The main objective of ODP Leg 175 was to obtain a series of high-resolution, undisturbed, and continuous sedimentary records of the late Neogene. A latitudinal mega-transect was drilled between 5° and 35°S, where each priority site was covered by threefold APC coring. It was the task of the stratigraphic coordinators to produce a composite section and a spliced record representing a continuous sediment record for each site. Beginning with DSDP Leg 94 (Ruddiman et al., 1987), investigators reported coring gaps between successive APC cores in a range as wide as 0.5 to 2.7 m (e.g., ODP Legs 108 [Ruddiman et al., 1988; Bloemendahl et al., 1988] and 121 [Farrell and Janecek, 1991]). Commonly, material is found to be missing between successive cores, even where there is nominal 100% recovery.
The methods used during Leg 175 were similar to those used previously for constructing composite depth sections (Hagelberg et al., 1992 [Leg 138]; Shipboard Scientific Party, 1995a [Leg 154]; Shipboard Scientific Party, 1996a [Leg 162]; Shipboard Scientific Party, 1997b [Leg 167]; and Shipboard Scientific Party, 1998 [Leg 171B]). The "Explanatory Notes" sections of these publications adequately describe the need for composite sections and spliced records and the overall approach taken to construct them during ODP legs.
During Leg 175, we used GRAPE density, magnetic susceptibility, and digital color reflectance data to document the exact correlation among cores from each of several holes at a particular site. The alignment of clearly correlated features from different holes inevitably necessitates depth-shifting cores. The end product of this shifting of records is a new depth scale (in meters composite depth [mcd]) that has the advantage of representing continuity, but the disadvantage of being longer than the distance actually cored (in mbsf). Typically, the length is expanded by about 10%, but the factor varies from ~5% to ~20%. The reason for this expansion is not fully understood, but gas expansion, decompression of the sediment (Moran, 1997), and distortion by the coring process are involved.
To obtain a meaningful representation of the whole section recovered, it is convenient to create a spliced record (Hagelberg et al., 1992) that is constructed by adding sequential intervals of core from any one of the holes recovered, proceeding down from the seafloor. Such a splice is useful both for (1) providing a continuous representation of the shipboard high-resolution records for time-series analysis and (2) providing a template that permits the sedimentary record recovered from different holes at a site to be sampled for shore-based analysis without wasting samples or analytical time.
A significant limitation of this approach was set by the expectation of high gas content for the sediments recovered from several sites (i.e., Lower Congo Basin, Mid-Angola Basin, and Walvis Basin), drilled during Leg 175. The gas generally causes substantial expansion of the sediment, which is not uniform along the cores or holes. In these cases, it is rarely possible to line up all the prominent features in two cores by using a linear depth offset. It is convenient to choose to align one core to another at that point that will be used to cross over from one core to the other in completing the splice. If the relationship between samples in parallel holes must be known very precisely, it may be necessary to map the data for each section of each hole onto the splice postcruise, creating another depth scale that is not linearly related to curated depth at the centimeter to decimeter scale (Hagelberg et al., 1995). Such a scale should be used for special purposes, and great caution is required to avoid generating unnecessary confusion.
During Leg 175, the core integration was performed using the software package SPLICER, developed by Peter deMenocal and Ann Esmay of the ODP Borehole Research Group at Lamont-Doherty Earth Observatory. The data sets used were MST magnetic susceptibility and GRAPE density data and output from the Minolta color scanner. In some core sections, changes in lightness (generalized color reflectance, characterized by the lightness L* and/or b*) are useful for correlation, whereas in other sections, color reflectance from individual channels (e.g., red/blue [650/450 nm] are more useful. However, we prefer to use the L*, b* output of the Minolta color scanner instead of the reflectance from individual color channels.
The use of a composite depth section dramatically improves our ability to correlate between sites and to describe changes in sedimentation rate, especially if biostratigraphy and magnetostratigraphy provide a dense network of precisely determined age control points. The sedimentation rates (mcd/m.y.) are artificially high by ~10% as a result of the stretched mcd scale. Note that to the extent that expansion caused by rebound can explain the growth of the mcd scale with depth, mass fluxes would be correct when calculated using shipboard densities and sedimentation rates determined by the mcd scale. The reason is that uniform sediment expansion caused by rebound would result in an increase in porosity. However, if expansion is caused by numerous small-scale gas voids, shipboard densities may be more similar to in situ densities, because they come from less disturbed intervals, and correction from mcd to actual depth may be more important.
It is important for some applications to rescale depths on the mcd scale back to true depth below seafloor. For example, to simulate a seismic section using physical properties measured in cores, it is vital that the composite depth be used to simulate the complete section, but this must be rescaled to true depths to correctly predict the depths of the reflectors in the sediment. This rescaling can be done most simply by a linear transformation based on the ratio of mcd to mbsf (Hagelberg et al., 1992) either over the full section or in intervals. At sites where there is significant expansion caused by gas, the relationship between mcd and mbsf may not be constant over the full depth range. In this instance, linear transformations can be determined over different depth intervals where expansion appears constant. Also, at sites where a single hole penetrated a greater distance than the other holes, it may be convenient to continue the composite depths and splice into the range where no true spliced record is possible. In this instance, there is only a constant offset between mcd and mbsf. A more complex procedure is to rescale on a core-by-core basis. This method was adopted for high-resolution conductivity measurements by Shipboard Scientific Party (1996a), by using their equations 20-22. Another method, possible when data are available from downhole logs, is to rescale the spliced record through alignment of MST data with downhole logs.
During Leg 175, one or more composite depth sections and spliced records were generated for each site, and the results are presented in the "Composite Section" section of each site chapter (this volume). Downhole plots of magnetic susceptibility, GRAPE den-sity, lightness (L*), chromaticity variable (b*), and red/blue (650/450 nm) are also shown. These plots show each core plotted at its appropriate composite depth alongside the spliced record. Also marked on each plot are the depths at which the splice moves from one core to another. The spliced records for magnetic susceptibility, L*, and b* also are plotted in each chapter. The spliced data and composite depth information for each core are available from the "Table of Contents".