The Ocean Drilling Program (ODP) is an international partnership of scientists and governments who have joined together to explore the structure and history of the earth beneath the ocean basins. On a typical ODP cruise, some form of coring is conducted. As a result, there are commonly several tens of meters to several kilometers of core available for scientific analysis, which begins with a variety of shipboard analyses and extends to postcruise shore-based studies.
Shipboard paleomagnetists provide the initial paleomagnetic and rock magnetic analysis of sediments and rocks recovered during an ODP cruise. This information is summarized in the Proceedings of the Ocean Drilling Program Initial Reports volumes and is often used by shipboard and shore-based scientists as the basis for subsequent sampling and investigations and as a primary data source for interpreting the geologic history of the drilling site. The shipboard paleomagnetists are responsible to the scientific community for collecting the appropriate data for characterizing the paleomagnetism and rock magnetism of sites drilled and for ensuring the data are accurate, reliable, and archived. Hence, the paleomagnetism laboratory on board the JOIDES Resolution contains state-of-the-art equipment to perform detailed paleomagnetic and rock magnetic measurements.
Paleomagnetic objectives for ODP are identified in "Paleomagnetic Objectives for the Ocean Drilling Program," a report from a 1986 JOI/USSAC workshop held at the University of California, Davis (Verosub et al., 1986). A copy of this report can be obtained from Joint Oceanographic Institutions, Inc. (JOI) (www.joiscience.org). Summarizing from this report, paleomagnetic objectives can be divided into four general areas:
Shipboard scientists collect, analyze, and compile data in accordance with ODP standards and formats. They assist in the production of shipboard scientific reports, which includes helping to write the site chapters for the Proceedings of the Ocean Drilling Program Initial Reports volumes. After the leg, they are responsible for analyzing their samples and reporting the results in manuscripts in the Proceedings of the Ocean Drilling Program Scientific Results volumes or in any recognized international journal that publishes in English, as outlined in the Publications Policy on the ODP Web site (www-odp.tamu.edu/publications/policy.html). All shipboard scientists are encouraged to assist the Co-Chief Scientists in preparing a press release for general distribution to news media immediately after the cruise.
This section is intended for paleomagnetists who have never participated in an ODP leg or who have not been on a leg for more than 2–3 years. First and foremost, you should prepare before getting to the ship. If you have preferred sampling tools, plastic sample boxes, data analysis software, and reference material, then start packing these in advance. If you do not have a strong preference, the ship has plenty of sampling tools and plastic boxes as described in "Discrete Samples" in "Coordinate Systems and Sampling Conventions." The shipboard library has many standard paleomagnetism textbooks, some of which we keep in the paleomagnetism laboratory for quick reference. The reference material overall is sparse. Plan to bring papers related to the specific science of the cruise in which you will be participating. If you have questions about equipment on the ship or you want to bring some of your own equipment or a computer, contact the staff scientist for your cruise as early as possible.
One of the main steps in being prepared is to understand what is done on the ship. Prior to sailing it is a very good idea to read the "Explanatory Notes" chapters from the Initial Reports volumes of several recent cruises, particularly those with coring objectives most similar to the leg on which you plan to participate. Focus on the paleomagnetism section and at least skim the other sections to understand core flow from the time the core is pulled up to the rig floor, as it passes through the laboratories, and until it is stored for shipping to one of the repositories. Initial Reports volumes are available on the Web (www-odp.-tamu.edu/publications/IR.HTML).
Each cruise has a scientific prospectus that gives background information about the cruise and outlines the cruise objectives, operations plan, logging plan, and sampling strategy. The prospectuses are available on the Web (www-odp.tamu.edu/publications/SCIPROSP.HTML). In addition, oncoming paleomagnetists should read as much of this technical note and the papers cited in it as possible.
All oncoming paleomagnetists would also benefit from communicating with the offgoing paleomagnetists and the offgoing paleomagnetism laboratory technician in port. If this is not possible, much can be learned from reading the draft version of the "Explanatory Notes" chapter of the previous leg, which is kept in black three-ring binders in the shipboard library. The paleomagnetism section of the site chapters and the technician's report for the previous leg and other past legs also provide valuable information. Look over the Long Core Labview manual. Ask the oncoming paleomagnetism laboratory technician for an overview of the laboratory. If possible, have him or her teach you how to operate the cryogenic magnetometer (also referred to as the superconducting rock magnetometer or long-core magnetometer), and then practice, learn about the noise level, and test the calibration of the instrument while in port. Remember that all the technicians are busy with port call activities, so they may not be available until after the ship is under way. Feel free to look around the laboratory to familiarize yourself with the equipment that is available, but use caution and discretion before using instruments or software unless you are very familiar with them or have been trained by the technician.
