2. Explanatory Notes¹

Shipboard Scientific Party²

Information in this chapter concerns only the shipboard operations and analyses used for producing the site chapters of the Leg 192 Initial Reports volume of the Proceedings of the Ocean Drilling Program. Methods used by various investigators for shore-based analyses of Leg 192 data will be described in the individual scientific contributions to be published in the Scientific Results volume and elsewhere.

The Leg Summary (Chapter 1) and separate sections of the site chapters were written by the following shipboard scientists (authors are listed below in alphabetical order; no seniority is implied).

Leg Summary: Godfrey Fitton, John Mahoney

Background and Objectives: Mike Coffin, Godfrey Fitton, John Mahoney

Operations: Ron Grout, Paul Wallace

Lithostratigraphy: Ken MacLeod, Hajime Naruse, Jim Ogg, Wietze van der Werff

Biostratigraphy: Jim Bergen, Paul Sikora

Igneous Petrology: Pat Castillo, Lynne Chambers, Bill Chazey, Marguerite Godard, Stephanie Ingle, Clive Neal, Takashi Sano, Paul Wallace, Rosalind White

Alteration: Neil Bannerjee, José Honnorez

Structural Geology: Wietze van der Werff

Paleomagnetism: Stuart Hall, Peter Riisager

Physical Properties: Maria Antretter, Loren Kroenke, Xixi Zhao

Downhole Measurements: Graeme Cairns, Mike Coffin

Core Descriptions: Shipboard Scientific Party

"Core Descriptions" includes summary core descriptions (barrel sheets), core photographs, and thin section descriptions.

Numbering of Sites, Holes, Cores, and Samples

Ocean Drilling Program (ODP) drill sites are numbered consecutively. Each site consists of one or more holes drilled while the ship was positioned over one acoustic beacon. For all ODP drill sites, a letter suffix distinguishes individual holes drilled at the same site. We assign the first hole drilled the site number modified by the suffix A; the second hole takes the site number and suffix B, and so forth. This procedure, which differs slightly from that used by the Deep Sea Drilling Project (DSDP) (Sites 1-624), prevents ambiguity between site- and hole-number designations. Distinguishing among holes drilled at a site is important because sediments or rocks recovered from different holes usually do not come from exactly equivalent positions in the stratigraphic column.

We measure the cored interval in meters below the seafloor (mbsf). The depth interval assigned to an individual core begins with the depth below the seafloor at which coring began and extends to the depth that coring ended. Each coring interval is generally 9.5 m, which is the length of a core barrel. Coring intervals may be shorter and may not necessarily be adjoining if separated by drilled intervals that are not cored. 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. In drilling hard rock, a center bit may replace the core barrel if it is necessary to drill without coring.

Cores taken from a hole are numbered sequentially from the top of the hole downward. Core numbers and their associated cored intervals in meters below the seafloor are unique in a given hole. Normally, 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 ~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 recovery may exceed the 9.5-m maximum. In hard-rock cores, this probably happens when a pedestal of rock fails to break off and be grabbed by the core catcher. When a subsequent core barrel is deployed, this pedestal enters the core barrel and, because it is from the previous cored interval, it can lead to >9.5 m of recovery from the next 9.5-m cored interval.

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 usually 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 are needed to accommodate the length of the core are used; for example, 4 m of core would be divided sequentially into two 1.5-m sections (1 and 2) and a 1-m section (3). If cores are fragmented and recovery is <100%, sections are numbered sequentially and the rest of the cored interval is regarded as void, whether or not shipboard scientists think that the fragments were contiguous when in situ. In rare cases, a section <1.5 m long 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, it is treated as a separate section. The core catcher is placed at the top of the cored interval in cases in which material is recovered only in the core catcher. However, information supplied by the drillers or by logging may allow more precise interpretation as to the correct position of core catcher material within an incompletely recovered cored interval.

By convention, when the recovered core is shorter than the cored interval, the top of the core is equated with the top of the cored interval. Samples taken from the cores are designated by distance measured in centimeters from the top of the section to the top and bottom of each sample.

