During Ocean Drilling Program (ODP) Leg 199 (Fig. F1), a series of eight sites was drilled in the central to eastern equatorial Pacific. The cruise was designed as a latitudinal transect, targeting sediments on Anomaly 25, just prior to the Paleocene/Eocene boundary. One of the primary objectives of ODP Leg 199 was the recovery of continuous, undisturbed, and high-resolution sediment records from the Paleogene. Throughout the entire Oligocene and parts of the Eocene and Miocene, high-resolution lithologic data from Site 1218 (8°53.378´N, 135°22.00´W; 4828 m present-day water depth) and Site 1219 (7°48.019´N, 142°00.940´W; 5063 m present-day water depth) show a very high correlation, at centimeter to decimeter length scales, even though the two sites are separated laterally by >740 km. For this contribution, we developed detailed and improved hole-to-hole, new site-to-site, and core-to-log correlations for Sites 1218 and 1219, thus providing stratigraphic information that forms the basis for additional studies.
For paleoceanographic interpretations and analysis of depth and time series, it is crucial that one is able to demonstrate the continuity of a given record. Thus, one has to be able to recognize the presence of hiatuses in the sedimentary record, and it is necessary to compensate for the limitations that result from the technology used to recover deep-marine sediment cores. ODP sediment cores are typically recovered in ~9.5-m-long sections. Depending on the consistency and hardness of the sediment encountered during drilling, different recovery methods are used (e.g., advanced piston coring [APC], extended core barrel [XCB] coring, or rotary core barrel coring [not used during ODP Leg 199]). Even with a nominal 100% recovery, recovery gaps are present between successive cores. These are at least on the order of tens of centimeters, typically 1–2 m, and rarely are the same strata recovered multiple times. Depending on the recovery method used, cores can also be affected by core fragmentation, slumping, core expansion due to unloading, core stretching and squeezing, and other core deformations, some of them related to the motion of the drill ship during coring (heaves, tides, etc.).
In order to obtain a complete geological record, multiple adjacent holes are cored at each site. By applying a depth offset of typically 2–5 m between cores from different holes during coring, one can then ensure that those intervals missing within a single hole can be recovered from an adjacent hole, which allows an evaluation of the length of core gaps, as well as the construction of a "spliced" representative record. Initially, each ~9.5-m-long core is assigned a depth according to the drill string length. This depth is denoted as meters below seafloor (mbsf). Subsequently, an attempt is made to correlate cores from different holes by using common features and diagnostic excursions in measurements of physical properties (bulk density, magnetic susceptibility, color reflectance, etc.) or magneto- and biostratigraphic records and events. Routinely, the depths of cores from different holes are adjusted to a common depth scale by adding a constant offset over the length of each core. This new depth scale is denoted as meters composite depth (mcd) and is generated shipboard during the cruise. The methodology for this shipboard stratigraphic correlation was pioneered during Legs 94 (Ruddiman et al., 1987) and 138 (Hagelberg et al., 1992). Similar methods were employed and developed further during 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., 1998), 189 (Exon, Kennett, Malone, et al., 2001), and 198 (Shipboard Scientific Party, 2002a). A new innovation of relating depth offsets between separate holes to precalculated tidal movements was introduced during Leg 202 (Shipboard Scientific Party, 2003).
Unfortunately, it has been frequently observed that cores are distorted in length within each ~9.5-m segment. This distortion is due to the coring methods used or to variations in accumulation rates between holes. This problem was discussed in detail by Hagelberg et al. (1995). For example, sediment inside the core can expand as a result of reduced environmental pressure following core recovery, leading to an expanded sedimentary sequence relative to its original length (Moran, 1997). The coring technology can also lead to sediment distortion (Skinner and McCave, 2003). The distortion in depth implies that geological events that are obviously synchronous in time, such as volcanic ash layers, are potentially not correctly aligned on the mcd scale.
This situation is not satisfactory, as far as a detailed stratigraphic correlation is concerned. Two strategies can be used to alleviate this problem. The one that is used shipboard is to construct a "spliced" record from available data by using the mcd scale and by switching the sampling between records from different holes. The splicing procedure requires a decision to be made with respect to what constitutes the "best" (or most representative) track and will depend on the purpose of the particular investigation. For example, one might want to choose, where available, the longest possible track down the core sequence, assuming that this record results in the highest possible resolution in time. Alternatively, one might want to select a "splice" that corresponds to the average increase in depth if, for example, the aim is the reconstruction of sediment flux variations. These arguments were put forward previously during Leg 171B (Shipboard Scientific Party, 1998).
An alternative to the construction of a "spliced" record is the calculation of a stacked record, treating data from different holes as the realization of the same geological section. The procedure for the generation of a common depth scale that allows stretching and squeezing on a centimeter scale was pioneered by Hagelberg et al. (1995) and results in a "revised meters composite depth" (rmcd) scale. In this case, it is necessary to align individual features between different holes at high resolution, allowing differential stretching and squeezing of depths within individual holes. This strategy was also employed by Pälike et al. (2001). As the rmcd scale implies that cores from individual holes are aligned not only at specific depths but ideally along the entire record, it is possible to "stack" (average) data from different cores covering the same stratigraphic interval. This method can facilitate the generation of an enhanced "signal-to-noise" ratio (i.e., "noise" in the data that is independent of the location of the holes can be reduced by averaging several measurements from the same stratigraphic level). New, improved sampling splices can be generated according to the rmcd scale, minimizing sampling waste and analytical time.
Figure F2 illustrates the relationship between the mbsf, mcd, and rmcd scales. In the left panel, gamma ray attenuation (GRA) bulk density data from three holes are shown on the shipboard mbsf depth scale, as determined by the length of the drill string. Note that features that clearly represent identical stratigraphic horizons are slightly offset from each other. The middle panel shows data on a common (mcd) scale as determined shipboard. This common depth scale is generated by matching one single point in depth between individual cores, not allowing any stretching and/or squeezing of depths within a single core. The shipboard mcd scale is used to create a single representative sample track, switching between holes ("splice"). The right panel shows an example of the revised composite depth scale developed here. Note that data are aligned on shorter-length scales compared to the shipboard mcd scale. The aligned data can then be used to generate a stacked record, reducing the noise component in the data (right panel). If the available data only allow 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.