The shipboard paleomagnetism laboratory is shown in Figure F1, and a virtual tour of the Bridge Deck, which includes the paleomagnetism laboratory, is available from the ODP Web site at www-odp.tamu.edu/drillship/index_6.html. Detailed descriptions of the equipment available in the shipboard paleomagnetism laboratory are provided in "Remanent Magnetization," "Magnetic Susceptibility," "Alternating Field and Thermal Demagnetization," "Laboratory-Induced Magnetization," and "Core Orientation." Two basic magnetic measurements are conducted on whole- and split-core sections and discrete samples: (1) magnetic remanence and (2) magnetic susceptibility.
Measurements of magnetic remanence include natural remanent magnetization (NRM), NRM after thermal or alternating field (AF) demagnetization, and remanence resulting from magnetizations induced in the laboratory, such as isothermal remanent magnetization (IRM), anhysteretic remanent magnetization (ARM), and partial anhysteretic remanent magnetization (pARM).
Magnetic susceptibility is measured routinely at a high resolution (2–10 cm) on whole-round core sections. Equipment to monitor the susceptibility of standard-sized discrete samples and to measure the anisotropy of magnetic susceptibility (AMS) is available for special rock magnetic projects.
The majority of the paleomagnetic equipment is housed on the starboard side of the core laboratory between the photo table and the core description area (Fig. F1). The whole-core susceptibility meter is installed on the multisensor track (MST) in the physical properties area. Whole-core susceptibility data are collected by the physical properties scientists (see Blum, 1997) and shared with paleomagnetism and other shipboard scientists (e.g., the stratigraphic correlator for hole-to-hole correlation) (e.g., Hagelberg et al., 1992).
The work-horse of the paleomagnetic shipboard laboratory is the 2G Enterprises superconducting rock magnetometer (SRM). It is used primarily for continuous remanence measurements on archive halves of cores. AF coils arranged on-axis with the magnetometer and set within the magnetometer's mu-metal shielding allow uniform demagnetization of the cores, so that both NRM and demagnetized remanences can be measured. The cryogenic magnetometer can also be used to measure and demagnetize discrete samples.
The Molspin Minispin spinner magnetometer is a basic field unit interfaced with a personal computer (PC) for control and data acquisition. After the upgrade of the SRM in 1996, the usage of the Molspin magnetometer is minimal.
The Bartington magnetic susceptibility meter has two sensors: one for discrete samples, and a loop for whole-core pass-through measurements. The 80-mm susceptibility loop is part of the MST (see Blum, 1997). The second susceptibility meter with a loop for discrete samples is available in the paleomagnetic laboratory.
The Kappabridge KLY-2 magnetic susceptibility system measures magnetic susceptibility (MS) and AMS of discrete samples. Because the shipboard environment is magnetically noisy, the sensitivity is less than in shielded rooms in shore-based laboratories but is sufficient for most shipboard purposes, especially when measuring igneous rocks. The instrument is a semiautomatic inductivity bridge, operated in conjunction with a Pentium PC.
A DTECH alternating field demagnetizer (model D-2000) is available for demagnetization of discrete samples of rock or sediment. The unit can demagnetize multiple samples (e.g., 5 of the 10 cm3 samples) at the same time at peak AFs of up to 200 mT. The D-2000 can also be used to impart an ARM, in which a direct current (DC) magnetic field is produced continuously across the AF demagnetizer coil, or a pARM, in which the user selects the demagnetization interval over which the field is applied.
A Schonstedt thermal demagnetizer (model TSD-1) is used for thermal demagnetization of dry samples over a temperature range of 0°–800°C. The instrument contains magnetically shielded heating and cooling chambers.
An ASC impulse magnetizer (model IM-10) is available for studies of the acquisition of IRM, the anisotropy of IRM, and the coercivity of remanence. The unit provides short-term fields of up to 1.3 T.
The Schonstedt Portable Three-Axis fluxgate magnetometer can measure small ambient fields, with a range of nT. The sensor fits into small spaces such as the sample access tube of the cryogenic magnetometer.
A Hall-effect magnetometer (model MG-5DP), capable of measuring DC and alternating-current (AC) fields over three orders of magnitude (coils and measurement of strong DC fields.
