Introduction

To ensure accuracy of paleomagnetic data and proper instrument functioning, a QAQC procedure was established for the SRM and the JR6 spinner magnetometer. 

We have two sets of paleomagnetic cube standards: PLEASE DO NOT DEMAGNETIZE THEM !!!

• the first set was kindly made by Scripps Institute of Oceanography for our use (IODPSTD01 to IODPSTD12): TO BE USED ONLY WITH THE SRM 

• the second set is homemade (IODPSTD13 to IODPSTD16): TO BE USED ON BOTH JR6 AND SRM

More information about the paleomagnetic cube standards is found at Paleomagnetic Cube Standards.

QAQC with the SRM

Every tray in the paleomagnetism laboratory has a weak magnetic signature. It is therefore important to monitor the magnetization of the trays used during an expedition.

Measuring an empty SECTION TRAY

During the course of an expedition, routine shipboard analyses include the measurement of the natural remanent magnetization (NRM) and its alternating-field (AF) demagnetization of ARCHIVE section halves with the SRM. Because marine sediments can be poorly magnetized, it is important to have an accurate and reliable background value of the SRM with a noise level as low as it is possible to achieve with the ship conditions.

Determining the noise level is fairly easy for the SRM. Follow the steps below to measure an empty section tray as ARCHIVE as part of the QAQC Paleomagnetism SOP. Recent expeditions have used the "A" section tray to measure archive section halves (Figure 1).
 
1) Clean the tray physically: Remove dirt, e.g., vacuum, Windex and wipe down the tray (Figure 1).

2) Clean the tray magnetically: Demagnetize the tray, typically using a field comparable or slightly higher than the highest field you plan to use on real samples. Generally, archive section halves are not AF demagnetized at a field higher than 30 mT. Therefore, a magnetic cleaning of about 50 mT is good enough. Up to 80 mT may be necessary; in general, try not to push the AF unit to its maximum as it damages the coils.
 
3) Measure the tray as section background. Use the same settings (i.e., interval, leader/trailer distance, measurement speed) that you plan to use on the samples that are to be measured. Make sure the measurement does not have any flux jumps or other anomalous features. If it does, remeasure.
 
4) Measure the empty section tray as an archive section. In IMS, select "START", choose "APC/XCB Archive". In the "Manual" tab, enter manually the SAMPLE ID (e.g., "EMPTY TRAY", "BLANK TRAY", "BLANK"), the LIMS ID must be "BLANK" in order to be uploaded as a QC standard to see in QCViewer. Set the section tray length at 155 cm, and then click "USE ME" (Figure 2). Measure the NRM only.

5) Assuming that the tray measurement was accurate and valid (no flux jumps), and the BLANK measurement was similarly accurate, the X, Y, and Z moments for the BLANK sample should be zero once the drift and tray (background) corrections are made; referred to as DBC= drift and background corrected in the data files (farther right columns). Of course, they are not zero, they are instead some (hopefully) very small number, which is the noise level. An example of this measurement is shown below (Figure 3). If the magnetometer is working well and the noise level is where it is suppose to be, the moments with the background and tray correction should be <5 x 10^-10 Am² from the SRM-Assessment report from Shanghai below (Shangai SRM Assessment):
[… the background correction is less than 5 x 10^-10 Am² at the knot and less than 2 x 10^-10 Am² elsewhere (Figure 7). For a typical core section, a moment noise level of 2 x 10^-10 Am² would equate to being able to measure split-core samples with intensities >1.5 x 10^-6 A/m or discrete samples with intensities >2.5 x 10^-5 A/m. This noise level is nearly identical to that which could be attained with the old SRM, e.g., see the Paleomagnetism Explanatory Notes for Leg 206 (Shipboard Scientific Party, 2003). ]

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Measuring an empty DISCRETE TRAY

It may happen that scientists want to measure discrete samples with the SRM during an expedition. It is therefore necessary to keep track of the background of the discrete tray although this tray is not routinely used. The steps are identical to the steps above for the ARCHIVE section tray. The discrete tray has its own Text_ID and flagged as a standard. DO NOT USE "BLANK" which is unique to the ARCHIVE section tray for the JR paleomagnetic lab.

Text_ID of the discrete tray:

QAQC CORELABMAGNETICS 1H DISCRETE TRAY 0/150

HRND11707741

Measuring DISCRETE CUBE samples

Install the tray designed to measure discrete samples. In IMS, confirm the sample preset as shown in Figure 4.

