Shipboard paleomagnetic scientists provided the first paleomagnetic analyses of sediments and rocks recovered by ODP. This information was used by shipboard and shore-based scientists as the basis for further sampling and study and for forming the first general conclusions about the geologic history of the drilling site. The shipboard Paleomagnetism Laboratory contained state of the art equipment that allowed scientists to perform detailed studies.
Main objectives for collecting paleomagnetic data included
Most of the PMAG data were collected with 2G Enterprises 760-R three-axis, pass-through cryogenic (superconducting) magnetometers. The 2G was equipped with an AF demagnetizer in-line with the cryogenic magnetometer, which allowed demagnetization and measurement of the remanent field on the same run. Normally, archive-half sections were run for NRM and at least one demagnetization step. Archive sections were typically not subjected to fields higher than 20 mT in the early years of ODP, but after September 1992 (Leg 147), the ODP panel overseeing scientific data collection agreed to allow the Shipboard Scientific Parties to partially demagnetize the core as high as necessary in an effort to remove drilling-induced overprint and isolate the characteristic remanence.
There were several changes in the PMAG data acquisition and data analysis software, and a series of independently written programs were used. Some of the programs were created under severe time constraints. Several programs may have been available to a Shipboard Scientific Party but there was no single preferential program defined for the analysis of data. After a new 2G 760R magnetometer was installed during the Leg 168 port call and the Janus database became operational during Leg 171, a new version of the data acquisition software was deployed at the end of Leg 172. This new program created a data file with the parameters that had been built into the Janus database.
In addition to the long-core measurements, the Paleomagnetism Laboratory on the JOIDES Resolution was equipped with a wide range of equipment that could be used for detailed studies of discrete samples. The available equipment included:
Discrete samples could be demagnetized in much higher fields or by other methods and run through the cryogenic magnetometer to measure the resulting fields.
One of the methods to help paleomagnetic scientists determine the ambient magnetic field was to measure the orientation of the core both vertically and horizontally. Two tools were used to collect core orientation data: the Eastman-Whipstock Multishot tool and the Tensor multishot tool. The older multishot tool, used during APC core drilling, required a special nonmagnetic drill collar, and orientation data were recorded on 10-mm movie film. The newer Tensor tool still required a nonmagnetic drill collar but could be used even during RCB drilling and collected the data from three magnetometers and two accelerometers digitally. The Tensor data acquisition and analysis software was a significant improvement for determining core orientation. After Janus became operational, an analysis program was developed that uploaded the analyzed data directly into the database.
PMAG data were stored in the S1032 database through Leg 129. Starting with Leg 130, data were written to files which were sent back to ODP/TAMU at the end of each cruise and archived on servers. Log sheets were used to keep track of the analyses of sections and discrete samples. The log sheets were sent back to shore to be microfilmed for archival storage.
The data model for PMAG data can be found in "Janus PMAG Data Model" in "Appendix I." Included are the relational diagram and the list of the tables that contain data pertinent to PMAG analyses, column names, and the definition of each column attribute. ODP Information Services Database Group was responsible for the migration of pre-Leg 171 data to Janus.
The discrete paleomagnetic data collected by shipboard paleomagnetic scientists were not migrated to the Janus database. Although data exist for the discrete sample analyses, the treatments that were applied to the samples were not well documented. It would require a significant amount of time to research each of the discrete analyses and determine whether the sample was just demagnetized in a higher intensity alternating field, thermally demagnetized, or subjected to other treatments such as PARM or IRM.
Most of the continuous paleomagnetic data are available through the cryogenic magnetometer Janus Web query. The PMAG 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, depth range, or latitude and longitude ranges. In addition, the PMAG query gives the user options of retrieving data by treatment type, demagnetization level, core geometry (archive or working half, whole core, etc.), continuous or discrete analyses only, and excluding a user-defined interval of the ends of the sections.
Table T22 lists the data fields retrieved from the Janus database for the predefined PMAG query. 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 the calculation used to produce the value. "Description of Data Items from PMAG Query" in "Appendix I" contains additional information about the fields retrieved using the Janus Web cryogenic magnetometer query and the data format for the archived ASCII files.
Some of the post-Leg 172 discrete sample data are also available. These discrete data have the same problem as the pre-Leg 172 discrete data—the treatments applied to the samples were not well documented. The Janus data model was modified to allow more formal documentation of discrete sample analyses. The treatments often used for studies on discrete samples were added so that scientists could document the treatment and demagnetization level in the data file without needing to put that information into a comment field. The changed database was deployed during Leg 191.
Although the post-Leg 172 uncorrected paleomagnetic intensity and uncorrected and corrected moment data are in the Janus database, the query currently does not have an option of retrieving those data. For additional information, contact the IODP/TAMU Data Librarian. Additional information about ODP PMAG data measurements can be found in Technical Note 34 (Richter et al., 2006).
A tremendous amount of PMAG data were collected by shipboard scientists during ODP. Almost 7.4 million measurements were made on cores recovered during 94 legs (discrete analyses not included), with >5 million of those measurement made since Leg 172 when the Janus database was operational and the new 2G Enterprises 760R magnetometer had been deployed. These numbers reflect the stabilization of PMAG data collection that allowed scientists to collect higher density measurements on sections and more reliable analytical tools for the analyses of data.
Several things can affect the quality of PMAG data. Type of cored material and the drilling method used to recover the core are major factors. The APC system used to recover softer, undisturbed sediments routinely give the best results because the core liner is usually full. The sediments, however, can also contain large quantities of gas, which creates voids in the cored material. Cores cut by XCB and RCB are often biscuits surrounded by drilling mud or irregularly shaped pieces. Voids, smaller diameter core, irregular pieces, and thin runny mud all affect the quality of the measurements.
Operator error may also be a source of errors. Throughout ODP, the operator manually entered core information into the data acquisition program. Typographical errors or entering 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. Much effort expended during verification of PMAG data has gone into finding sections that may have been misidentified or the demagnetization level was wrong. Scientists used log sheets to document what was being analyzed. The log sheets were not routinely returned to ODP/TAMU until Leg 181 and were very helpful when analyses were properly documented. Some runs have been renamed to different sections. The evidence for misidentification had to be conclusive. The following clues were used to find incorrectly identified analyses:
ODP Technical Note 18 (Stokking et al., 1993) contains an extensive discussion of some of the other problems with paleomagnetic data collection. ODP Technical Note 34 (Richter et al., 2006) describes paleomagnetic data collection since the Janus database became operational.
Although the Janus data model and the data acquisition program allow better documentation of discrete sample analyses, most of the post-Leg 191 discrete sample data files did not contain the treatment information. It is not known whether there was a problem with the data acquisition code not writing that information in the file, or whether the scientists did not use the tools in the data acquisition program set up to add that information.