tie point, which gives the preferred correlation, is selected between data from this first core and a core in a second hole. All the data from the second hole below the correlation point are vertically shifted to align the tie points between the holes. After choosing an appropriate tie point and adjusting the depths, the shifted section becomes the next "reference" section, and a tie is made to a core from the first hole. Working downhole in an iterative fashion, each core is then vertically shifted. The tie points are added to the SPLICER "affine" table (which records all the depth adjustments that define composite depth scale) in units of mcd. The affine table that relates mbsf to mcd values along with the applied linear offsets for each core is presented in tabular form in the "Composite Depths" section of each site chapter. The composite depth table of Site 1218 is given as an example in Table T5. The last two columns in each table give, for each core, the cumulative depth offset added to the ODP curatorial sub-bottom depth (in mbsf) and the composite depth (in mcd), respectively. The depth offset column allows the calculation of the equivalent depth in mcd by adding the amount of offset listed to the depth in mbsf of a sample taken in a particular core. Table T6 shows a typical example for the SPLICER file that defines the switching across holes to generate a spliced composite section. The composite depth and splice tie point tables from each site chapter are also presented in ASCII (see the "ASCII Tables" contents list).
Figure F8 illustrates how the creation of a composite record allows the alignment of the most prominent lithology features. In the left panel, MS data from three holes at Site 1218 are shown on the mbsf depth scale, whereas in the middle panel the data are shown on a common depth (mcd) scale. The right panel shows the generated spliced record, indicating where the sample track was switched from one hole to the next. If the available data only allow an ambiguous or imprecise correlation, or where multiple hole data were unavailable, no additional depth adjustments were made. In this case, the cumulative offset remains constant for all subsequent cores below.
Splice tie points were made between adjacent holes at identifiable, highly correlated features, attempting to construct the splice crossover points at transitions rather than peaks. Each splice was constructed beginning at the mudline at the top of the composite section and working downward. Typically, one hole was chosen as the backbone for the record, and cores from other holes are used to patch in the missing intervals in core gaps (Fig. F8). Intervals were chosen for the splice such that section continuity was maintained, whereas disturbed intervals were avoided.
The final alignment of the adjacent holes could be slightly different from the best overall visual or quantitative hole-to-hole correlation because of the constraint that a constant offset be applied to each core by the SPLICER software. Thus, in addition to creating an additional revised mcd scale for time-series analysis and other studies that require the stacking of data from multiple holes, investigators intend to rescale the spliced record to downhole logging data (Harris et al., 1995) as part of postcruise studies. Although the difference between the lengths of the mcd and mbsf scale will not normally be significant as far as uncertainties in geological flux calculations are concerned, this rescaling process would be required for some studies (e.g., the calibration of seismic studies that make use of physical properties measurements).