Information assembled in this chapter will help the reader understand the basis for our preliminary conclusions and also enable the interested investigator to select samples for further analysis. This information concerns only shipboard operations and analyses described in the site reports in the Leg 198 Initial Reports volume of the Proceedings of the Ocean Drilling Program. Methods used by various investigators for shore-based analyses of Leg 198 data will be described in the individual contributions published in the Scientific Results volume and in publications in various professional journals.
The separate sections of the site chapters were written by the following shipboard scientists (authors are listed in alphabetical order; no seniority is implied):
Three standard coring systems were used during Leg 198: the advanced piston corer (APC), the extended core barrel (XCB), and the rotary core barrel (RCB). These standard coring systems and their characteristics are summarized in the "Explanatory Notes" chapters of various previous Initial Reports volumes as well a number of technical notes. The Leg 139 Initial Reports volume (Davis, Mottle, Fisher, et al., 1992) includes a particularly detailed description. In addition, the motor driven core barrel (MDCB) system was deployed in an attempt to improve recovery in alternating chert/chalk layers. All systems were applied to maximize recovery of the sediments cored. Most cored intervals were ~9.6-m long, which is the length of a standard core barrel. However, the MDCB system cores 4.5-m intervals. In other cases, the drill string was drilled, or "washed ahead," without recovering sediments to advance the drill bit to a target depth where core recovery needed to be resumed.
Drilled intervals are referred to in meters below rig floor (mbrf), which are measured from the kelly bushing on the rig floor to the bottom of the drill pipe, and meters below seafloor (mbsf), which are calculated. When sediments of substantial thickness cover the seafloor, the mbrf depth of the seafloor is determined with a mudline core, assuming 100% recovery for the cored interval in the first core. Water depth is calculated by subtracting the distance from the rig floor to sea level from the mudline measurement in mbrf. This water depth usually differs from precision depth recorder measurements by a few to several meters. The mbsf depths of core tops are determined by subtracting the seafloor depth (in mbrf) from the core-top depth (in mbrf). The resulting core-top datums in mbsf are the ultimate reference for any further depth calculation procedures.
When cores are split, many show signs of significant sediment disturbance, including the concave-downward appearance of originally horizontal bedding, haphazard mixing of lumps of different lithologies (mainly at the tops of cores), fluidization, and flow-in. Core deformation may also occur during retrieval because of changes in pressure and temperature as the core is raised, and during cutting and core handling on deck.
Numbering of sites, holes, cores, and samples follows the standard Ocean Drilling Program (ODP) procedure. A full curatorial identifier for a sample consists of the leg, site, hole, core number, core type, section number, and interval in centimeters measured from the top of the core section. For example, a sample identification of 198-1207A-1H-1, 10-12 cm, represents a sample removed from the interval between 10 and 12 cm below the top of Section 1, Core 1 (H designates that this core was taken with the APC system) of Hole 1207A during Leg 198. Cored intervals are also referred to in "curatorial" mbsf. The mbsf of a sample is calculated by adding the depth of the sample below the section top and the lengths of all higher sections in the core to the core-top datum measured with the drill string.
A sediment core from less than a few hundred mbsf, in some cases, may expand upon recovery (typically 10% in the upper 300 mbsf), and its length may not necessarily match the drilled interval. In addition, a coring gap typically occurs between cores, as shown by composite depth construction (see the Legs 138, 177, and 189 Initial Reports volumes [Shipboard Scientific Party, 1992, 1999, 2001]). Thus, a discrepancy may exist between the drilling mbsf and the curatorial mbsf. For instance, the curatorial mbsf of a sample taken from the bottom of a core may be larger than that of a sample from the top of the subsequent core, where the latter corresponds to the drilled core-top datum.
If a core has incomplete recovery, all cored material is assumed to originate from the top of the drilled interval as a continuous section for curation purposes. The true depth interval within the cored interval is not known. This should be considered as a sampling uncertainty in age-depth analysis and correlation of core facies with downhole log signals.
General core handling procedures are described in previous Initial Reports volumes and the Shipboard Scientist's Handbook and are summarized here. As soon as cores arrived on deck, gas void samples (if present) were taken by means of a syringe for immediate analysis as part of the shipboard safety and pollution prevention program. Core catcher samples were obtained for biostratigraphic analysis. When the core was cut in sections, whole-round samples were taken for shipboard interstitial water analysis. In addition, headspace gas samples were immediately extracted from the ends of cut sections and sealed in glass vials for light hydrocarbon analysis.
Before splitting, whole-round core sections were run through the multisensor track (MST), and thermal conductivity measurements were taken. The cores were then split into working and archive halves (from bottom to top), so investigators should be aware that older material could have been transported upward on the split face of each section. When short pieces of sedimentary rock were recovered, the individual pieces were split with the rock saw and placed in split liner compartments created by sealing spacers into the liners with acetone.
Coherent and reasonably long archive-half sections were measured for color reflectance using the archive multisensor track (AMST). All archive-half sections were run through the cryogenic magnetometer, described visually and by means of smear slides and thin sections, and photographed with both black-and-white and color film. Close-up photographs were taken of particular features for illustrations in site chapters, as requested by individual scientists. During Leg 198, a digital imaging track system equipped with a line-scan camera was installed for the first time and used extensively.
The working half was sampled both for shipboard analysis—such as physical properties, carbonate, and bulk X-ray diffraction (XRD) mineralogy—and for shore-based studies. Both halves of the core were then put into labeled plastic tubes, sealed, and placed in a cold-storage space aboard the ship. At the end of the leg, the cores were transferred from the ship into refrigerated containers and shipped to the ODP Gulf Coast Core Repository in College Station, Texas.
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
Ms 198IR-102