Paleomagnetics Technical Report

A. Roth

Summary

Since Expedition 390C sailed without a science party, the main objectives were to install reentry systems and collect preliminary cores and data for the South Atlantic Transect, with the intent of returning for Expeditions 390 and 393. For the Paleomagnetic Laboratory, this entailed measuring core sections on the Superconducting Rock Magnetometer (SRM) from sites U1556A, U1557A, U1558A, and U1559A with all APC cores magnetically oriented except for U1559A.

All magnetic orientation tools (MOTs) appeared to be functioning properly (Icefield tools deployed – no Flex-IT tools). Both Icefield and Flex-IT Keyed End-Seal Snubbers (KESSes) were properly aligned and documented.

No other magnetic measurements were required during the expedition, but most instruments were calibrated and used solely for characterizing the new Pmag Play Cube standards. The exception was the Kappabridge, which was only tested but for communications and basic functionality, but not calibrated nor properly exercised.

General lab diagnostics and maintenance were carried out at the beginning of the expedition (BOX) and the end of the expedition (EOX). The SRM was routinely monitored throughout the expedition.

The only significant issues were communication problems between instruments and computers after system updates and initial difficulty with the SRM and the SQUIDs having unstable output. Both are detailed below, but more in depth writes up are available on Confluence under the lab notebooks.

Measurements

Core Orientation

All three Icefield MI5 magnetic orientation tools (MOTs- 2007, 2043, and 2052) were deployed for all APC cores from U1556A, U1557A, U1557B, and U1558A but not for U1559. Log files from RigWatch were used to create standpipe pressure plots with data every 10 seconds to approximate the sampling interval for the MOTs. From these charts the firing time was determined and used to calculate the magnetic tool facing (MTF) for each core. The values chosen to average were focused on the final intervals just before the core was shot as this point should best represent the actual orientation. The picks for the cores that had the ACP-T run proved challenging as the core was shot immediately and then the wait period was 10 minutes after the shot (normally ~5 minutes before the shot). In this case the pick was biased to the values immediately following the shot.

SRM

All split core archive sections were measured on the SRM (0, 5, 10, 20 mT AF demagnetization steps) at 2 cm intervals. Data was collected for all sections regardless of potential disturbances, gaps, etc. Since the sites were reported to have such low sedimentation rates (<10 m/Ma), it wasn’t uncommon to see multiple reversals within any given section and thus it was not realistic just use the corrected declination data to verify that the MOTs were functioning correctly.

Core Liner Set Screws

For all oriented APC cores, the core liner is fixed relative to the core barrel by a set screw on the core barrel which is inserted into a small hole in the liner between the working-half lines. At least 40% of oriented core liners had evidence of significant set screw migrations to a 2^nd location (many had multiple locations). As with X384 photos were taken to document most of these core liners and set screws holes. No interpretation is offered but the photos are located in ~\IODP_Share\PMag\Core_Orientation\Set_Screw_Photos .

Comments and Issues

SRM

The SRM field was trapped in port in Kristiansand, and the SRM profile looked acceptable, with values under 20 nT at the SQUIDs, but the background measurements were dominated by extreme drift and the SQUIDs appeared very unstable. A second field trap was attempted, with the shield and SQUIDs reaching higher temperatures, allowing the field to be re-trapped but also releasing any potential trapped flux in the SQUIDs. Again, the profile looked good, with even lower values, but still the drift was extremely large and the SQUIDs remained unstable. After several more attempts at re-trapping/flux release at higher and higher temperatures, it became apparent that the Fluxgate probe itself (cable as well) had acquired a strong magnetization (10^2 A/m2).

The Fluxgate probe and cable were demagnetized according to the instrument manual. Then the field was successfully re-trapped. At this point the SQUIDs were stable and the background drift was no longer an issue. The z axis SQUID still had significant noise (the final field trap was performed during transit) but everything was within acceptable levels of noise and drift.

The field was trapped and re-trapped at least 4 times, and more complete write up can be found in the Confluence Lab Notebooks (along with details on the Fluxgate demagnetization). http://confluence.ship.iodp.tamu.edu:8090/x/9ARmB

Some general conclusions are the following:

1. The fluxgate probes and cables can acquire a significant magnetization.

   1. AF demagnetization on the SRM (80 mT) remove most of what is possible to be removed.

   2. AF demagnetization on the D-2000 (up to 200 mT) did not demagnetize the probe and cable any further and the residual moment was still ~10^-2 A/m2.

   3. Fluxgate probes/cables should be stored alone in the mu-metal shield bin and since other components could magnetized them.

   4. The magnetization of the probe and cable are easily measure on the SRM by sending them through as a sample (SHLF) and ideally should be checked before trapping the field.

   5. A newly demagnetized probe will require the Fluxgate unit to have its zero-offset adjusted with the probe in a “zero-field” environment (according to the manual).

   6. The actually fluxgate probe x, y, and z pick up positions are not physically at the same locations and are not properly labeled on the probe (consult manual for actual positions).

2. A clean SRM profile does not necessary mean that the trapped field is in fact low (in the case that the probe added a significant component to the net field) nor does it necessarily mean that the SQUIDs are stable.

3. Better null field values can be achieved by:

   1. Positioning the probe in the middle of the tray away from the highly magnetized tray ends

   2. Once the field is minimized on the Fluxgate at the 10 mOe continuing to the higher 1 mOe sensitivity is possible and will typically yield better results.

4. The cold-head gradient currently seems consistently large and, in the future, should probably be adjusted prior to minimizing the individual axes (as stated in the SRM instrument manual for the initial step prior to minimizing the three axes).

After the field was successfully trapped, the X, Y, and Z demagnetization fields were measured with the hall probes. Peak demagnetization field where within reason (X: 2.5%, Y: 0.7%, and Z: 6.8%) with the actual field consistently higher than chosen field.

