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doi:10.2204/iodp.proc.342.102.2014

Stratigraphic correlation

The scientific objectives of Expedition 342 required the recovery of complete stratigraphic sections; however, recovery of a continuous section from a single borehole is technically impossible. Despite 100% or more nominal recovery, core recovery gaps occur between successive APC and XCB cores (e.g., Ruddiman et al., 1987; Hagelberg et al., 1995; Lisiecki and Herbert, 2007). Tides, ship heave, core expansion, and coring deformation also prohibit complete recovery in a single hole. Luckily, tidal variations (calculated using OTPS software from volkov.oce.orst.edu/tides/otps.html) during the expedition were <0.8 m for all sites (Egbert and Erofeeve, 2002). However, several storms transpired during operations and occasional damaged core liners impeded depth corrections. Depth corrections were also affected by differences in mudline depths between adjacent holes (likely because of seafloor bedforms) and local variations in stratigraphic thicknesses. Construction of a complete stratigraphic section requires the construction of a composite depth scale for multiple holes and assembly of stratigraphic intervals from two or more holes cored at the same site into a spliced record. By offsetting the recovery depth of cores below seafloor between each hole, it is possible to maximize the probability that adjacent holes recover most core gaps within previous holes. Expedition 342 cored at least two holes at all sites in order to construct the splice where stratigraphic control and recovery allowed.

The Expedition 342 composite depth and splice construction methodology followed a number of previous ODP legs and IODP expeditions (e.g., Hagelberg et al., 1992; Curry, Shackleton, Richter, et al., 1995; Jansen, Raymo, Blum, et al., 1996; Lyle, Koizumi, Richter, et al., 1997; Wefer, Berger, Richter, et al., 1998; Gersonde, Hodell, Blum, 1999; Wang, Prell, Blum, et al., 2000; Lyle, Wilson, Janecek, et al., 2002; Mix, Tiedemann, Blum, et al., 2003; Zachos, Kroon, Blum, et al., 2004; Channell, Kanamatsu, Sato, Stein, Alvarez Zarikian, Malone, and the Expedition 303/306 Scientists, 2006; Pälike et al., 2009, 2012). After generating a composite depth scale, it is possible to produce a stratigraphically continuous and complete splice using representative intervals from multiple holes. Ideally, this spliced record does not include core recovery gaps or intervals with coring deformation.

Core composite depth scale

The goal of constructing a composite depth scale is to place coeval, laterally continuous stratigraphic features into a common frame of reference, which is achieved by shifting the depths of individual cores from the original CSF-A scale to the CCSF scale in order to maximize correlation between holes. In the composite depth scale, the rule is to shift the depths of individual cores by a constant amount (affine transformation). Core-by-core depth shifting does not permit corrections for expansion or contraction within cores. This method provides good first-order correlation between cores from different holes without adding more subjective, and potentially erroneous, interpretations and also preserves the “true” relationship between curated core sections and the depth scale.

The CCSF depth scale overcomes many of the inadequacies of the CSF-A depth scale. The CSF-A scale is unique to each hole and derives from the length that the drill string advanced. Between-hole variations in mbsf can occur because of ship heave (not compensated for in APC coring), tidal variations in sea level, and other sources of error. In contrast, the CCSF scale is built by using the uppermost sediment (mudline) in the first core from a given hole as the “anchor” in the composite depth scale. The mudline anchor core is the only one with equivalent depths on the CSF-A and CCSF scales. From this anchor, physical property data generated on cores (e.g., bulk density and magnetic susceptibility) allow correlation among holes. For each core, we added a constant depth offset to the mbsf depth that best aligned the physical property variations to equivalent cores in adjacent holes.