The Tensor tool measures the orientation of cores recovered by the advanced hydraulic piston corer (APC) with respect to the downhole ambient magnetic field. The instrument contains a three-axis magnetometer and two perpendicular gravity sensors that record the orientation of the core liner with respect to magnetic north and vertical.
After cores arrive on deck, they are cut into 1.5-m-long sections and stored in racks for temperature equilibration. The first measurement station is the MST in the core receiving area, where MS, compressional wave (P-wave) velocity, gamma ray density, and natural gamma radiation are measured on whole-core sections (see Blum, 1997).
After cores are split either with a wire line or a saw, the half cores are designated as archive-half cores and working-half cores. A single line on the outside of the core liner indicates the archive half, and a double line indicates the working half (a mnemonic aid is that "A," as in "archive," has one pointed end = single line, whereas "W," as in "working," has two pointed ends = double line). Figure F2 shows the relative core orientation conventions (+x is vertical upward from the split-core surface of archive halves, +y is left along split-core surface when looking upcore, and +z is downcore) established to place core measurements in a relative reference frame. The same coordinate system is used for physical properties and structural measurements. Placement of this relative coordinate system in a geographical reference frame can be achieved in some instances with the Tensor tool, a downhole orientation device (see "Core Orientation" or the ChRM. It may also be possible to reorient core pieces by matching images of the core exterior with downhole logging images of the borehole wall, such as those obtained with the Formation MicroScanner or Ultrasonic Borehole Imager logging tools. On the ship, nondestructive measurements are made on the archive-half cores, whereas the working-half cores are available for measurements that physically disturb parts of the cores and for the removal of samples for shipboard and shore-based studies.
The archive-half core is used for visual core description, paleomagnetic measurements using the cryogenic magnetometer, color reflectance measurements, and photography. After core photographs have been taken, the archive-half cores are stored in plastic tubes and refrigerated.
The working-half core is used for physical property measurements that require inserting probes into the sediment (P-wave velocity, vane shear strength, and thermal conductivity). Afterward, it is sampled by the paleomagnetic and physical properties scientists before sampling for general shore-based projects on the sampling table for immediate shipboard measurements.
Figure F2 illustrates the coordinate system of the SRM in comparison to the archive- and working-half coordinate systems for reference. The magnetometer software handles the conversion into the core coordinate system, but the raw superconducting quantum interference device (SQUID) voltages are provided in the magnetometer coordinate system (see "Magnetic Susceptibility").
Hard rocks are sampled with a nonmagnetic drill (standard 2.45 cm diameter or other smaller diameter bits that are available on the ship) on a drill press in the sample preparation area. Alternatively, cubes may be cut in a variety of sizes, with the standard being 2 cm x 2 cm x 2 cm, although smaller sizes may be preferred for strongly magnetized rocks or when core material is rare.
There are currently three different nonmagnetic plastic sampling boxes in use for soft sediment sampling (Fig. F3):
Internal volumes were estimated by filling the cubes with water and then measuring the volume of water in a graduated cylinder. Calculation of volumes from caliper measurements of the interval dimensions are slightly larger than the values given above. For all practical purposes, the sample volumes are 7 cm3 for any of the plastic sample boxes when they are filled.
Samples are typically collected in the boxes in two ways. The box can be pushed into the sediment and removed with a U-shaped wire mounted on a handle (Fig. F4). Alternatively, an extruder can be pushed into the sediment to remove the sample, which is then extruded into a plastic box with the aid of a piston (Fig. F4). Extruders that are longer than the sample box is deep work better because they allow the bottom portion of the sediment (that closest to the core liner) to be removed, leaving a flat and less deformed surface for inserting into the sample box. It is also possible to retain just enough sample in the extruder to always fill the sample box by marking the appropriate distance on the piston. Extruders with sharp cutting edges work especially well in more indurated sediments, where pushing sample boxes or drilling or cutting samples may not be practical. The azimuthal orientation of the extruded samples is 180° opposite (equivalent to the orientation of the archive-half) to the orientation of the pushed samples. Caution: The extrusion process introduces one additional step during sampling, which can lead to orientation errors if not done carefully.
