Tunes Expedition Leg 08


R/V Thomas Washington TUNES08WT Weekly Report 7 January 1992. FROM: H. Paul
Johnson. RE: Weekly Cruise Report, TUNES08WT - Cruise started with a delay due
to the late arrival of the sea surface magnetometer cable to the ship. We left
port for engineering tests at 1600 1 January, and proceeded approximately 50
miles due west of Guam (to nearest deep water). Deep-tow magnetometer was tested
successfully. Returned to Guam on 2 January to pick up magnetometer cable.
Proceeded to survey area, arrival on 6 January. We deployed deep-tow
magnetometer, water gun, seismic streamer and sea surface magnetometer and began
survey of Jurassic Magnetic Quiet Zone boundary area. This morning (7 Jan), the
(old) sea surface magnetometer cable and seismic streamer became tangled in the
.680 cable (probably due to rough weather and slow speed of the tow), and
required approximately 12 hours to clear; both sea surface magnetometer and
streamer were destroyed. Deep-tow magnetometer is working well, and we are
proceeding with survey. Major concern at present is developing tropical storm
Axel to the south east, near Majuro.


R/V Thomas Washington TUNES08 Weekly Cruise Report 15 January 1992 (local) - At
the present time, we have completed our surface geophysical survey (surface
magnetometer, single channel seismic profiles, SEABEAM and gravity) of the
proposed Deep-tow magnetometer lines in the transition between the Jurassic
Quiet Zone (JQZ) and the well-defined magnetic anomalies of the Japanese
Lineations. After completion of the surface geophysical survey, we began a
deep-tow magnetometer survey, from well inside the JQZ, in a NW direction,
toward established magnetic anomaly M28. As of 15 January, we have successfully
completed 250 nautical miles of deep-tow magnetometer survey, and are currently
over proposed Anomaly M38 (approximate age 175 million years). All survey
equipment is working well, and we expect to continue with the deep-tow survey on
this line until 19 January, when we will have reached Anomaly 28 at 160 MY.
Present projections indicate that the time available will allow us to complete a
second parallel deep-tow line, offset approximately 30 nautical miles to the
east. In contrast to our report of last week, all is going well, the weather is
good and morale is high. (H. Paul Johnson, U of WA)


R/V Thomas Washington TUNES08WT Weekly Report FROM: H. Paul Johnson 21 January
1992 - During the past week, we have completed almost 600 nautical miles of
deep-tow track, along our designated Profile 1, from the oldest Jurassic
Magnetic Quiet Zone crust out to basement that is younger than magnetic anomaly
M28. At the present time, we are underway on our second (and final) deep-tow
profile, parallel to line 1 with an offset to the east of approximately 32
nautical miles. The magnetometer is operating at an approximate water depth of
5000 meters (with 8200 meters of wire out), and we are maintaining an average
deep-tow speed of 2.6 knots over-the-ground. Our preliminary interpretation of
the observed crustal magnetization is that the geomagnetic field was undergoing
a transition from a time of stable constant positive polarity with long-period
intensity fluctuations (the early Jurassic Quiet Zone), to a period of 'normal'
magnetic anomaly reversals (the traditional M-anomaly sequence). The transition
period between these two states is characterized by a growing instability in the
field that eventually develops into small amplitude, high frequency reversals.
These small amplitude reversals appear to develop systematically into the larger
scale reversals of the M-sequence. Although we will need the confirmation of
similar crustal magnetization behavior on the second parallel track line, this
appears to be a robust result; the quality of the deep-tow magnetic data exceeds
our highest expectations. At the present time, the weather is excellent, the
equipment all works, the cruise track is straight, and the survey is on
schedule.


R/V Thomas Washington/KGWU NR 1 280030Z Jan92 Weekly Report TUNES08WT - We are
presently in-transit to Majuro, at the completion of our deep-tow magnetometer
survey of the Jurassic Quiet Zone. During that program, we completed over 1000
miles of deep-tow magnetometer measurements, from well within the Jurassic Quiet
Zone (in crust older than ODP site 801C at 166 Ma) out to magnetic anomaly 28.
Our average speed during the deep-tow profiles was 2.5 knots over-the-ground. In
addition to the deep-tow data, we acquired single channel seismic (water gun)
data, 3.5 kHz sub-bottom profiler records, Sea Beam bathymetry, and surface
magnetometer data. We successfully completed 2 parallel survey lines,
approximately 500 nautical miles each, at an offset distance of 32 nautical
miles. The deep-tow magnetometer data from the two lines are well correlated
with each other and have been 'upward continued' to match the sea surface
magnetometer data. Our early interpretation (described in the previous weekly
report) regarding the magnetic field behavior during the transition from the
single polarity Jurassic Quiet Zone to 'normal' magnetic anomalies of the
M-sequence has been confirmed by the completion of the second line. The deep-tow
magnetometer continued to function well during the entire survey period, and was
still working after recovery on deck. During transit to Majuro, we are
continuing to take Sea Beam and surface magnetometer data. If our speed
continues at the present rate (and conditional on being able to maintain our
scheduled arrival in port), we hope to acquire surface magnetic\data and single
channel seismic data over ODP Site SYL-3 (Leg 143), which is located directly on
our transit path to Majuro. The weather is good, morale is very high. (H. Paul
Johnson, UW)