For Expedition 342, we primarily based the CCSF scale and the splice on stratigraphic correlation of data from the STMSL, WRMSL, SHMSL, and NGRL (see “Physical properties”). The measurement interval is 2.5 cm for the multisensor logger data and 10 cm for the NGRL data. We used STMSL magnetic susceptibility and GRA density only for preliminary stratigraphic correlation in Microsoft Excel with the goal of directing drilling operations to avoid between-core gaps in one hole becoming aligned with those in the hole being cored. Unfortunately, real-time correlation was not always possible because many of the drilled strata had very low magnetic susceptibility (<20 IU). In these cases, we made few “live” depth adjustments and interpreted the overlap between holes primarily based on the top depths of cores reported on the drill floor.

These preliminary susceptibility and GRA density data sets were subsequently replaced with data series collected on the WRMSL, which was used to measure all cores after they equilibrated to room temperature. We used WRMSL magnetic susceptibility and GRA bulk density as well as NGR, color reflectance, and the intensity of magnetization following 20 mT AF demagnetization to assign tie points between cores from adjacent holes. All of these measurements are described in “Physical properties” and “Paleomagnetism.” We used additional data as appropriate, such as biozones and core images (see “Biostratigraphy” and “Lithostratigraphy”). Of all the shipboard data sets considered, we typically considered magnetic susceptibility our prime variable for sediments with >40 wt% calcium carbonate. In clay-dominated sediments, our choice of the prime variable depended on the signal-to-noise ratio of all data sets.

To generate the CCSF depth scale, we imported the raw stratigraphic data into Microsoft Excel and culled as necessary to remove anomalous data influenced by edge effects at section boundaries or by coring disturbance. We identified additional one-off outliers by eye and removed these from the record; however, all raw data (including outliers) are available through the LIMS database. We used plots of physical property data for all available holes to assess the stratigraphic continuity of the recovered sedimentary sequences and to add appropriate depth offsets between holes to build the CCSF depth scale and splice.

It is impossible to align all correlative features within cores without squeezing or stretching depth intervals. The appearance of apparently compressed or expanded intervals when comparing 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 are compressed, although this effect is lithology dependent. When there was only a small overlap between cores, we chose tentative ties rather than appending cores (which inserts arbitrarily large coring gaps).

The CCSF depth scale denotes the new, depth-shifted data. Each site chapter contains an affine table that summarizes the depth offsets of all cores. These tables are necessary for converting depth in mbsf to depths in mcd. The mcd depth for any point within a core equals the mbsf depth plus the cumulative offset.

Splicing

After depth shifting to align stratigraphic features, we built a splice from segments selected from multiple holes to form a complete record at each site. The splice includes core sections from adjacent holes such that coring gaps in one hole are filled with core intervals from an adjacent hole. The shipboard splice is a guide for core sampling for detailed depth-continuous paleoceanographic studies. A table and figure presented in each site chapter summarize the intervals from each hole used to construct the splice.

The choice of tie points (and hence of a splice) is partly a subjective exercise. Our method in the construction of each splice followed two key rules. First, where possible we avoided using the top and bottom ~50 cm of each core, where disturbance resulting from drilling artifacts (even if not apparent in physical property data) is most likely. Second, we attempted to incorporate those portions of the recovered core that are most representative of the overall stratigraphic section. Splice ties connect two points in two splice intervals at the same CCSF depth.

The length of the spliced section (on the CCSF 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 CSF-A scale. Reasons for this difference include sediment expansion resulting from elastic rebound, stretching during the coring process, gas expansion during the core recovery process, and curation practices, in which slurry commonly occurring at the tops of cores is curated as part of the core (e.g., Moran, 1997; Acton et al., 2001; Lisiecki and Herbert, 2007).

Ideally, the splice is continuous to the bottom of the deepest core recovered from the deepest hole. In practice, however, the splice is continuous until gaps align across all holes, the data quality no longer allows reliable correlations between holes, or not all holes were drilled to equivalent depths. It is not possible to directly tie cores below an interval with aligned core gaps between multiple holes into the overlying and continuous CCSF scale. However, it is sometimes possible to correlate cores from two or more holes below the base of the continuous CCSF scale in order to create a floating splice. In this case, we add the average differential offset of the last several cores to the floating cores.

