Magnetic susceptibility is the degree to which a material can be magnetized in an external magnetic field. Magnetic susceptibility is used mostly as a relative indicator for changes in composition because of its high sensitivity to iron-bearing minerals. These changes can be linked to paleoclimate-controlled depositional processes. The high precision and sensitivity of susceptibility loggers makes this measurement extremely useful for core to core and core to downhole log correlations.
Early in ODP, magnetic susceptibility measurements were taken with a manually controlled susceptibility meter, so measurements on whole cores or archive halves were usually limited. The MSL was integrated into the MST to measure the susceptibility of the whole-round core sections, which significantly increased the number and density of measurements. Later in ODP, a split-core track was built to measure point susceptibilities with a magnetic susceptibility probe.
Magnetic susceptibility measurements have been taken throughout ODP, starting from Leg 101. Details about changes to the magnetic susceptibility data acquisition system are sketchy. During Leg 101, the susceptibility meter had a discrete point susceptibility sensor and a 100-mm pass-through susceptibility loop. By Leg 115, the susceptibility meter was using an 80-mm pass-through loop with an operating frequency of 0.465 kHz. The susceptibility meter was integrated into the MST during Leg 124 and became known as the MSL. The Bartington Instruments MS2C system was installed as part of the MST upgrade during Leg 169. The 80-mm loop had an operating frequency of 0.565 kHz and an alternating field intensity of 80 A/m (0.1 mT). Table T16 briefly summarizes known changes to the magnetic susceptibility data acquisition systems.
The Bartington instrument output values are relative, volume-specific susceptibilities that must be corrected before they can be reported as absolute susceptibilities. However, no calibration or correction was implemented to volume correct the raw susceptibility values. Bulk susceptibility values were collected and reported in SI units but should be considered dimensionless.
Prior to Leg 131, there is little information about data collection procedures for magnetic susceptibility. The cores were stored on a rack to allow them to equilibrate to room temperature because MSL measurements are sensitive to temperature. Starting around Leg 163, drift correction was implemented. Instrument drift may occur during the period of a section scan, which is usually ~10 min. Assuming that drift was linear over the time of interest, the Bartington instrument was zeroed at the beginning of each run, and a zero-background measurement was taken at the end. Using the elapsed time information collected at each analysis location, the susceptibility value can be drift corrected.
More complete information about ODP magnetic susceptibility measurements can be found in Chapter 4 of Technical Note 26 (Blum, 1997).
Whole-core magnetic susceptibility data were collected digitally using computer data acquisition programs except for discrete measurements made before Leg 108. For data collected through Leg 130, the data were uploaded to the S1032 database. The discrete measurements made before Leg 108 were collected on paper forms and encoded on shore. Beginning with Leg 131, the raw data files were saved and returned to ODP/TAMU for archival on the ODP data servers.
The data model for magnetic susceptibility can be found in "Janus Magnetic Susceptibility Data Model" in "Appendix G." The relational diagram and list of the tables that contain data pertinent to magnetic susceptibility, column names, and the definition of each column attribute are included. ODP Information Services Database Group was responsible for the migration of pre-Leg 171 data to Janus. The migration of these data was done in conjunction with the other MST data sets (GRA, PWL, and NGR). Each change in format was documented and added to the MST migration program. Additional information about the migration of PWL data or original file formats can be requested from the IODP/TAMU Data Librarian.
Magnetic susceptibility data can be retrieved from Janus Web using a predefined query. The MSL query Web page allows the user to extract data using the following variables to restrict the amount of data retrieved: leg, site, hole, core, section, specific run numbers, depth range, or latitude and longitude range. In addition, the user can use the output raw data option in the query to extract the raw measurements and data acquisition parameters. Because there are >4.7 million MSL data records in Janus, a user must restrict the amount of data requested.
Table T17 lists the data fields retrieved from the Janus database for the predefined MSL query with the output raw data option. The first column contains the data item, the second column indicates the Janus table or tables in which the data were stored, and the third column is the Janus column name or calculation used to produce the value. "Description of Data Items from Magnetic Susceptibility Query" in "Appendix G" contains additional information about the fields retrieved using the Janus Web MSL query and the data format for the archived ASCII files.
There are several things that can affect the quality of MSL data. One of the most significant factors for magnetic susceptibility measurements is contamination of the cores by metal fragments. Metal shards come from drill bits, fittings, and rusty drill pipe (Sager, 1986). The nature of drilling makes it very difficult to totally eliminate this problem. Drilling method and type of cored material also have major effects. Undisturbed sediments with no drilling disruption or voids will typically give the highest quality measurements. Table T18 summarizes how much of the different types of core were analyzed on the MSL systems.
Because of the lack of a calibration procedure for the MSL system, equipment problems may not be immediately identified. For example, during Leg 130 three different susceptibility loops with different diameters and frequencies were used. Data from Site 803 were collected with an 80-mm loop with a frequency of 0.47 kHz. The Site 803 data show some excessive drift, as this loop was not working properly. For Core 130-804A-3H, a change was made to a 100-mm loop with a frequency of 0.86 kHz. The loop was changed again to an 80-mm loop with a frequency of 0.565 kHz for Core 130-806B-26H. This loop was used until the end of the cruise. Susceptibilities from the three loops are different and not directly comparable.
Core sections were run through the MST system before the liners were opened and the core curated. During the curation process, core material was often shifted. In sedimentary cores, voids may have been closed. Gassy cores may have small voids that continue to enlarge after analysis. Sections may not be completely full, and material may have spread throughout the liner. After curation, this material was shoved up to close voids and the section's curated length was less than what was originally analyzed. The effects can be seen when looking at the data for a section, though the effects are not necessarily as dramatic as GRA or PWL because (1) there are reasonable susceptibility values beyond the curated length of the section (null depth values) and (2) there are zero or negative susceptibility values within the section indicating the measurement interval could be within a void or have less than a full liner.
Hard rock cores can be continuous cylinders with a uniform diameter or can be broken into small irregular pieces. The curation process shifts hard rock pieces, sometimes even shifting core material from its original liner section to an adjacent section liner. Where the core material was in its liner during analysis and where it was eventually placed after curation can be very different. MSL data for these types of cores should be used with caution.
Another important factor that needs to be considered is operator error. Throughout ODP, the operator manually entered core information into the data acquisition program. Typographical errors or entering the wrong information occasionally happened, and some mistakes were not identified. Sometimes, the Scientific Party noticed the error and corrected it for the data included in the Initial Reports volume, but the original files were not corrected. During verification of the migrated MSL data, a lot of effort was expended to find sections that may have been misidentified. Some runs were renamed to different sections. The evidence for misidentification had to be conclusive. The following clues were used to find incorrectly identified analyses: