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Stratigraphic correlation

Composite depths

The scientific objectives of Expedition 302 include examination of the sedimentary record at high resolution as well as recovery of a geological record that is as stratigraphically complete as possible. Core recovery from a single hole is insufficient to accomplish these goals because of recovery gaps between adjacent cores, even with a nominal 100% recovery. To obtain a complete sedimentary record, multiple adjacent holes are cored with an offset in depth of typically 1–2 m between cores from different holes to ensure that intervals missing in a single cored hole can be recovered from an adjacent hole.

Several aspects of Expedition 302 necessitate a modified approach toward hole-to-hole stratigraphic correlation and generation of composite depths compared to previous, non-MSP cruises. In particular, the shorter core length (~4.5 m at full recovery, compared to ~9.5 m with IODP) and a limited number of planned holes (two to three) make it more challenging to ensure complete overlap of cores. During Expedition 302, the main mode of stratigraphic correlation was actually a site-to-site correlation, as the drill sites are close to each other and are tied together confidently by seismic stratigraphy.

The offset in depth required in subsequent holes must rapidly be determined before and during coring. The continuity of recovery is assessed by developing composite sections that align prominent features in physical property data from adjacent holes. The procedure and rationale for this stratigraphic correlation is described in this section and follows the methodology pioneered during ODP Legs 94 (Ruddiman et al., 1987) and 138 (Hagelberg et al., 1992). Similar methods were employed and developed further during ODP Legs 154 (Curry, Shackleton, Richter, et al., 1995), 162 (Jansen, Raymo, Blum, et al., 1996), 167 (Lyle, Koizumi, Richter, et al., 1997), 171B (Norris, Kroon, Klaus, et al., 1999), and more recent ODP legs such as 199 (Shipboard Scientific Party, 2002; Pälike et al., 2005). During ODP Leg 202 it was found that vertical tidal movements can have a significant impact on the predicted meters below seafloor depth for a given stratigraphic interval (Shipboard Scientific Party, 2003). For ACEX, tidal movements were expected to be small, and this was confirmed precruise for the anticipated drilling days and locations, with predicted tides of less than a few tens of centimeters (L. Erofeeva, pers. comm., 2004; Padman and Erofeeva, 2004).

Data employed for stratigraphic correlation

Onboard stratigraphic correlation requires closely spaced data that can be generated rapidly. After each whole-core section had equilibrated to room temperature (typically 3–5 h), it was run through the custom-modified Geotek MSCL (see “Petrophysics”). The MSCL generates high-resolution (down to 2 cm intervals) measurements of magnetic susceptibility (MS), gamma ray attenuation (GRA) bulk density, P-wave velocity, and electrical resistivity (RES). Natural gamma radiation (NGR) and color reflectance data were not measured during the offshore phase and were instead measured during the onshore phase at BCR.

The MSCL was run in two separate modes: rapid and standard. In the rapid mode, the MSCL was run using two MS loops at a depth resolution of 2 cm; other parameters were measured at a depth resolution of 4 cm. The cores were measured quickly after retrieval, and their temperature may not have necessarily equilibrated with the laboratory temperature. Thus, the data are not absolute values of MS and were only used for initial offshore stratigraphic correlation. In the standard mode, all MSCL parameters (MS, bulk density, P-wave velocity, and RES) were measured at a depth resolution of 2 cm. As the core flow was lower than expected, both MS loops were used to measure susceptibility values at the same positions downcore. These duplicate measurements were used for quality control and were averaged where appropriate. The data files saved from the Geotek (version 7.3) software (both raw and processed) were copied across to a dedicated stratigraphic correlation computer via the network, where data from individual cores were assembled into a continuous downhole record with custom written conversion scripts. These scripts also attached mbsf depths for each measurement, as the output from the Geotek software is limited to downcore depths. For the detailed setup of the Geotek scanner, see “Petrophysics.” During the coring operation, it became clear that geochemical data could be used for stratigraphic correlation purposes. Ammonia concentrations (see “Geochemistry”) were loaded into the stratigraphic correlation software and complemented the stratigraphic correlation process.

During the onshore phase, cores were run through an MSCL with an NGR emission sensor. NGR data were generated at 6 cm intervals and proved useful in the correlation of cores, with a distinct signal throughout the stratigraphic intervals recovered. In addition, measurements of color reflectance in the 400–700 nm wavelength range and calculated L*a*b* values were made at a 5 cm depth resolution on the split archive half of the core using a Minolta spectrophotometer (see “Petrophysics”). The red-green parameter (a*), in particular, proved useful for correlation of distinct color changes between the lithologies.

Cores split longitudinally into working and archive halves during the onshore phase allowed for a detailed examination of core quality and verification of correlatable features. Cores were assessed for signs and levels of core disturbance, flow-in, and other coring-related features during the core description (see “Lithostratigraphy”) and line image color scanning phase. The results of these assessments are summarized on the barrel sheets (see “Core Descriptions”) in a Core Disturbance column. An example of the types of core disturbance encountered is given in Table T2.