U-channel samples (Tauxe et al., 1983; Nagy and Valet, 1993; Weeks et al., 1993) have become increasingly popular because of the large amount of paleomagnetic and rock magnetic data that can be obtained rapidly and at a high resolution when the samples are measured in a the narrow-access long-core SRM (often referred to as a U-channel magnetometer). The revised sampling policy from July 1998 (last updated June 1999) allows the sampling of the "temporary" archive halves, which brought the opportunity to sample spliced core sections of considerable length. More than 3300 U-channels have been taken since Leg 138, with >2300 of them taken just within the last 3 years. They are usually sampled postcruise in the repository; however, a few U-channels are usually stocked on the ship for shipboard sampling. U-channels are collected by pushing rigid U-shaped plastic liners (2 cm x 2 cm cross-section; up to 1.5 m in length) into the split core sections (Fig. F5). Each sample is taken out of the core by guiding a fishing line under the U-channel. A plastic cap is then placed on the U-channel, and the ends are sealed with tape to minimize dehydration of the sediment. To further prevent dehydration, the U-channels can be stored similarly to the split-core sections, which are put in D-tubes with wet sponges in the plastic end caps. Typically four or five U-channels fit within a D-tube.
Data acquisition programs on the ship have been written in a variety of programming languages. In some cases, the acquisition programs are provided by the instrument vendor, and in other cases, they have been written by ODP staff to provide interfaces more applicable to the unique shipboard environment. Over the past several years, Labview has been used as the programming language for many of the acquisition programs. Data acquired by the acquisition programs for most instruments are uploaded into the ODP database.
The SRM control program Long Core (version 3, at the time this was written) is written in Labview (PC) for 2G Enterprises by Bill Mills and customized for ODP. The Minispin program PMagic (version 1.2) is written in Visual Basic by Jakub Rehacek, and the DTECH-2000 control program has been supplied by the vendor. The control program for the Kappabridge is a Fortran program supplied by the vendor, Geofyzika Brno. Neither the Minispin nor the Kappabridge are interfaced with the database. Instead, it is the role of the shipboard paleomagnetists to ensure that data collected with these instruments are archived in the Initial Reports volume.
Once data are acquired with the SRM and located on a local drive, they must be uploaded to the Oracle database. Although this procedure could be fully automated and become part of the data acquisition program, it was decided that an interactive user quality control should separate the two functions. Invalid or erroneous data are frequently acquired, particularly on highly automated systems such as the SRM. The user has the option to delete such data from the local directory before triggering upload to the database, which avoids excessive editing within the database, a process that involves significantly more risk and effort. The data upload is handled through a separate upload utility (CRYOEDIT; www-odp.tamu.edu/isg/appsdev/docs/cryoedit.pdf) and is the responsibility of the ODP technical support representative, but scientists may learn the procedure and operate it themselves.
The ODP Oracle database is designed specifically for ODP's unique shipboard environment and user needs. The system includes >250 data tables in a complex relational scheme, capturing data from the initiation of a leg through core recovery and curation, operational aspects, physical and chemical analyses, core description, and sampling. The paleomagnetic data model is relatively simple. Data for the SRM are contained in four tables (Fig. F6) with related tables for sample identification and depth data shared with other laboratories. MS as part of the physical properties area database model contains five tables (see Blum, 1997). The Tensor tool data tables are presented in Figure F7. Updated data models can be viewed on the Web (www-odp.tamu.edu/database/janusmodel.htm) and are included in Technical Note 37 (ODP Information Technology and Data Services, 2007). Details of the file formats and example data files are given in "Appendix A."
Discrete MS data, AMS data, and magnetic data acquired with the spinner magnetometer are not routinely collected and are not stored in the database. AMS data are usually few and are reported in the Initial Reports volume. Spreadsheets containing these data are stored at ODP and can be obtained from the ODP Data Librarian. As noted above, the shipboard paleomagnetists should also ensure that data collected with these instruments are archived in the Initial Reports volume.
Access to the database is provided by standard Web queries either through the shipboard network or on shore through the Internet (Fig. F8) (www-odp.tamu.edu/database). Standard queries allow the download of discrete and long-core data, MS values, and Tensor tool core orientation data in a format that can be used for data analysis and representation with commercial plotting software.
In the relational ODP database, redundancy of information is minimized for efficient data management. For example, site, hole, core, and section information is entered in specific tables linked in a logical way, and all measurement locations in a particular section are linked to the <Section> table. Similarly, if a core specimen is extracted for shipboard or shore-based analysis, the basic curatorial information is accessed through the <Sample> table, which is linked to the <Section> table, and so on. In the paleomagnetic database models presented in the following sections, the field <section_id> alone or with the fields <interval_top> and <interval_bottom> are the links to the more specific information in the appropriate tables. The <Sample> and <Section> tables are listed in Figure F9.
