In this chapter, we describe the shipboard procedures and observations that led to our preliminary conclusions. These data are also useful for selecting samples for shore-based research. This information concerns only shipboard operations and analyses described in the site reports in the Leg 183 Initial Reports volume of the Proceedings of the Ocean Drilling Program. Methods used by various investigators for shore-based analyses of Leg 183 data will be described in the individual scientific contributions to be published in the Scientific Results volume.
The separate sections of the site chapters were written by the following shipboard scientists (authors are listed in alphabetical order):
In addition to each site chapter, summary core descriptions ("barrel sheets" and images of each core) are presented (see the "Core Descriptions" contents list).
Ocean Drilling Program (ODP) drill sites are numbered consecutively and refer to one or more holes drilled while the ship was positioned over one acoustic beacon. Multiple holes may be drilled at a single site by pulling the drill pipe above the seafloor (out of the hole), moving the ship some distance from the previous hole, and then drilling another hole.
For all ODP drill sites, a letter suffix distinguishes holes drilled at the same site. The first hole drilled is assigned the site number modified by the suffix "-A"; the second hole takes the site number and suffix "-B," and so forth. Note that this procedure differs slightly from that used by the Deep Sea Drilling Project (DSDP; Sites 1 through 624), but it prevents ambiguity between site- and hole-number designations. It is important to distinguish among holes drilled at a site because recovered sediments or rocks from different holes usually do not come from equivalent positions in the stratigraphic column.
The cored interval is measured in meters below seafloor (mbsf). The depth interval assigned to an individual core begins with the depth below the seafloor at which the coring began and extends to the depth that the coring ended. Each coring interval is generally up to 9.5 m long, which is the length of a core barrel. Coring intervals may be shorter and may not necessarily be adjacent if separated by drilled intervals. In soft sediments, the drill string can be "washed ahead" with the core barrel in place, without recovering sediments. This is achieved by pumping water down the pipe at high pressure to wash the sediment out of the way of the bit and up the space between the drill pipe and the wall of the hole. If thin, hard rock layers are present, then it is possible to get "spotty" sampling of these resistant layers within the washed interval and thus to have a cored interval >9.5 m. When drilling hard rock, a center bit may replace the core barrel if it is necessary to drill without core recovery.
Cores taken from a hole are numbered serially from the top of the hole downward. Core numbers and their associated cored intervals in meters below seafloor usually are unique in a given hole; however, this may not be true if an interval must be cored twice because of caving of cuttings or other hole problems. Maximum recovery for a single core is 9.5 m of rock or sediment contained in a plastic liner (6.6-cm internal diameter) plus about 0.2 m (without a plastic liner) in the core catcher (Fig. F1). The core catcher is a device at the bottom of the core barrel that prevents the core from sliding out when the barrel is being retrieved from the hole. In certain situations (e.g., when coring gas-charged sediments that expand while being brought on deck) recovery may exceed the 9.5-m maximum.
A recovered core is divided into 1.5-m sections that are numbered serially from the top (Fig. F1). When full recovery is obtained, the sections are numbered from 1 through 7, with the last section possibly being shorter than 1.5 m (rarely, an unusually long core may require more than seven sections). When less than full recovery is obtained, as many sections as needed to accommodate the length of the core will be recovered (e.g., 4 m of core would be divided into two 1.5-m sections and one 1-m section). If cores are fragmented (recovery is <100%), sections are numbered serially and intervening sections are noted as void, whether shipboard scientists believe that the fragments were contiguous in situ or not. In rare cases, a section <1.5 m may be cut to preserve features of interest (e.g., lithologic contacts).
By convention, material recovered from the core catcher is placed below the last section when the core is described and is labeled "core catcher" (CC); in sedimentary cores, this is treated as a separate section. The core catcher is placed at the top of the cored interval in cases where material is recovered only in the core catcher. However, information supplied by the drillers or by other sources may allow for more precise interpretation as to the correct position of core-catcher material within an incompletely recovered cored interval.
When the recovered core is shorter than the cored interval, the top of the core is equated with the top of the cored interval by convention to achieve consistency when handling analytical data derived from the cores. Samples removed from the cores are designated by distance, measured in centimeters from the top of the section to the top and bottom of each sample removed from that section.
A complete identification number for a sample consists of the following information: leg, site, hole, core number, core type, section number, piece number (for hard rock), and interval in centimeters measured from the top of the section. For example, a sample identification of "183-1135A-25R-1, 10-12 cm" would be interpreted as representing a sample removed from the interval between 10 and 12 cm below the top of Section 1, Core 25 (R designates that this core was taken during rotary drilling) of Hole 1135A during Leg 183.
All ODP core and sample identifiers indicate core type. The following abbreviations were used: R = rotary core barrel (RCB); B = drill-bit recovery; C = center-bit recovery; S = sidewall sample; W = washed-core recovery; and M = miscellaneous material. Only RCB cores were cut during Leg 183.