Composite section development

IODP sample and core depths are recorded in mbsf from the first core in each hole that shows a true mudline, and consecutive depth measurements are determined by the length of the drill string. Several factors can lead to a deviation of the relative distance of geological features in the core from their true in situ stratigraphic separation. For example, sediment inside the core can expand as a result of reduced effective stress following core recovery, leading to an expanded sedimentary sequence relative to its original length (Moran, 1997). Thus, sediment can be missing between cores, even if a nominal 100% recovery is achieved. In addition, variations in ship motion, tides, and heave can result in stretching and/or squeezing of the recovered sediment in particular intervals and even multiple recovery of identical stratigraphic intervals in consecutive cores. To allow the placement of geological data from different holes according to their stratigraphic position, physical property measurements are correlated to create a meters composite depth (mcd) scale. The generation of a mcd scale attempts to match coeval stratigraphic features, as recorded by the MSCL and color reflectance measurements, at the same level by generating a composite record from different holes. This process requires depth shifting cores relative to each other. During this step, the total length on the mcd scale is typically expanded by 10% compared to the mbsf scale, although this factor can vary between ~5% and 15% (Hagelberg et al., 1992; Norris, Kroon, Klaus, et al., 1998). Site-to-site and hole-to-hole correlation was facilitated by using the graphical and interactive UNIX platform software, Splicer (version 2.2), which was developed by Peter deMenocal and Ann Esmay of the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG) (available on the World Wide Web at​BRG/​ODP). Significant modifications were made for this expedition (by H. Pälike) to run the program on Mac OSX 10.3. The log-integration software Sagan was also modified.

Splicer allows data sets from adjacent holes to be correlated simultaneously, making use of interactive cross-correlation computation. After prominent features are aligned and referenced to a new mcd scale, a spliced record is generated by switching holes to avoid core gaps or disturbed sediment, resulting in a continuous record. Another output of the composite depth scale is generation of a template that forms the basis for sediment sampling along a complete section, thus minimizing duplicate sampling and saving analytical time. For this reason, it is highly desirable to apply a constant offset to an entire core such that the depth increments along individual cores on the mcd scale are linearly related to the curated core depth increments. Splicer only allows the application of a constant offset for each core. However, composite depth scales created using the Splicer method are usually not satisfactory to create a stacked record from different holes because not every individual feature present across different holes can be aligned. Hence, if the aim is to increase the signal-to-noise ratio of quasi-periodic cycles that might be recorded in the sedimentary record, it is important to allow stretching and squeezing within individual sections so that data from different holes on the individual cycle level are aligned.

The procedure for generating a common depth scale that allows stretching and squeezing on a fine level was pioneered by Hagelberg et al. (1995) and results in a revised mcd (rmcd) common depth scale. Generation of rmcd scales is typically conducted as part of postcruise work. Composite depth generation using Splicer first begins by assigning the core with the best record of the upper few meters of a hole as the top of the composite record, and it typically contains the mudline. This core is assigned an mcd identical to its mbsf depth. A tie point, which gives the preferred correlation, is selected between data from this first core and a core in a second hole. All the data from the second hole below the correlation point are vertically shifted to align the tie points between the holes. After choosing an appropriate tie point and adjusting the depths, the shifted section becomes the next “reference” section and a tie is made to a core from the first hole. Working downhole in an iterative fashion, each core is then vertically shifted. Where there is no overlap, consecutive cores are appended. The tie points are recorded in a Splicer output (“affine”) table in units of mcd. The affine table that relates mbsf to mcd values along with the applied linear offsets for each core is presented in tabular form in “Stratigraphic correlation” in the “Sites M0001–M0004” chapter. A shortened composite depth table for Site M0004 is given as an example in Table T3. The last two columns in each table give, for each core, the cumulative depth offset added to the IODP curatorial subbottom depth (in mbsf) and the composite depth (in mcd), respectively. The depth offset column allows the calculation of the equivalent depth in mcd by adding the amount of offset listed to the depth in mbsf of a sample taken in a particular core. Table T4 shows a typical example for the Splicer file that defines the switching across holes to generate a spliced composite section. The composite depth and splice-tie point tables from each site chapter are also available in ASCII.

Figure F6 illustrates how creating a composite record allows alignment of the most prominent lithologic features (example from Leg 199, Site 1218; Pälike et al., 2005). In the left panel, MS data from three holes at one site are shown on the mbsf depth scale, whereas in the middle panel, the data are shown on a common depth (mcd) scale together with a generated spliced record, indicating where the sample track was switched from one hole to the next. The right panel shows the rmcd scale, where individual physical property features were aligned by stretching and squeezing, together with a final stacked record from all holes. If the available data allow only an ambiguous or imprecise correlation or if multiple hole data were unavailable, no additional depth adjustments were made. In this case, the cumulative offset remains constant for all subsequent cores. On several occasions cores had a recovery >100% percent, resulting in overlapping mbsf depths. For these cores, we applied small vertical offsets to avoid overlap, while generally trying to keep mcd values close or identical to mbsf depths where there was no overlap between holes. Splice tie points were made between adjacent holes at identifiable, highly correlated features and were placed at transitions rather than peaks. Each splice was constructed beginning at the mudline at the top of the composite section and worked downward. Typically, one chooses one hole as the backbone for the record and cores from other holes are then used to patch in the missing intervals in core gaps (Fig. F6). Intervals were chosen for the splice such that section continuity was maintained, whereas disturbed intervals were avoided. The final alignment of the adjacent holes could be slightly different from the best overall visual or quantitative hole-to-hole correlation because of the constraint that a constant offset be applied to each core by the Splicer software.

Technological advancements

During Expedition 302, it was important to use the stratigraphic correlation to make coring decisions in near real time. This was achieved by introducing several modifications to the standard IODP protocol. Crossover between shifts was done over the Internet connection (mirroring the correlation computer screen between ships). Stratigraphic correlators were able to view the status of the splice on computers while they talked by mobile telephone between the drilling vessel Vidar Viking and the operations center vessel Oden.

In addition to the MSCL data, lower-resolution core catcher sample data (age, grain size, color, and density) were used to support this work. Scientists aboard Oden provided these supplemental data by regularly updating an Excel spreadsheet with this information on the central server, which was available across vessels by means of wireless data transfer and networking. A faster throughput of cores through the MSCL was achieved by incorporating two MS loops calibrated and run at slightly different frequencies.