Multisensor track (MST) and color reflectance data collected from Holes 1215A and 1215B were used to determine depth offsets in the composite section. Magnetic susceptibility (MS), GRA bulk density, P-wave velocity, and color reflectance measurements were the primary parameters used for core-to-core correlation at Site 1215. MS, P-wave velocity, and color reflectance data were collected at 2-cm intervals and GRA bulk density at 4-cm intervals on all APC cores recovered from Holes 1215A and 1215B (see "Physical Properties" and "Lithostratigraphy" for details about MST and color reflectance data). The data used to construct the composite section and determine core overlaps are presented on a composite depth scale in Figure F10. The depth offsets that comprise the composite section for Holes 1215A and 1215B are given in Table T5.
The composite data from Site 1215 provide a continuous sedimentary section to ~50 meters composite depth (mcd) (interval 199-1215A-6H-6, 76 cm; 46.96 mbsf; 50.80 mcd). The composite depth scale is ~8% longer than the true sedimentary section, as indicated by the drill string mbsf measurement, and reflects the reduced environmental pressure following core recovery, leading to an expanded sedimentary sequence relative to its original length (Moran, 1997). The expansion experienced at Site 1215 is slightly lower than the typical values of 10%-15% found on other ODP legs (Moran, 1997). Hole 1215B has many intervals with flow-in and drilling disturbance that in most cases are associated with intervals of chert. Therefore, the composite sedimentary sequence was established from MST and color reflectance data, after we culled intervals that showed evidence of disruption. Disturbed intervals removed before construction of the composite section are listed in Table T6.
The lithologic units (see "Lithostratigraphy") are clearly reflected in the MST data and color reflectance data (Fig. F10). The carbonate-poor lithologic Unit I (approximately the top 26 m) is most easily correlated between the two holes by matching MS and P-wave velocity data. GRA density and color reflectance (L*) data reflect a decreasing illite:smectite ratio and increases in ferromanganese oxides downcore in the upper part of lithologic Unit I (see "Physical Properties"). The composite section aligns several prominent ash layers that are present in both holes. MS values are lower in the carbonate-rich lithologic Unit II, and in this interval GRA bulk density and color reflectance data vary according to cyclical changes between light yellow to orange layers and thinner, more clay-rich darker layers. The interval most readily correlated between holes is represented by material from Cores 199-1215A-6H and 199-1215B-5H (~43-54 mcd). The close match of cycles in this interval is illustrated in Figure F11.
The interval between ~30 and 50 mcd shows a clear succession of cycles on a decimeter scale that appears modulated in amplitude as well as thickness. This amplitude modulation, when combined with biostratigraphic time control (see "Biostratigraphy"), is consistent with a climatic forcing related to precession (~19- to 23-k.y. period). Postcruise work will establish the possibility of extracting an orbitally related signal from the data.
Stretching and compression of sedimentary features in aligned cores indicates distortion of the cored sequence. Because much of the distortion happens in individual cores on depth scales of <9 m, it is not possible to align every feature in the MST and color reflectance records accurately by simply adding a constant to the mbsf core depth. Core scale changes will require postcruise processing to align smaller sedimentary features. Only after allowing variable adjustments of peaks in each core can we make an accurate estimate of core gaps.
Following construction of the composite depth section for Site 1215, a single spliced record was assembled for the aligned cores over the upper 50 mcd, primarily by using cores from Hole 1215A as the "backbone" and patching across core gaps with cores from Hole 1215B (Fig. F12; Table T7). Intervals having significant disturbance or distortion were avoided when possible. The Site 1215 splice can be used as a sampling guide to recover a single sedimentary sequence between 0 and 50 mcd, although it is advisable to overlap a few decimeters from different holes when sampling in order to accommodate anticipated ongoing development of the depth scale.