The "Explanatory Notes" chapter is designed to document the primary procedures and methods employed by the various shipboard laboratories in order to understand the basis of the preliminary interpretations. This information concerns only shipboard operations and analyses described in this volume.
Descriptions of individual drilling sites, summaries of operations, and preliminary results and interpretations are contained in the site chapters. This volume should be treated as a publication to which all the scientists listed in the front pages have contributed. The following shipboard scientists, listed in alphabetical order, made the principal contributions to the following sections:
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. In some cases, the ship may return to a previously occupied site to drill additional holes or to log or deepen an existing hole. Such is the case for Hole 801C, which was first drilled during Leg 129 and was deepened during Leg 185.
For all ODP drill sites, the letter suffix distinguishes holes drilled at the same site. For example, the first hole to be drilled is assigned the site number modified with 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 Deep Sea Drilling Project (DSDP) (Sites 1-624) but prevents ambiguity between site- and hole-number designations. It is important to distinguish between holes drilled at a site because recovered sediments or rocks from different holes do not necessarily 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 begins with the depth below the seafloor at which the coring began and extends to the depth that the coring ended. Each cored interval is generally up to 9.5 m long, which is the length of a core barrel. Maximum recovery for a single core interval 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 (e.g., when coring gas-charged sediments that expand while being brought on deck), recovery may exceed the 9.5 m maximum. During Leg 185 several cores in Hole 801C (e.g., Cores 185-801C-21R, 22R, and 26R) were taken without core liners to minimize jamming by fractured rock.
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, or short sections, 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 numbered; for example, 4 m of core would be divided into two 1.5-m sections and one 1-m section. If cores are fragmented (recovery <100%), sections are numbered serially and intervening sections are noted as void, whether or not shipboard scientists think that the fragments were contiguously in situ. 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 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, by convention the top of the core is equated with the top of the cored interval 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 "185-1149A-10R-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 10R designates that this core was taken during drilling of Hole A with the rotary core barrel, at Site 1149, during Leg 185.
All ODP core and sample identifiers indicate core type. The following abbreviations are used: H = hydraulic piston corer (HPC, also referred to as APC, or advanced hydraulic piston corer), X = extended core barrel (XCB), R = rotary core barrel (RCB), D = diamond core barrel (DCB), W = washed-core recovery, and M = miscellaneous material.
As soon as a core is retrieved on deck, a sample is taken from the core catcher and given to the paleontological 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, whereas 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 taken from the ends of cut sections while on the catwalk and sealed in glass vials for light hydrocarbon analysis. Each section is then 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. 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. Additionally, during Leg 185 sediment samples were taken on the deck for microbiology studies. Special handling of these samples is specified in "Sampling".
Next, the cores are carried into the laboratory, where the core liners are labeled with an engraver to permanently mark the full designation of the 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 band saw or diamond saw. During Leg 185, 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 recorded into CORELOG 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 magnetic studies and physical properties analysis, calcium carbonate (coulometric method), and organic carbon (carbon-nitrogen-sulfur [CNS] elemental analyzer) analyses.
The archive half of each core 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 the archive multisensor track (AMST) digital camera for color shots (see "Color") and a regular camera for black-and-white pictures. Close-up photographs (black-and-white and color) are taken of particular features for illustrations in the site summaries, as requested by individual scientists.
Both halves of the core are then placed into labeled plastic tubes, sealed, and transferred to cold-storage (4°C) space aboard the drilling vessel. At the end of the leg, the cores are usually transferred from the ship in refrigerated airfreight containers to cold storage (4°C) at the ODP Gulf Coast Repository at Texas A&M University. However, the cores obtained during Leg 185 were inadvertently shipped in unrefrigerated containers. Worst-case estimates indicate that the cores warmed from 4° to 12°C over the 10 days of shipment.
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 CORELOG. The core then is cut into 1.5-m-long sections and transferred to the laboratory. Special handling of rock samples taken for microbiology studies is detailed in "Sampling".
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 then is split into archive and working halves in such a way that important features (e.g., veins, cooling unit boundaries, breccias) are equally represented in the two 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 185 was possible only by using paleomagnetic or downhole logging techniques.
The working half of the core is sampled for shipboard physical properties (PP) measurements, magnetic studies, X-ray fluorescence (XRF), X-ray diffraction (XRD), and thin-section studies. Nondestructive PP 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. Records of all samples are kept by the curator at ODP. The archive half then is photographed using the AMST digital camera (see "Basement"), and black-and-white and color film. Close-up photographs (black-and-white and color) are taken of particular features for illustrations in each site summary, as requested by individual scientists. Both halves of the core then 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 185 cores, they are housed at the ODP Gulf Coast Repository at Texas A&M University.
The measurement of depth for ODP cores and logs is subject to several uncertainties. Depths are assigned to cores and samples as a means of fixing them in space, but the "depth" values are only approximations. We do not measure depth, but provide an approximation of depth on the basis of length measurements of drill pipe and other tubulars before they become part of the drill string. The resulting vertical measurement is subject to several errors and inaccuracies.
