The recovery of complete sediment sections of APC- and XCB-cored intervals was crucial to the paleoceanographic objectives of Leg 208. Drilling of parallel holes at Sites 1262 through 1267 was planned to ensure that intervals missing from one APC/XCB hole as a result of recovery gaps between cores could be recovered in an adjacent hole. Composite depth sections have been developed for all multiply cored sites to confirm their continuity of recovery.
Adjustments to the shipboard mbsf depth scale are required for several reasons (e.g., Ruddiman et al., 1987; Hagelberg et al., 1992). Elastic rebound and gas expansion of the sediment following core recovery causes the cored sediment sequence to be expanded relative to the cored interval. As a result, the composite depth scale grows downhole relative to the mbsf scale, typically on the order of 10%–20%. In addition, the ship's motion, which is due to tides and heave, can affect the in situ depth at which a core is cut. Portions of the sediment sequence are usually missing, even between cores that have >100% nominal recovery.
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 within any core. A horizontal feature present in recovered material from several holes will have, in the absence of local stratigraphic variations, approximately the same mcd but will most likely have different mbsf depths. Horizontal features will have exactly the same mcd in two holes at the correlation tie points. Most other intervals will not be precisely correlated (offsets of a few millimeters to several centimeters) because of differential stretching and squeezing within cores. APC recovery gaps between cores in one hole typically range from 0.5 to 2 m and rarely exceed 5 m. After establishing an mcd scale, complete stratigraphic records are spliced from the data of multiple holes.
The methods used during Leg 208 to construct composite depth and spliced sections were similar to those used during Leg 138 (Hagelberg et al., 1992) and subsequent paleoceanographic ODP legs (e.g., Legs 154, 160, 161, 162, 167, 171B, 172, 175, 177, 178, 181, 182, 184, 189, 198, 199, 202, and 207). For each site, the hole-to-hole correlation was based on 2.5- or 5-cm spaced hole core measurements of magnetic susceptibility (MS), gamma ray attenuation (GRA) bulk density, compressional wave (P-wave) velocity measured on the P-wave logger (PWL), and natural gamma radiation (NGR) measured on the whole-core MST as well as spectral reflectance and point-sensor MS measurements on split cores (see "Physical Properties"). A new application, Splicerintegrator, was used to retrieve the MST and color reflectance data from the ODP online database (Janus) and to upload the affine and splicer tables into the database. The mcd scale was constructed using the program Splicer (version 2.2), available on the World Wide Web from the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG) (www.ldeo.columbia.edu/BRG/ODP). Splicer allows various data sets from several holes at a given site to be correlated simultaneously.
Correlations were done visually by selecting a tie point from primarily the MS data and/or color reflectance data in one hole and comparing it directly with data from another hole. Features were aligned by adjusting the coring depths in mbsf, measured from the length of the drill string advance, on a core-by-core basis. No stretching, squeezing, or any other depth adjustments were made within an individual core. The core that had the most pristine record of the upper portion of the upper few meters of the sedimentary record, particularly the mudline, was chosen as the first (anchor) core of the composite section. The mcd of the first core was thus the same as its mbsf depth. A tie point that gave the preferred correlation was selected between data from this core and a core in an adjacent hole. All data from the second hole below the correlation point were vertically shifted to align the tie points horizontally between the holes. Once the depth adjustment was made, the shifted section became the reference section and a tie was made to a core from another hole. The process continued downhole, vertically shifting the data from one core at a time relative to data from the other hole. The tie points were added to the "affine" table, which records all of the depth adjustments that define the composite depth scale in mcd. The composite depth scale for each site is presented in tabular form in the "Composite Depth" section of each site chapter. For each core, the depth adjustment required to convert from the mbsf depth scale to the mcd scale is given as the cumulative depth offset added to the ODP curatorial subbottom depth (in mbsf).
For some intervals of some sites, and in the case of Site 1266 for almost the complete succession, several factors limited the success of building a complete composite section. In these cases, we applied a constant growth rate, based on the average growth rate of either overlying or underlying spliced intervals or, in the case of Site 1266, the average growth rate of previous Leg 208 sites was used to construct a (partially) spliced composite depth section.
During the composite section construction procedure, a spliced record was assembled that provides a single representative sedimentary record suited for postcruise sampling and studies. Splice ties were established as close to the composite depth tie points as the Splicer program allowed. Intervals were chosen for the splice so that section continuity was maintained and disturbed intervals were avoided. Tables that give the splice tie points for the construction of the spliced records are presented in each site chapter. By definition, splice tie points always connect features with exactly the same composite depths. Other intervals may not correlate perfectly between adjacent holes because the cores are stretched and squeezed differentially during the coring process. Further adjustments to the composite depth section by detailed correlation that includes expanding and compressing the depth scale within individual core intervals are required to align all features exactly.