As soon as a core is retrieved on deck, a sample is taken from the core catcher and given to the paleontology laboratory for an initial age assessment. Then the core is placed on a long horizontal rack, and gas samples may be taken by piercing the core liner and withdrawing gas into a vacuum tube. Voids within the core are sought as sites for gas sampling. Some of the gas samples are stored for shore-based study, but others are analyzed immediately as part of the shipboard safety and pollution-prevention program. Next, the core is marked into section lengths, each section is labeled, and the core is cut into sections. Headspace gas samples are scraped from the ends of cut sections on the catwalk and sealed in glass vials for light hydrocarbon analysis. Each section then is sealed at the top and bottom by gluing on color-coded plastic caps: blue to identify the top of a section, and clear for the bottom. A yellow cap is placed on the section ends from which a whole-round sample has been removed. These caps are usually attached to the liner by coating the end liner and the inside rim of the cap with acetone, and then the caps are taped to the liners.
Next, the cores are carried into the laboratory where the sections again are labeled. An engraver is used to permanently mark the full designation of each section. The length of the core in each section and the core-catcher sample are measured to the nearest centimeter. This information is logged into the shipboard CORELOG database program. After cores have equilibrated to room temperature (~3 hr), they are run through the multisensor track (MST), thermal conductivity measurements are performed on relatively soft sediments, and the cores are split.
Cores of soft material are split lengthwise into working and archive halves. The softer cores are split with a wire or saw, depending on the degree of induration. Harder cores are split with a diamond saw. During Leg 183, the wire-cut cores were split from the bottom to top; thus, investigators should be aware that older material may have been transported up the core on the split face of each section.
The working half of the core is sampled for both shipboard and shore-based laboratory studies. Each extracted sample is logged into the sampling computer database program by the location and the name of the investigator receiving the sample. Records of all removed samples are kept by the curator at ODP. The extracted samples are sealed in plastic vials or bags and labeled. Samples are routinely taken for shipboard physical properties analysis and for calcium carbonate (coulometric analysis) and organic carbon (CNS elemental analyzer) analyses.
The archive half is described visually. Smear slides are made from sediment samples taken from the archive half. Most archive sections are run through the cryogenic magnetometer. The archive half then is photographed using both black-and-white and color film, a whole core at a time. Close-up photographs (black and white) are taken of particular features for illustrations in the summary of each site, as requested by individual scientists.
Both halves of the core then are placed into labeled plastic tubes, sealed, and transferred to cold-storage space aboard the drilling vessel. At the end of the leg, the cores are transferred from the ship in refrigerated air-freight containers to cold storage at the Gulf Coast Repository of the Ocean Drilling Program at Texas A&M University.
Igneous-rock cores are handled differently than sedimentary cores. Once on deck, the core-catcher sample is placed at the bottom of the core liner, and total core recovery is calculated by shunting the rock pieces together and measuring to the nearest centimeter. This information is logged into the shipboard CORELOG database program. The core is then cut into 1.5-m-long sections and transferred into the laboratory.
The contents of each section are transferred into 1.5-m-long sections of split-core liner, where the bottom of oriented pieces (i.e., pieces that clearly could not have rotated top to bottom about a horizontal axis in the liner) are marked with a red wax pencil. This is to ensure that orientation is not lost during the splitting and labeling processes. Important primary features of the cores also are recorded at this time. The core is then split into archive and working halves. A plastic spacer is used to separate individual pieces and/or reconstructed groups of pieces in the core liner. These spacers may represent a substantial interval of no recovery. Each piece is numbered sequentially from the top of each section, beginning with number 1; reconstructed groups of pieces are assigned the same number, but are lettered consecutively. Pieces are labeled only on the outer cylindrical surfaces of the core. If the piece is oriented, an arrow is added to the label pointing to the top of the section. Because pieces are free to turn about a vertical axis during drilling, azimuthal orientation during Leg 183 was possible only by using paleomagnetic or downhole logging data.
In splitting the core, every effort is made to ensure that important features are represented in both halves. The working half is sampled for shipboard physical properties measurements, magnetic studies, X-ray fluorescence (XRF), X-ray diffraction (XRD), and thin-section studies. Nondestructive physical properties measurements, such as magnetic susceptibility, are performed on the archive half of the core. Where recovery permits, samples are taken from each lithologic unit. Some of these samples are minicores. The archive half is described using the visual core description (VCD) form and is photographed before storage.
The working half of the hard-rock core then is sampled for shipboard laboratory studies. Records of all samples are kept by the curator at ODP. The archive half is described visually, then photographed with both black-and-white and color film, one core at a time. Both halves of the core are then shrink-wrapped in plastic to prevent rock pieces from vibrating out of sequence during transit, placed into labeled plastic tubes, sealed, and transferred to cold-storage space aboard the drilling vessel. As with the other Leg 183 cores, they are housed at ODP's Gulf Coast Repository.
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.