A complete identification number for a sample consists of the following information: leg, site, hole, core number, core type, section number, piece number (assigned only for igneous rock and sedimentary rock interbedded with igneous rock), and interval in centimeters measured from the top of the section. For example, as in Figure F1, a sample identification of "192-1183A-1R-5, 80-85 cm" indicates a 5-cm sample removed from the interval between 80 and 85 cm below the top of Section 5 of Core 1 (R indicates that this core was taken during rotary drilling) of Hole 1183A during Leg 192.

Sediments

As soon as a core is retrieved on deck, a sample is removed from the core catcher and taken to the paleontology laboratory for an initial age assessment. Then the core is laid out on a long horizontal rack on the catwalk adjacent to the drilling floor. The core is marked into section lengths, each section is labeled, and the core is cut into sections. Headspace gas samples are taken from the ends of cut sections on the catwalk and sealed in glass vials for light hydrocarbon analysis as part of the shipboard safety and pollution-prevention program. During Leg 192, we only took headspace gas samples of sediment at Site 1185. The plastic core liner containing 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 to identify the bottom. The caps are usually attached to the liner by coating the end of the liner and the inside rim of the cap with acetone.

Next, the sections of core are carried into the laboratory, and each is labeled again using an engraver to mark the full designation of the section permanently onto the plastic core liner. The length of the core in each section and of the core catcher sample is 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 passed through the multisensor track (MST). Thermal conductivity measurements are made on relatively soft sediments and the cores are split.

Cores of soft material are split lengthwise into working and archive halves. Softer cores are split with a wire or saw, depending on the degree of induration. Harder cores are split with a band saw or diamond saw. During Leg 192, we split the wire-cut cores from the bottom to top; thus, older material may have been transported up the core on the split face of each section.

The working half of the core is next sampled for both shipboard and shore-based laboratory studies. Each sample extracted is logged into the sampling computer's database program and identified by the sample's location and the name of the investigator receiving the sample. The curator at ODP keeps records of all samples removed from the core. Samples are sealed in plastic vials or bags and labeled. Samples for shipboard physical properties (PP) measurements and for calcium carbonate (coulometric) analysis are taken routinely.

The archive half of the core is described visually and smear slides are made from sediment samples taken from this half. Most archive sections are run through the cryogenic magnetometer. The archive half then is photographed using both black-and-white and color film, an entire core at a time. Close-up photographs (color and black-and-white) of particular features are taken as requested by individual scientists for illustrations in the Initial Reports volume.

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 ODP's Gulf Coast Repository at Texas A&M University.

Igneous and Metamorphic Rocks

Igneous rock cores are handled differently from sedimentary cores. To minimize contamination of cores with platinum-group elements and gold, scientists and technicians removed all jewelry from their hands and wrists before handling the core. Once on deck, the core catcher sample is placed at the bottom of the core liner, and total core recovery is calculated by pushing the rock pieces together and measuring the total length to the nearest centimeter. This information is logged into the shipboard CORELOG database program. Then the core is cut into 1.5-m-long sections and transferred to the laboratory.

The contents of each section are transferred into 1.5-m-long sections of split core liner where the bottoms 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 ensures that orientation is not lost during splitting and labeling. Important primary features of the cores are recorded at this time. The core then is split into archive and working halves. A plastic spacer separates 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 pieces are all assigned the same number but with a consecutive suffix letter (e.g., Piece 1A, 1B, etc.). Pieces are labeled only on the outer cylindrical surfaces of the core. If the piece is oriented, an arrow pointing to the top of the section is added to the label. Because pieces are free to rotate about a vertical axis during drilling, relative azimuthal orientation during Leg 192 was possible only by using paleomagnetic data.

In splitting the core, every effort is made to ensure that important features are represented in both halves. The archive half is described visually. Most archive sections are run through the cryogenic magnetometers. The archive halves are then photographed with both black-and-white and color film, one core at a time. Nondestructive physical properties measurements, such as magnetic susceptibility, are also performed on the archive half of the core. The working half is sampled for shipboard physical properties measurements, paleomagnetic studies, inductively coupled plasma-atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD), and thin section studies. The working half of the hard-rock core then is sampled for shore-based laboratory studies. The curator at ODP keeps records of all samples. After samples are taken, both halves of the core are 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 192 cores, they are housed at ODP's Gulf Coast Repository at Texas A&M University.