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doi:10.2204/iodp.proc.303306.102.2006

Composite section

Paleomagnetic and paleoclimatic objectives of Expedition 303 necessitated the recovery of complete stratigraphic sections, yet stratigraphers have demonstrated that a continuous section is rarely recovered from a single ODP or IODP borehole because of core-recovery gaps between successive APC and extended core barrel cores, despite 100% or more nominal recovery (Ruddiman et al., 1987; Hagelberg et al., 1995). Construction of a complete section, referred to as a composite splice, requires combining stratigraphic intervals from two or more holes cored at the same site. To maximize the probability of bridging core-recovery gaps in successive holes, the depths below the seafloor from which cores are recovered are offset between the holes. This practice ensures that most intercore intervals missing within a given hole are recovered in at least one of the adjacent holes. During Expedition 303, three or more holes were cored at all but one site and used to construct composite sections.

Our composite sections and splice construction methodology follows one that has been successfully employed during a number of previous ODP 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, Wang, Prell, Blum, et al., 2000; Mix, Tiedemann, Blum, et al., 2003; Zachos, Kroon, Blum, et al., 2004; among others). The assembly and verification of a complete composite stratigraphic section requires the construction of a composite depth scale. Other depth scales (discussed in “Compositing,” below) can be implemented using logging data and/or combining splicing and driller depths.

Construction of a continuous spliced section is a two-step process involving compositing and splicing.

Compositing

Cores from the various holes must first be composited, which entails stratigraphically correlating and depth shifting cores relative to each other. Such correlation enables development of a composite depth scale referred to as meters composite depth. The mcd scale differs from the traditional (hole specific) depth scale, called the meters below seafloor scale. Mbsf is based on the length the drill string is advanced on a core-by-core basis and is often inaccurate because of ship heave (which is not compensated for in APC coring), tidal variations in sea level, and other sources of error. 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 typically the only one in which depths are the same on both the mbsf and mcd scales. From this anchor, core logging data are correlated among holes downsection. For each core, a depth offset (a constant) that best aligns the observed lithologic variations to the equivalent cores in adjacent holes is added to the mbsf depth in sequence down the holes. Depth offsets are often chosen to optimize correlation of specific features that define splice levels in cores from adjacent holes.

For Expedition 303, the mcd scale and the splice are based on the stratigraphic correlation of data from the IODP whole-core Fast Track, the MST, and the AMST. Core-logging data were collected at 2.5, 5, or 6 cm intervals. We used magnetic susceptibility (the data are referred to as MSCL if collected using the Fast Track or MS-MST if collected using the MST), gamma ray attenuation (GRA) bulk density, natural gamma radiation (NGR), and reflectance (L*, a*, and b*). All of these measurements are described in “Physical properties.”

The raw stratigraphic data were imported into the shipboard Splicer software program (version 2.2) 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.

Because depth intervals within cores are not squeezed or stretched by Splicer, not all correlative features can be aligned. Stretching or squeezing between cores from different holes may reflect small-scale differences in sedimentation and/or distortion caused by the coring and archiving processes. The tops of APC cores are generally stretched and the bottoms compressed, although this is lithology dependent. In addition, sediment (especially unconsolidated mud, ash, sand, and gravel) occasionally falls from higher levels in the borehole onto the tops of cores as they are recovered, and, as a result, the top 20–100 cm of many cores is not reliable.

Correlations among cores from adjacent holes are evaluated visually and statistically (by cross-correlation within a 2 m depth interval). Depth-shifted data are denoted by mcd. Each site chapter includes a table that summarizes the depth offsets for each core. These tables are necessary for converting 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 because depth adjustments are applied linearly to individual cores; no adjustments, such as squeezing and stretching, are made within cores. Such fine-scale adjustment is possible postcruise (e.g., Hagelberg et al., 1995).