All vertical measurements of cores, logs, and other downhole measurements (except the precision depth recorder [PDR]) are referenced to the passage of measured lengths of pipe past the driller's datum (dual elevator stool [DES]), which is the last point at which reference marks on the pipe can be seen. Sea level is the permanent datum, and the distance from sea level to the driller's datum can be calculated for each site by subtracting the current ship's draft from the fixed distance between the keel and the DES—if a distance below sea level is desired. Nonetheless, official depth measurements recorded by drillers and operations personnel are based on meters below the rig floor (mbrf).
By convention, core and sample depth are given as meters below seafloor (mbsf) by the scientific party. Though the seafloor datum is convenient and helpful in visualizing the geologic setting, it is a poor datum because its depth below sea level usually is not accurately determined. Seafloor "depth" usually is established by the recovery of a partially filled core barrel containing the seafloor interface and then subtracting the length of the core from the drill pipe length to the bottom of the cored interval. It is subject to error in the assumption of 100% core recovery and to errors of drill-pipe depth (discussed below). A more accurate, though seldom used, determination is to view contact between the core bit and seafloor with the reentry television camera and to record concurrently the drill-string measurement at the DES. Sometimes neither of those determinations is available, and seafloor depth is estimated by PDR, deflection of the driller's weight indicator, or assumption of the same depth as an offset hole.
Inaccuracies in measuring drill-string length apply to all depth measurements. They arise from several sources, with the most significant usually being the stretching of the drill string. In the past, most DSDP/ODP penetrations have been single-bit holes, and the relative depth placement of cores and logs has been of greater interest than measurement of water depth. Because most of the drill string length has been in the water column, the difference in stretch from the top of the hole to the bottom has been negligible. Stretch can be considerable, however, with long drill strings. For example, a 6-km drill string with a heavy bottom-hole assembly or casing string can stretch as much as 15 m (making seafloor depth appear 15 m shallower than it is). When multiple drill strings and varying bottom-hole assembly (BHA) weights are used in deep-water reentry installations, depth discrepancies on the order of meters can occur.
Currently, no corrections are applied for pipe stretch. Even if there were, several other inaccuracies would still be inherent in applying drill-pipe length to depth. Thermal contraction of more than 1 m would apply to the above drill string if the pipe were measured at warm tropical air temperature and then lowered into cold seawater. The tidal range in the open ocean can vary by several meters. The normal search of the dynamic positioning system can induce apparent gain and loss of drill-string depth on the scale of meters, which can change on a core-to-core basis. Other factors include vessel heave, which makes the driller's reference mark a moving target, displacement of the pipe by ocean currents, and change in vessel draft during site occupancy.
Vertical measurement to the centimeter may be useful in measuring samples and locating them relative to other samples within the core, but the vertical location of the core relative to adjacent cores or to the seafloor is unknown even to the meter.
Precision depth recorder (PDR) readings are routinely taken upon arrival at a new site. Though the PDR data are curated and sometimes used as a check on water depth as recorded on seismic records, they are not used for the depth determination of cores or measurements. The readings are taken only to ensure that the drill string is not run into the seafloor before it is time to spud the hole. In many cases, the corrected PDR reading may actually be a more accurate depth than the drill-pipe measurement because of the pipe stretch factor mentioned above. That would only apply to areas of level seafloor because the JOIDES Resolution broad-beam PDR gives inaccurate readings in sloping bathymetry.
Another measurement that must be kept in perspective is that of geographic coordinates of latitude and longitude. Navigation and site location are by the Global Positioning System (GPS). For military security reasons, random errors of up to 70 m are introduced by the U.S. Department of Defense into GPS fixes. (Differential GPS is not available on the open ocean.) Those random errors can be averaged out if enough fixes are taken over time, and that is the methodology used to determine the locations of ODP holes.
The coordinates recorded for ODP holes represent the average location of the GPS antenna (corrected to the ship's moonpool) while the hole was occupied, and not the precise location of the hole. The hole cannot be assumed to be located directly beneath the moonpool. The drill string tends to swing with a pendulum motion below the ship, and the point on its trajectory at the moment of spud is not known. The ship moves about within a circle of ~1% of water depth, and it may not have been at the averaged location when the hole was spudded. Also, the string can be offset some distance laterally by ocean currents and sustained winds.
One minute of latitude represents 1 nmi (6080 ft, or 1852 m). One minute of longitude represents that distance at the equator and proportionately less at all other latitudes. Thus 0.1´ is 600 ft or less, 0.01´ is 60 ft or less, and 0.001´ is 6 ft or less (<2 m). The value of the third decimal of latitude/longitude is therefore highly questionable for site location. The newer practice of recording coordinates in decimals of degrees can be evaluated, considering that 1° of latitude is 60 nmi and 0.0001° is 36 ft or 11 m.
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