Measurement locations or sampling locations are identified by leg, site, hole, core, core type, section, and interval (measured in centimeters with 0 cm starting at the top of each section). For example, an SRM measurement made at 40 cm from the top of the third section from the second core collected with the APC in Hole E at Site 1062 during Leg 172 would be identified as Sample 172-1062E-2H-3, 40 cm. Similarly, a plastic box sample taken from the same section, but from the interval that is 50 to 52 cm below the top of the section, would be referred to as sample 172-1062E-2H-3, 50–52 cm.
The actual depth of the sample in the borehole can be calculated in a number of ways. The standard method is to place the top of the core at the top of the cored interval, where the cored interval is determined by drill pipe measurements and is given in units of meters below seafloor (mbsf). Each cored interval is generally 9.5 or 9.6 m long, which is the length of a core barrel, although the length may be shorter than this.
Of course, mbsf depth is only an estimate of the true depth below seafloor and may differ from the true depth for several reason. Ship motion, tides, heave, and deviation of the drill hole from vertical can all cause errors in drill pipe measurements. Depth errors may also result from biases in core recovery estimates, which commonly exceed 100% for APC cores. These artificially high recovery percentages probably result from decompression of sediments (Farrell and Janecek, 1991; Hagelberg et al., 1995; MacKillop et al., 1995; Moran, 1997), entrance of excess sediment into the core barrel as some of the sediment displaced by the walls of the coring shoe is forced inward (pp. 93–96 of Hvorslev, 1949), and to curation practices, in which soupy core material commonly occurring at the top of many cores are curated as part of the core (Acton et al., 2001). In reality, much of the soupy material results from sediment falling into hole or from sediment being stirred at the bottom of the hole. This happens as the roller cone bit, which is part of the bottom-hole assembly (BHA), advances from the top of the core previously recovered to the top of the core that is next to be recovered. If the ship heaves upward as the piston strokes into the sediment, then the debris in the hole can be recovered. Additional expansion of the upper part of each core can occur because the top of the core is exposed to circulating water, particularly as the water jets from the BHA are cleaning out the hole.
Incomplete recovery also results in potential depth errors because ODP curation convention assumes the top of the core corresponds to the top of the cored interval. For example, in the case where there is only 1 m recovered from a 9.6-m-long cored interval, the top of the core may have come from as much as 8.6 m deeper than that estimated by the mbsf convention.
Duplicate recovery within a hole, in which the piston corer repenetrates the same sediment sequence either by piercing the side wall of the borehole or by the BHA shifting laterally in very water saturated and unconsolidated sediment, can also cause depth errors (e.g., Robinson, 1990). Depth errors also result from core deformation, such as "suck-in" that occurs when sediment is sucked up into the core liner. Smaller depth errors result from minor core distortion that occurs to some degree in most APC cores, such as bowed or sheared sediment near the core liner caused by friction as the sediment passes through the coring shoe and into the core liner (pp. 93–100 of Hvorslev, 1949; Acton et al., 2002).
If a complete stratigraphic section is to be constructed, multiple holes are drilled at the same site and a composite section is developed using the meters composite depth (mcd) scale. Examples of composite depth scales and background information on their construction is available (e.g., Prell, Gardner, et al., 1982; Ruddiman et al., 1987; Ruddiman, Kidd, Thomas, et al., 1987; Ruddiman, Sarnthein, Bauldauf, et al., 1988; Alexandrovich and Hays, 1989; Robinson, 1990; Farrell and Janecek, 1991; Hagelberg et al., 1992, 1995; Curry, Shackleton, Richter, et al., 1995; Acton et al., 2001; Barker, 2001). The mcd scale is constructed with its own rigorous conventions and should not be confused with a true depth scale. Most mcd scales are expanded by 2%–20%. Contraction of the mcd scale to one closer to a true depth scale can be done by correlating the complete stratigraphic section constructed from the mcd scale with downhole logging data, where depths are determined by a wireline measurement.
Additional corrections can be applied to derive a more accurate approximation to depth below seafloor. These and other depth issues are explained in detail in a workshop report (Blum et al., 1995). The redefined concepts are integrated in the new database, which features a depth map that allows the rapid calculation of any depth type provided that pertinent data have been acquired and entered.