Splicing

Once all cores have been depth-shifted and stratigraphically aligned, a composite section is built by splicing segments together from multiple holes to form a complete record at a site. It is composed of core sections from adjacent holes so that coring gaps in one hole are filled with core intervals from an adjacent hole. The splice does not contain coring gaps, and an effort has been made to minimize inclusion of disturbed sections. The shipboard splice is ideally suited to guide core sampling for detailed paleoceanographic studies. Each site chapter includes a table and a figure that summarize the intervals from each hole used to construct the splice. Additional splices may be constructed postcruise as needed.

The choice of tie points (and hence of a splice) is a somewhat subjective exercise. Our method in the construction of a splice followed three rules:

1. Where possible, avoid using the top and bottom 1 m of cores, where disturbance resulting from drilling artifacts (even if not apparent in core logging data) is most likely;

2. Attempt to incorporate those portions of the recovered core that are most representative of the overall stratigraphic section of the site; and

3. Try to minimize tie points to simplify sampling.

The length of the spliced section (on the mcd scale) at a given site is typically ~5%–15% greater than the length of the cored section in any one hole, as indicated by the mbsf scale. This expansion is commonly attributed to sediment expansion resulting from elastic rebound, stretching during the coring process, gas expansion during the core recovery process, and other factors (e.g., Moran, 1997).

Ideally, the base of the 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 or the data quality does not allow reliable correlations between 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 sometimes be correlated with each other to create a floating splice. In this case, an mcd was assigned to the section below the splice by adding the greatest cumulative offset to the first core below the splice and beginning the floating splice from that point in the section.

Corrected meters composite depth

We also provide corrected meters composite depth (cmcd) in our depth conversion tables. This scale is intended to correct the mcd scale for empirically observed core expansion. A growth factor (GF) is calculated by fitting a line to mbsf versus mcd. A cmcd datum is produced by dividing mcd by GF over a sufficiently long interval so that random variations in drill pipe advance due to ship heave, tides, and other factors are negligible. This operation produces a complete stratigraphic sequence that is the same length as the total depth cored. The cmcd scale is a close approximation of the actual drilling depth scale.

Fast Track

The Oregon State University Fast Track for measuring magnetic susceptibility on cores as soon as possible following recovery was first introduced during ODP Leg 202. During Expedition 303, we tested the first deployment of the IODP MSCL Fast Track system, which uses two magnetic susceptibility loops on a single track to speed up analysis time. This setup was not satisfactory (see “Physical properties”) and we quickly converted the Fast Track to a single sensor measuring at 6 cm intervals beginning with Hole 1302B. This helped us to make drilling adjustments aimed at ensuring the recovery of a complete stratigraphic section while allowing us to run the MST at a slower rate to optimize data quality (longer measurement time yielding greater analytic precision and, in many cases, higher spatial resolution) because we did not need its data to assess recovery of a complete section at most sites.

Depths in splice tables versus Janus depths

The depth of a core interval recorded for a tie point in a splice table is not always the same as the depth for the same core interval returned by most database queries. This is because the tie point depth is based on the liner length, which is measured when the cores are cut into sections on the catwalk. The cores are analyzed on the MST almost immediately after this liner-length measurement. At some later time, typically 10–36 h after being analyzed by the MST, core sections are split and analyzed further (see “Core handling and analysis” in “Introduction”). At this time, the section lengths are measured again and are archived as “curated lengths.” General database queries return depths based on the curated liner lengths. Because the sections are usually expanding during the period between the two measurements, the curated length is almost always longer than the initial liner length. Thus, the depths associated with the MST data used to construct the splice table are not identical to the final depths the database assigns to a given interval. This leads to small differences (usually between 0 and 5 cm) between the mbsf and mcd recorded in a splice table and the depths reported in other places for the same core interval. We have chosen not to change these depths to be compatible with Janus because this would not improve their accuracy. For consistency, we recommend that all postcruise depth models use or build on mcd values provided in the Janus database.

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