Changes in CSF-A depth scales

The mbsf depths assigned to WRMSL measurements in the LIMS database are not always the same as the mbsf depths used in the construction of the splice table. Below-seafloor depths associated with measurement on the WRMSL derive from liner length, measured when the cores are cut into sections on the catwalk. At some later time, typically 10–36 h after measurement on the WRMSL, core sections are split and analyzed further (see “Core handling and analysis”). At this time, section lengths are sometimes remeasured because the sections may have expanded during the period between the whole-round core logging measurements and the section-half analyses, or shifted during splitting and handling. If section length has changed by >1 cm, the database record is updated, which means the CSF-A depth scale is slightly changed. General database queries return depths based on the last updated section length, and the depths associated with the WRMSL data at the time of measurement are therefore not identical to the final depths assigned to a given interval in the database. For each site, we updated our final composite section and splice using the final section lengths. This means that, for a particular stratigraphic feature, mbsf depths used in the splice may not align precisely with mbsf depths reported for WRMSL measurements.

Drift deposits

The drilling of drift deposits complicated the construction of shipboard composite sections for some of the sites on Expedition 342. Side-scan sonar images of the seafloor near the Titanic wreckage location (proximal to the Expedition 342 sites) show clear erosional scours. This indicates persistent deep western-boundary currents, coming from the Gulf of Mexico, the Labrador Sea, and/or the East Greenland Sea, have influenced deposition and erosion on a local scale. These currents generally follow the bathymetric contours of continental margins but can interact with topographies (e.g., ridges, seamounts, etc.) that focus and redirect flow, resulting in spatially variable current energies. The Deep Western Boundary Current redistributes deep-sea sediments to form drift deposits (mainly biogenic mud, clay, silt, and rarely sand) or to erode preexisting sediments. The thickness of surficial Pleistocene deposits varied, in some cases, by a few meters—easily enough to distort hole-to-hole correlations based upon mudline depths. As a result of local erosion/deposition of sediment on the seafloor, the mudline is typically not the strongest anchor for our composite depth scales. In contrast, prominent color and/or sediment compositional changes as deep as 10 m below the sediment surface provide the strongest anchors between holes at a given site.

Evidence from studies of the seafloor and shallow subsurface using seismic reflection profiles indicates that some drifts are characterized by lateral discontinuity because of the effects of changes in local sedimentation and erosion through time. For example, fields of migrating mudwaves (as high as 50 m with wavelengths of hundreds of meters to kilometers) can develop in drift systems (Wynn and Masson, 2008). The seismic character of the drifts targeted in this expedition is generally uniform, sometimes transparent, but subtle reflector geometries suggestive of mudwaves are also visible. The lateral discontinuity of sedimentation rates (including hiatuses) might add further complexity.

Shore-based splice construction

In addition to the possibility of lateral discontinuities between holes at a single site, the homogeneity of drift sediments typically corresponds to low variability in shipboard physical properties. The absence of clear, prominent, meter-scale features in physical properties and the low magnetic susceptibility in clay-rich lithologies impeded shipboard stratigraphic correlation and splice construction for some sites, particularly Sites U1404 and U1405. As a result, we devised a strategy to revise tentative splice tie points using data collected from XRF analyses at TAMU in College Station, Texas, and at the Scripps Institution of Oceanography in La Jolla, California.

XRF elemental measurements are generally assumed to show a larger signal-to-noise ratio compared to shipboard physical property measurements and have been used to improve shipboard composite depth scale and splice construction (Röhl and Abrams, 2000; Evans et al., 2004; Westerhold et al., 2007, 2008; Westerhold and Röhl, 2009). XRF scanning allows the construction of high-resolution, nondestructive estimates of elemental composition from the surfaces of split core sections (Jansen et al., 1998; Richter et al., 2006). Constraints imposed by time and cost caused us to prioritize collection of XRF measurements across intervals where shipboard correlation was ambiguous and across intervals with heavy sampling requests. Individual site chapters include descriptions of the XRF scanning strategy for each site as well as the extent to which the collected data were used to revise the composite depth scale and splice as of January 2013.

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