Run a discrete tray background then load the standards into the sample holders on the discrete sample tray.

All discrete samples should be measured in the SRM as if they were collected from a WORKING half. The Text_ID of the standards are associated to a working half section. The position of the discrete samples is Working Top-Away (Position #1 in Figure 5). CAUTION: the standards made by Scripps (IODPSTD01 to IODPSTD12) are oriented as if they were taken from an ARCHIVE half (Position #2 in Figure 5). See Paleomagnetic Cube Standards for more details. Make sure to put the Scripps samples (IODPSTD01 to IODPSTD12) in the discrete tray in the right position: upside-down in the position WTA, top away (arrow of the cube away from the SRM) and +X down (Position #3 in Figure 5) to have these standards as if they were taken from a WORKING half. 

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QAQC with the JR6

Four standards, IODPSTD13 to IODPSTD16, can be measured with the JR6 spinner magnetometer for QAQC. DO NOT USE THE STANDARDS IODPSTD01 TO IODPSTD12 IN THE JR6!!!

It is good practice to measure the IODP standards at the beginning of an expedition and regularly during the expedition. Create a file with a file name such as "QAQC name_of_expedition" (e.g., QAQC X397.jr6) which will contain all the IODP standards measurements for an expedition. Data can be appended to this file which is to be uploaded with MUT2. Insert the IODP standards inside the JR6 and measure their magnetization as regular samples (Figure 6).

Upload with MUT2

Empty trays

When an empty tray (either section or discrete) is measured as a QAQC standard, the data file is saved in C:\DATA\IN folder, along with regular archive section half measurements. A typical empty section tray for QAQC appears as "BLANK_BLANK_other_information_NRM.SRM". The empty discrete tray has its own Text_ID flagged as a standard.

Open MUT2 and select the data files corresponding for QAQC (Figure 7). The project name does not matter, the data file will be automatically uploaded as QAQC. For safety, one can choose to select "999" as Project Name. Click "Upload". Like regular archive section measurements, the data file is copied in the "DATA\archive" folder. Copy all "BLANK" and discrete tray measurements in the "Diagnostic Data" folder for the pmag lab record to monitor the level of the background noise of the SRM and of the trays.

Discrete cube samples

IODP cube standards are measured with the SRM and the JR6. Standards measured with the SRM are saved as regular discrete samples and data are saved in a .DSC file. For standards measured with the JR6, as for regular measurements, three files are necessary for the QAQC data to be uploaded: .jr6, .csv and .txt files (Figure 8). Similarly to the SRM QAQC files, the project name does not matter. Click "Upload" after selecting the QAQC files.

Verification with QCViewer

QCViewer is a web application and as all other IODP applications is best seen with Firefox. QCViewer is found at http://web.ship.iodp.tamu.edu/QCViewer/#/qcviewer.

More information on QCViewer and QAQC SOP in general is found at Laboratory QAQC & Error Analysis.

Displaying the data

To display the data with QCViewer:

1. Select the appropriate group, i.e. "Paleomagnetics (PM)" (Figure 9, #1)

2. Select the template name corresponding to the standard you want to display (Figure 9, #2). There are 21 different templates. For the empty section tray, select "BLANK TRAY" (example shown in Figure 9). For a discrete cube standard, select "IODP STDXX SRM" or "IODP STDXX JR6", depending on the instrument used for the measurement (Figure 10). XX stands for the standard number, i.e. 01 to 16.

3. Select the date range of interest (Figure 9, #3). Start and end dates are mandatory. Different time clocks are used, e.g., UTC vs JR time. It is well advised to set an end date at Day+1 to make sure that the display includes the most recent measurements.

4. Select a display mode (Figure 9, #4). There are two display modes: (i) by date order and (ii) by sequence order. Date order might be more appropriate for discrete cube samples, while sequence order is best for tray.

5. Click "View Data" (Figure 9, #5) - it may take a few seconds for the display to show.

Downloading the data

To download the QAQC data, there are two options:

1. Use the CSV link available on QCViewer for each panel to download the data of interest for a given time period (Figure 9)

2. Use the "Measurements on Standards" application (Figure 11):

   1. Select the analysis (JR6A for QAQC data measured with the spinner and SRM for QAQC data measured with the SRM)

   2. Select a start and end date. Choose Day+1 to make sure that the data include the most recent measurements.

   3. Click "Show Standards"

   4. Copy the Text_ID of the standard of interest in the designated box and click "Get measurements on standards". A .csv file will be downloaded containing all data from this sample during the time window selected.