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doi:10.2204/iodp.proc.342.202.2017

Material and methods

The shipboard composite depth scale and splice for Site U1406 were revised using primarily XRF core scanning data. XRF elemental counts were generated using an Avaatech XRF Core Scanner at the Gulf Coast Repository (Texas, USA), equipped with an Oxford Instruments 100W Neptune Rh X-ray tube and a Canberra X-PIPS detector. The X-ray tube was run at 10 kV and 1000 µA, the measurement area was set with a cross-core slit of 10 mm and a downcore slit of 12 mm, and measurements were taken at 2 cm intervals with a live counting time of 20 s. For some intervals where XRF data alone do not yield satisfactory correlations, we correlated intervals using physical properties data (i.e., magnetic susceptibility, GRA, NGR, and color reflectance; see the "Site U1406" chapter [Norris et al., 2014c]). We cleaned the XRF and physical properties data by removing all data points in intervals with drilling disturbance (e.g., core tops) and visual outliers.

The primary depth scale representing the position of a sample or measurement is core depth below seafloor, Method A (CSF-A), in IODP nomenclature (Fig. F2; also available as IODP Depth Scales Terminology v.2 at http://www.iodp.org/policies-and-guidelines). This scale represents the distance from seafloor to a target level within a recovered core. Because cores may expand upon recovery by up to 20%, the IODP database also provides the CSF-B depth scale, where each core with >100% recovery is scaled back into the cored interval but other cores with <100% recovery are not scaled. During Expedition 342, only the CSF-A depth scale was used, equivalent to the mbsf depth scale used during the Deep Sea Drilling Project and Ocean Drilling Program (ODP). The main advantage of the CSF-A scale is that it affords straightforward calculation of sample positions within holes, cores, and sections. The caveat is that it allows for overlaps at core boundaries due to core expansion upon recovery.

The composite depth scale, core composite depth below seafloor (CCSF; equivalent to meters composite depth in ODP nomenclature), is defined by adding offsets to the CSF-A depths of cores to align correlative features in cores from multiple holes, simultaneously eliminating overlaps and filling in core gaps. We named the composite depth scale CCSF-A to emphasize that the CCSF depth scale is the composite version of the CSF-A scale. The core depth shifts that define the CCSF-A scale are based on one correlative feature between any two cores; this depth scale does not involve stretching and squeezing because it is an affine transformation. Here, we revise the shipboard offsets given to cores in the "Site U1406" chapter (Norris et al., 2014c) and present a revised CCSF-A depth scale for Site U1406. In cases where no correlation tie points could be established, we shifted the tops of cores 1 m below the bottom of the previous core, instead of the +0.01 m used for the shipboard splice (see the "Site U1406" chapter [Norris et al., 2014c]). This 1 m shift is based on offsets of cores where we could establish good correlation tie points. Thus, we made a more generous (yet still speculative) accommodation for unrecovered sediments.

We only revised the shipboard splice for Site U1406 (used to guide postexpedition sampling) where we collected new data (Cores 342-U1406A-1H and 342-U1406B-1H and Sections 342-U1406B-2H-1 and 342-U1406C-1H-1 through 1H-5) or where existing data resulted in reinterpretations of core offsets. The construction of the revised splice follows the same procedures as that of the shipboard splice (see the "Methods" chapter [Norris et al., 2014b]). First, we avoid disturbed intervals and do not use the top and bottom ~50 cm intervals of a core where possible. Second, we select intervals from cores that are most characteristic of the stratigraphic interval as represented in the three available holes.

Despite the close proximity between drill holes from a single IODP site, aligned cores from different holes can have different thicknesses for the same stratigraphic interval. This difference may be the result of differential physical compression or expansion of cores during the coring process, or it may represent stratigraphic heterogeneity at the drill location that can be attributed to varying bed form thicknesses. Note that the standard IODP practice for drilling adjacent holes at a given site is to offset the ship by 20 m. Distance between drill sites at the seafloor are likely to be larger, especially where ocean currents are vigorous, as in the case of Expedition 342. In any case, this circumstance requires scaling (i.e., the stretching and squeezing) by mapping multiple stratigraphic features within cores to correlate off-splice intervals to the splice. The scaled off-splice intervals constitute an adjusted CCSF depth scale (CCSF-M). Method M denotes that off-splice intervals have been mapped (hence “M”) to the splice using a scaling factor. By definition, any interval on the splice will have the same numerical depth on the revised CCSF-A scale and the CCSF-M scale. Selected tie points (“mapping pairs”) with well-defined features (e.g., clear maximum or minimum of a data record) control the mapping of off-splice intervals, with linear interpolation between mapping pair tie points.

Compound and composite core images were created using Code for Ocean Drilling Data (CODD), an updated version of the ODP Macro software (Wilkens et al., 2009, 2017). In this program, all cropped core section images collected by the Section Half Imaging Logger (SHIL) are combined into one compound core image based on the CSF-A depth scales for each hole. These compound core images are converted to the revised CCSF-A depth scale using the revised offset table. Subsequently, using the splice intervals and the mapping pair tie points, the images are presented on the CCSF-M scale.

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