All COMs were randomly off on 2020-10-10. In order to get the COMs reset properly, all settings had to be completely deleted and then recreated. The formatting of the COMs setting on confluence was changed to make it easier to read and match what a user would actually see on the computer’s control panel settings.

MagSpy was not working at the BOX and the developer worked with shore to re-instate a prior version of the IMS that now functions properly again.

Magnetic Orientation Tools (MOTs)

After discovering the misalignment of one of the Keyed End-Seal Snubbers (KESS) on X384, likely the cause of years of problematic data, a protocol was needed for easy, accurate, and repeatable alignment of the KESSes. After some basic design work, prototype Mule Shoe and T-Bar shoe holders (for both Icefield and Flex-IT MOTs) were 3D printed and attached to an extruded aluminum slot rail to create an alignment jig. Once satisfied with the functionality of the proto-type jig, the shoes were properly fabricated from aluminum billet on the CNC and then assembled into the final “MOT KESS alignment jig”. A SOP was written for aligning both the Icefield and Flex-IT KESSes. Several IODP techs assisted aligning all of the KESSes while also vetting the SOP (located on Confluence under the Lab Notebooks).

A digital logbook was created for the MOTs that tracks basic orientation run data, but also tool type and number, KESS and pressure barrel numbers, and specific monitoring platform (e.g., which Palm Pilot). A separate log should be generated for each expedition and the blank is stored on ~\IODP_Share\Pmag\Core_Orientation. The paper is logbook is still used for quick notes and ease, but all data needs to be entered into the digital logs.

After many hours of tool use, it was noted that the AA batteries very still at 1.7-1.8 V (without load) and thus had many more runs in them. While investigating the Icefield specs for power consumption, it was noted that the tools are only rated to 3,500 m H2O while in the tool’s pressure barrel (only 300m without) and we are using these tools at times in 5000 m H2O depth with standpipes exceeding 2800 PSI above that hydrostatic pressure. However, the tool and its pressure barrel are actually in another pressure barrel that is closed at sea level and thus never experience any elevated pressures at all.

Kappabridge

The Kappabridge was not actually used on this expedition, but was responsive after all computer software updates.                     

JR-6

The JR-6 was used to characterize Pmag Play Cube standards (see below) and appeared to be properly functioning. Comparisons of the magnetic moments measured on the SRM versus JR-6 were within ~15% of each other, with the JR-6 consistently higher. Differences in volume assumptions for the instruments’ internal calculations may account for some or all of this, but since the samples were measured on the SRM as section taped to SHLF tray in various positions, imprecise placement is also likely to cause discrepancies as well.

D-2000 AF Demagnetizer

The AF Demagnetizer was used to demagnetize (up to 200 mT) Pmag Play Cube standards (see below) and was properly functioning.

ACS Impulse Magnetizer

The IMS-10 and IMS-10-30 were tested to impart IRMs (<80mT) on Pmag Play Cube standards (see below) and were properly functioning.

Thermal Demagnetizer

The Thermal Demagnetizer was used at 120°C for several hours to dry new Pmag Play Cube standards (see below) and was properly functioning.

Haskris

The Haskris water was changed and the reservoir cleaned at BOX and EOX. The previous condensation catchment system was redesigned to more efficiently capture all water dripping off of the heat exchange coils. Since a significant portion of the water spilling on the floor and evaporating is now being contained, the catchment bin fills

up very quickly and needs emptying around every 2- 3 (depending on relative humidity and ship roll and heave). A basic water sensor and circuit was designed to alert the technician when water needs to be emptied.

Standards

The lab was lacking standards that could be run on both the SRM and the JR-6 for cross calibration, but also standards that could be magnetized and demagnetized. Samples were collected into J-cubes from two different sources of play core mud (Mud A: MA01-04, Mud B: MB-01-02, and a mixture of the two: MC01-02). These samples were then dried out in the Thermal Demagnetizer at 120°C and the resulting voids were filled in with epoxy. The resulting suite of standards (referred to as Pmag Play Cubes) were then subjected to various IRMs (<80mT to avoid flux jumps on the SRM), measured on the SRM and JR-6, with AF demagnetization steps on the SRM (up to 80 mT) and on the D-2000 (up to 200 mT). The experiment was repeated to show that samples can be demagnetized and then restored to the previous magnetic state. Obviously long-term alteration may be an issue, but on the shorter times scales the Pmag Play Cubes are standards that can be used repeatedly throughout the lab (including IRMs, ARMs, and AF demagnetization steps).

Lab Computers

Monthly software updates for all Pmag computers were completed on 21-10-2020 and again on 16-11-2020. Both times all COMs correctly mapped on SRM and functioned properly. The Kappabridge responded and appeared to be functioning proper, though a calibration was not attempted. JR-6 had COM issues initially both times but went away after the second computer restart. JR-6 calibration and empty sample holder corrections were both successful.

Mut Uploads

All SRM data was routinely uploaded to LIMS via MUT. Core Orientation data was more of a challenge due to matching the exact header and data format. The previous example file on confluence was not really helpful as it was just basic text with white space characters within header elements, between headers, and after lines, instead of comma separated values with carriage returns. A file from X384 was used as an example to get the exact format and is now link in the SRM user guide for future use.

Data

All X390C data was backed up to Data1 and removed from local storage. All X384 SRM data was verifying to have been properly uploaded and then removed locally as well. Diagnostic data for all instruments (background measurements etc.) were organized in ~\Diagnostic_Data folders for SRM and Core orientation and further organized by expedition on local storage. The intent being to keep the instrument diagnostic data locally (non-sample/non moratorium data) in case of future issues with data and or instrumentation.