COMPOSITE DEPTHS

To recover the most complete sedimentary record possible, multiple holes were cored during Leg 207 at Sites 1257–1261. Continuity of recovery was assessed by constructing composite depth sections for all sites. Adjustments to the shipboard mbsf depth scale are required for several reasons, listed below, and are discussed in more detail in Ruddiman et al. (1987), Farrell and Janecek (1991), and Hagelberg et al. (1992, 1995). Rebound of the sediment under reduced pressure caused the cored sediment sequence to be expanded relative to the drilled interval. In addition, random variations in ship motion and heave may have affected the true in situ depth of each core. Portions of the sediment sequence are usually missing in section breaks, even between successive cores having 100% recovery. As a result of these factors, the mcd depth scale increases downhole relative to the mbsf scale, typically on the order of 10% (e.g., Alexandrovich and Hays, 1989; Farrell and Janecek, 1991; Hagelberg et al., 1995; Lyle, Koizumi, Richter, et al., 1997; Acton et al., 2001). A composite depth scale places coeval laterally continuous stratigraphic features into a common frame of reference by shifting the mbsf depth scales of individual cores to maximize the correlation between holes. The individual cores are shifted vertically without permitting expansion or contraction of the relative depth scale in any core. After establishing an mcd scale, more complete stratigraphic records are spliced from the data from multiple holes.

The methods we used during Leg 207 were similar to those used to construct composite depth sections during 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), 178 (Acton et al., 2001), 189 (Exon, Kennett, Malone, et al., 2001), and 199 (Lyle, Wilson, Janecek, et al., 2002). At each site, closely spaced (2.5- to 7.5-cm interval) measurements of magnetic susceptibility, gamma ray attenuation (GRA) bulk density, and natural gamma ray (NGR) emissions were made on the MST soon after the core sections had equilibrated to room temperature. For some holes, noncontact resistivity (NCR) data were also collected on the MST. Susceptibility values are presented as raw meter values (instrument units), which can be converted to SI volume susceptibility units by multiplying by ~0.68 x 10–5 (Mayer, Pisias, Janecek, et al., 1992; Blum, 1997). Measurements of spectral reflectance were made at 2.5-cm resolution on all split cores (see "Color Reflectance Spectrophotometry" in "Lithostratigraphy"). All data were entered into the shipboard Janus database. Data profiles from each hole were compared to determine if coring offsets were maintained between holes. Integration of at least two different physical properties allowed more reliable hole-to-hole correlations than would be possible with a single data set. The physical properties most useful for correlation varied among sites and lithologies.

The mcd scale was constructed using the software program SPLICER (version 2.2), which is available on the World Wide Web from Lamont-Doherty Earth Observatory–Borehole Research Group (LDEO-BRG) at www.ldeo.columbia.edu/BRG/ODP. SPLICER allows data sets from several holes at a given site to be correlated simultaneously. Corresponding features in data sets from adjacent holes were aligned based on graphical and mathematical cross correlations using an iterative process. Correlations were first made visually by selecting a tie point from data in one hole and comparing it directly with data from another hole. Distinctive features and trends were aligned by adjusting the ODP coring depths in mbsf on a core-by-core basis. No depth adjustments can be made in an individual core using SPLICER. Postcruise refinements to the composite depths will be made where it is necessary to stretch, condense, or break individual cores.

Cross-correlation coefficients for all data sets were calculated in SPLICER. Depth adjustments were chosen that provided the best correlation in a preferred data set or the best compromise of correlation coefficients among some or all the data sets. The values of the cross-correlation coefficient vary from +1 to –1, with +1 indicating perfect correlation (such as would be obtained by comparing identical data sets) and –1 indicating anticorrelation (such as would be obtained by comparing a data set to its inverse). Values near zero indicate poor or no correlation. Each time a depth adjustment is made in SPLICER, the coefficient is recalculated, allowing the user to determine the preferred correlation. The window over which the coefficient is calculated is adjustable. The default window length of 2.00 m on either side of the selected tie point was used for most correlations. This window was reduced to 1.00 m as needed to focus on features of interest or to avoid spurious features such as those biased by coring disturbances.

Correlation began by selecting the core that had the most pristine record in the upper few meters of the sedimentary record, with particular emphasis on the mudline, if recovered. This first core is defined as the top of the composite section and its mcd depth is the same as its mbsf depth. A tie point that gives the preferred correlation was selected between data from this core and a core in a second hole. All of the data from the second hole below the correlation point were shifted vertically to align the tie points between the holes. Once the appropriate tie was determined and the depth adjustment was made, the shifted section became the reference section and a tie was made to a core from the first hole. The process continued downhole, vertically shifting the data in one hole relative to data from the other hole. By tying points of different mbsf depths, SPLICER vertically adjusted the individual sections of the cores and brought the chosen features into the common mcd depth scale. The tie points were added to the SPLICER "affine" table. This table records the depth adjustments for each core, and these adjustments define the composite depth scale in mcd.

The composite depth section for each site is presented in tabular form in the "Composite Depths" section of each site chapter. A portion of the composite depth table of Site 1258 is given as an example in Table T8. For each core, the cumulative depth adjustment (offset) required to convert from the ODP curatorial subbottom (mbsf) depth scale to the mcd scale is given, along with the resulting composite depth (in mcd) for the top of each core. The depth offset column facilitates conversion of sample depths that are recorded in ODP curatorial subbottom depth (in mbsf) to composite depth (in mcd). The equivalent depth in mcd is obtained by adding the amount of offset listed to the depth in mbsf of a sample taken in a particular core.

The need for a composite section to verify stratigraphic continuity is illustrated in Figure F8. In the left panel, magnetic susceptibility data from two holes at Site 1258 are shown on the mbsf depth scale. In the middle panel, the same records are shown after depth scale adjustment so that correlative features are aligned. The correlation of lithologic parameters between parallel holes and associated depth adjustments for individual cores were optimized in such a way that a single record could be sampled from the aligned cores without any additional depth scale changes. The right panel shows the resulting spliced record. Where the amount of offset necessary to align features was ambiguous or imprecise for all lithologic parameters or where multiple hole data were unavailable, no additional depth adjustments were made. In these cases, the total amount of offset between mbsf and mcd depths is equal to the cumulative offset from the overlying cores.

Where core recovery in adjacent holes spanned gaps in the sedimentary sequence, it was possible to assemble a continuous representative record (a "splice") using the composite depths. Splice tie points were made between adjacent holes at identifiable highly correlated features. 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 were used to patch in the missing intervals in the core gaps. Intervals were chosen for the splice so that section continuity was maintained. Disturbed intervals were avoided where possible. In some cases, gaps were not spanned by recovery in any hole at a site. Here, discrete "hanging" splices were assembled. The composite splices provide a representative continuous record of each lithologic parameter (e.g., magnetic susceptibility, GRA bulk density, NGR, NCR, and spectral reflectance) for a given site. The splice can also serve as a template for sample collection for paleoceanographic studies.

Tables that give the tie points for construction of the spliced records are presented in each site chapter. An example is given in Table T9 (a portion of the splice table for Site 1258). By identifying intervals where features present in the multiply cored holes were most highly correlated, it was possible to construct a spliced record that avoided duplication or omission of individual features or cycles. Splice tie points always connect features with exactly the same composite depths. As a result, the final alignment of the adjacent holes could be slightly different from the best overall visual or quantitative hole-to-hole correlation. Further adjustments to the composite depth section by expanding and compressing the depth scale in individual core intervals are required to align all features exactly. 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 (rmcd) scale. Typically, the generation of revised mcd scales is carried out as part of postcruise work.

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