We have assembled material in this chapter to help the reader understand the basis for our preliminary scientific conclusions and also to provide the interested investigator with the information needed to select samples for further analysis. This information concerns only shipboard operations and analyses described in the site reports in the Leg 181 Initial Reports volume of the Proceedings of the Ocean Drilling Program. Methods used by various investigators for shore-based analyses of Leg 181 data will be described in the individual scientific 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):
Information concerning sedimentary stratification in uncored or unrecovered intervals may be inferred from seismic data, wireline-logging results, and from an examination of the behavior of the drill string, as observed and recorded on the drilling platform. Typically, the harder a layer, the slower and more difficult it is to penetrate. A number of other factors may determine the rate of penetration, so it is not always possible to relate the drilling time directly to the hardness of the layers. Bit weight and revolutions per minute, recorded on the drilling recorder, also influence the penetration rate.
When cores are split, many show signs of significant sediment disturbance, including the concave-downward appearance of originally horizontal bands, haphazard mixing of lumps of different lithologies (mainly at the tops of cores), and the near-fluid state of some sediments recovered from tens to hundreds of meters below the seafloor. Core deformation probably occurs during cutting, retrieval (with accompanying changes in pressure and temperature), and core handling on deck.
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 each hole 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 Program (DSDP) (Sites 1 through 624), but 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 exactly equivalent positions in each hole.
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 that the coring operation began, and extends to the depth that the coring operation ended (Fig. F1). Each cored interval is generally 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 greater than 9.5 m.
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, ideally are unique in a given hole. However, this may not be true if an interval is cored twice, if the borehole wall caves in, or other hole problems occur. Full 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. F2). 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 many advanced hydraulic piston core (APC) or extended core barrel (XCB) cores, recovery exceeds the 9.5-m theoretical maximum by as much as 0.60 m. The cause of this expansion is not fully understood. The recovered core in its liner is divided into 1.5-m sections that are numbered serially from the top (Fig. F2). When full recovery is obtained, the sections are numbered from 1 through 7, with the last section generally being shorter than 1.5 m. Rarely, a core may require more than seven sections; this is usually the result of gas expansion having caused voids within some sections. When less than full recovery is obtained, as many sections as are needed to accommodate the length of the core will be recovered; for example, 4 m of core would be divided into two 1.5-m sections and a 1-m section. If cores are fragmented (recovery less than 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 less than 1.5 m may be cut to preserve features of interest. Sections less than 1.5 m in length are also sometimes cut when the core liner is severely damaged.
By convention, material recovered from the core catcher is placed immediately 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. In cases where material is only recovered in the core catcher it is assigned the depth of the top of the cored interval (this convention differs from that used in the early days of deep-sea drilling), although information from the driller or other sources may indicate from what depth it was actually recovered.
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 section. For example, a sample identification of "Sample 181-1119A-11H-6, 10-12 cm" would be interpreted as representing a sample removed from the interval between 10 and 12 cm below the top of Section 6, Core 11 (H designates that this core was taken by the advanced hydraulic piston corer) of Hole 1119A during Leg 181. A computer routine was available to calculate depth in mbsf from any correctly formulated ODP sample designation; this avoided inconsistencies that could have arisen on those occasions where some sections were cut to nonstandard lengths. Although depth mbsf is an invaluable convention, it is not ideal especially for high-resolution work; for further explanation, see "Composite Depths."
All ODP core and sample identifiers indicate core type. The following abbreviations are used: H = hydraulic piston core (HPC; also referred to as APC, or advanced hydraulic piston core); X = extended core barrel (XCB); W = wash-core recovery; and M = miscellaneous material. APC and XCB cores were cut during Leg 181.
As soon as each core was retrieved on deck, a sample taken from the core catcher was given to the paleontological laboratory for an initial age assessment. Special care was taken in transferring the core from the drill floor to a long horizontal rack on a catwalk near the core laboratory so that the core did not bend or twist excessively. The core was capped immediately, and gas samples were taken by piercing the core liner and withdrawing any gas into a vacuum tube. Voids within the core were sought as sites for gas sampling. Some of the gas samples were stored for shore-based study, but others were analyzed immediately as part of the shipboard safety and pollution-prevention program. Next, the core was marked into section lengths of 1.5 m, each section was labeled, and the core was cut into sections. Interstitial water (IW) and whole-round samples were also taken at this time. In addition, headspace gas samples were taken from the end of cut sections on the catwalk and sealed in glass vials for light hydrocarbon analysis. Afterward, each section was 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 its bottom. A yellow cap was placed on the section ends from which a whole-round sample was removed. The caps were usually attached to the liner by coating the end liner and the inside rim of the cap with acetone and then attaching the caps to the liners.
The cores were then carried into the laboratory where the sections were labeled with an engraver to permanently mark the complete designation of the section. The length of the core in each section and the core-catcher sample were measured to the nearest centimeter. This information was logged into the shipboard ORACLE database (JANUS).
Whole-round sections from APC and XCB cores were routinely run through the multisensor track (MST). The MST includes the gamma-ray attenuation porosity evaluator (GRAPE), a P-wave logger, natural gamma-ray emission measurement, and a volume magnetic susceptibility meter. The core-catcher sample is not run through the MST track, for which reason we avoided using it for final biostratigraphic work. Before MST analysis, the temperature of each core was measured. After the core had equilibrated to room temperature (~3 hr), soft sediments were measured for thermal conductivity. After MST analysis, cores were split lengthwise into working and archive halves. Soft cores were split with a wire. Hard cores were split using a diamond saw. The wire-cut cores were split from the bottom to top so that sediment below the voids or soupy intervals that were sometimes present at the top of Section 1 would not be drawn into the voids.
Archive core halves were described visually and then run through the ODP Minolta color scanner and the cryogenic magnetometer. Finally, the cores were photographed with both black-and-white and color film, a whole core at a time. Close-up photographs (black-and-white and/or color) were taken of particular features, as requested by individual scientists, for illustrations in the summary of each site.
The working half of the core was measured first for sonic velocity. After physical properties and paleomagnetic sampling, the working half was sampled for reconnaissance-level and low-resolution shipboard and shore-based laboratory studies. Most sampling for detailed high-resolution paleoceanographic and paleoclimatic studies was deferred until after the cruise to optimize the sampling with the stratigraphic information from biostratigraphy, paleomagnetic stratigraphy, and lithologic correlations.
Each sample taken either for shipboard or shore-based analysis was logged into the ORACLE database (JANUS) by the location and the name of the investigator receiving the sample. Records of all of the samples removed are kept by the curator at ODP headquarters. The extracted samples were sealed in plastic vials or bags and labeled. Samples were routinely taken for shipboard physical properties and paleomagnetic analysis.
Both halves of the core were placed into labeled plastic tubes, which were then sealed and transferred to cold-storage space on board the drilling vessel. At the end of the leg, the cores were transferred from the ship in refrigerated air freight containers to cold storage at the ODP core repository in College Station.
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 181IR-102