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Stratigraphic correlation

During the coring operations of Expedition 302, it became clear that the aim of recovering multiple core copies across the same stratigraphic intervals for hole-to-hole correlation could not be achieved to the extent planned. However, material recovered from separate but closely spaced sites allowed a limited amount of site-to-site correlation, based primarily on physical property data (gamma ray attenuation [GRA] bulk density, magnetic susceptibility [MS], P-wave velocity, and electrical resistivity [RES]), generated with the multisensor core logger (MSCL) (see “Petrophysics”) but also aided by high-resolution geochemical pore water measurements of ammonia concentrations, alkalinity, and other pore water measurements (see “Geochemistry”). Additional shore-based measurements of color reflectance values and natural gamma radiation (NGR) counts proved useful to refine stratigraphic correlations during the shore-based part of the expedition. For the final shore-based splice, inclination data provided by postcruise paleomagnetic studies proved important. When the cores were split, many of them showed signs of disturbance and/or “flow-in,” which could not be identified aboard the ship. These intervals were systematically tabulated and are given in Table T24. Additional data acquired from the onshore phase led to revisions in the original splice. In terms of recovered stratigraphy, the bulk of the material was recovered from Hole M0002A in the upper half of the stratigraphic record and from Hole M0004A in the lower half. Apart from the deeper Core 302-M0004B-3X, which provided overlap with Cores 302-M0002A-49X and 50X at ~216 mbsf, only in the upper ~30 m were multiple copies of cored intervals recovered, allowing for construction of a composite depth scale and spliced record for this short interval.

Depth offset determination data

Three main sources of information were used to determine the vertical stratigraphic offset between cores. The first information was the drillers depths in relation to the mudline. Because of problems with the piston coring device, which was only used for Cores 302-M0001A-1H and 2H, 302-M0003A-1H through 3H, and 302-M0004C-1H through 4H, the mudline was identified confidently only for Core 302-M0004C-1H, which represents the shallowest stratigraphic interval recovered from all holes. Those cores recovered with the extended corer system showed variable recovery rates. In cases where core recovery was low, the absolute depth of the core material could be from anywhere within the cored interval, which occasionally included material from the previous, shallower core and resulted in >100% recovery. Thus, the real stratigraphic depth was adjusted by using the closely spaced physical property measurements from the MSCL (see “Petrophysics”). Because the core flow was slower than expected, it was possible to measure all four physical properties at a resolution of 2 cm, with repeat measurements for MS, allowing data quality control. GRA bulk density and MS data were determined shipboard for all cores except Core 302-M0004C-6X, which did not fit through the MS loops. This core was remeasured during the onshore phase of the expedition. For this core, the absolute magnitude of P-wave velocity, GRA bulk density, and NGR data need to be corrected for sediment volume effects, whereas MS measurements require correction for sensor-type effects (see “Petrophysics”).

Finally, after the high-resolution pore water squeeze and Rhizone samples were analyzed (see “Geochemistry”), the ammonia, alkalinity, and manganese profiles were found to exhibit a characteristic linear change below the top few meters and allowed determination of stratigraphic offsets for cores where MSCL data alone gave ambiguous results. Shore-based paleomagnetic measurements allowed further refinement of stratigraphic offsets.

Data pruning

Raw data, comprising all MSCL measurements, are available in electronic form. According to routinely performed standard calibrations, these data were processed with the Geotek software (see “Petrophysics” in the “Methods” chapter). All sensor values clearly showed the influence of section as well as core breaks. To avoid correlation of section breaks, the Geotek measurements of P-wave signal amplitude and liner thickness were used to programmatically remove data points from near core breaks and across section end caps, resulting in the removal of 4–6 cm at the top and bottom from each section.

MSCL, color, and NGR data, plotted against composite depth, are shown in Figure F13A, 13B, 13C, and 13D for Site M0002; Figure 14A, 14B, 14C, and 14D for Site M0003; and Figure 15A, 15B, 15C, and 15D for Site M0004.

Composite depths and splice

With limited core recovery, an almost complete composite depth section could only be established for the upper ~30 m using a site-to-site integration. Data for the top 50 mcd are shown in Figure F16A (GRA bulk density), F16B (MS), F16C (P-wave velocity), F16D (RES), F16E (L*), F16F (chromaticity a*), F16G (chromaticity b*), F16H (NGR), and F16I (top 30 mcd, magnetic inclination). The stratigraphic ties, described in the following section, result in depth offsets for each core as summarized in Table T25 and allow the definition of a stratigraphic splice as given in Table T26. Composite splice data are shown in Figure F17A and F17B.

A mudline was established for Core 302-M0004C-1H. There was no multiple core recovery to verify the interval between this core and Core 302-M0004C-2H, although the sediment from the top of Core 302-M0004C-2H appears to be identical to the lithology at the base of Core 302-M0004C-1H. Core 302-M0002-1X was matched to an interval covered by Core 302-M0004C-1H, based on NGR and color data. The top core from Hole M0002A also shows very low ammonia pore water concentrations, suggesting that it lies within the stratigraphic interval covered by Core 302-M0004C-1H. Cores 302-M0003A-1H and 302-M0004C-2H both show a characteristic dark layer overlying an olive-green clay, with distinct signals in the chromaticity parameter a*, GRA bulk density, P-wave velocity, and NGR data and supported by magnetic inclination data. Chemical pore water profiles strongly support a continuous stratigraphic succession from Cores 302-M0004C-1H and 2H, and Core 1H also shows higher P-wave velocities than Core 2H.

A strong tie can be made between Cores 302-M0003A-2H through 302-M0004C-3H and 4H, bridging the gap between these two cores. Core 302-M0004A-1H can be tied to Core 302-M0003A-3H. Core 302-M0003A-3H can be linked to Core 302-M0004C-4H and extends the semicontinuous splice to ~19 mcd. Starting from the dark layer found in Cores 302-M0004C-2H and 302-M0003A-1H, chromaticity a* data, which reflect red-green variations, show a distinct sinuisoidal succession from red values across Cores 302-M0004C-2H and 3H and 302-M0003A-1H and 2H to more green values at the base of Cores 302-M0004C-3H and 302-M0003A-2H, followed by more red values in Cores 302-M0004C-4H and 302-M0003A-3H. This succession culminates in a distinct shift to more green sediments downcore, recorded in Cores 302-M0004A-2X, 302-M0004C-6X, and 302-M0002A-5X. This distinct marker is found at ~21 mcd. Below this marker, Core 302-M0004A-3X ties to Core 302-M0002A-6X. Core 302-M0002A-6X can be tied to Core 302-M0004A-3X based on magnetic inclination data, extending the splice to ~28 mcd.

At ~215 mcd, a single Core 302-M0004B-3X can be tentatively tied to Core 302-M0002A-49X and 50X and partially fills a stratigraphic interval not recovered otherwise.

Drillers depths suggest overlap between Holes M0004A and M0002A at ~268 mcd, but this overlap cannot unequivocally be confirmed with MSCL data. For the cores below the top ~40 mcd, most depths were determined such that mcd = mbsf for simplicity. The only departure from this strategy was necessary when subsequent cores overlapped because of >100% recovery. In those cases, one or both of the overlapping cores was slightly depth adjusted to avoid overlap where MSCL measurements would clearly indicate that no such overlap existed.

Postcruise work will involve a more detailed correlation of individual features from the top 40 m, allowing stretching and squeezing in cores, helped by high-resolution image line-scan data. It must be noted that the offshore interpretation was achieved with no knowledge of the actual core quality and was revised during the onshore phase after cores were split and additional measurements were taken. The variable core quality meant that several cores had to be excluded from composite depth development. Downhole logging data collected from Hole M0004B between ~218 and 65 mbsf might allow the placement of cores that fall within this interval to be constrained more tightly.