Date: 04 Jun 96 17:07:17 EDT
From: Rachel Haymon <102332.3430@CompuServe.COM>
To: "dufour, rose"
Subject: Cruise Plan
HAYMON/MACDONALD CRUISE PLAN SUMMARY
We primary propose is to survey the narrow axial zone of the ultrafast-spreading
EPR at 17*18'-42'S using the fiber-optic ARGO II near-bottom optical/acoustic
system and the AMS-120 sonar system. Our purpose is to test the hypothesis
(based on ARGO data from EPR 9-10*N) that along-strike thermal gradients set up
by the segmented pattern of magma supply to fast-spreading MOR's exert primary
control on the distribution and types of hydrothermal vents and vent biota, as
well as on variations in fine-scale volcanotectonic characteristics along the
axial zone. On the 4th order scale at EPR 9-10N, this magmatic control of
hydrothermal discharge is manifested by the concentration of high-temperature
vents along eruptive fissures. EPR 17*18'-42'S is a superb area for further
investigation of relationships between magmatic processes and other axial
processes. Along a segment of ridge only 45 kms long, seismic data show that
the axial magma chamber (AMC) changes along strike from a flat-topped body at
relatively constant depth to a peaked cupola ("spike") that intrudes to within
0.8 km of the seafloor at 17*27'S. This represents the most extreme along-srike
variations in thermal gradients that we know of on the MOR, and contrasts with
the flat-topped AMC at EPR 9-10N. The survey we propose is designed (and will
be interpreted) within the context of seismic reflection/refraction data,
SeaMARC II and MR1 imaging, SeaBeam bathymetry, gravity and magnetic data,
submersible observations, and extensive petrologic/geochemical data that exist
already for the proposed study area and adjacent ridge flanks. These data show
that this apparently unsegmented portion of the EPR is actually partitioned into
at least six 4th-order segments (our proposed survey may reveal more), and that
the axial zone exhibits along-strike changes in morphology and some extreme
along-strike changes in axial lava compositions. Ridge morphology and some
visual observations indicate recent eruptive activity in part of the proposed
survey area. Beyond testing ideas about coupled magmatic/hydrothermal
segmentation along the MOR, we will also observe how hydrothermal and other
axial zone processes are affected by ultrafast spreading rates and extreme
along-strike thermal/magmatic gradients. We will determine the nature of the
axial troughs found along portions of the axial zone in the proposed study area
(axial summit caldera or graben?) and investigate the development of these
important axial features. To this end, we will carry out a secondary Argo
survey of a hydrothermally-active portion of the axial summit graben on the
segment south of the main survey area (at approx. 18.5S. Finally, we will
provide a baseline survey of the fine-scale segmentation and distribution of
vents and biota along a ridge segment destined for future seismic and
submersible studies.
We propose to carry out a 36 day cruise that consists of: 2 days of AMS-120
surveying, 19 days of ARGO-II surveying; 2 days for ARGO II maintenance checks
(required every three days, with a turnaround time of 6 hours); 2 days for
deployment/surveying/ recovery of acoustic transponders used in navigation; and
11 days of transit (from Tahiti to the survey area, and then to Easter Island).
We are not proposing any ship time for additional dredge sampling of basalts or
hydrothermal deposits (beyond that of Sinton et al., 1991) because the length of
the cruise would be prohibitively long.
SeaMARC II records indicate that the axial zone is relatively narrow (<400 m)
throughout the survey area. To achieve sufficient density of coverage, we plan
to drive fourteen 45 km-long, axis-parallel lines through the axial zone with
line spacings of 10-30 m. This will provide us with 100% saturation coverage
where the axial zone is <100 m wide, ranging down to a minimum coverage of 45%
where the axial zone widens to 400 m. [For the 83-km long ARGO survey at EPR
9-10*N, we achieved a maximum of 80% coverage where the axis was narrowly
defined by an ASC <200 m wide, and 40% coverage for the southern third of the
survey area where the location of the ridge axis was less well-defined by the
structure of the axial zone].
Accurate navigation is absolutely necessary to achieve the close line spacings
required for our proposed survey and to determine the relative locations of
fine-scale features with respect to each other. For the proposed survey we will
follow the procedures that we established for our 1989 ARGO I survey to attain a
navigational precision of +5 m throughout the survey area. At the outset of the
survey we will lay out a line of 11 bottom-moored acoustic transponders spaced 1
km west (or east) of the ridge axis and ~5 km apart. These will be surveyed in
using GPS navigation and accurate depths at the drop points. Navigation of the
ARGO II vehicle can then be acquired by ranging off successive pairs of
transponder as the vehicle travels along strike.
For the proposed program, Haymon will be chief scientist at sea, and Macdonald
will be co-chief scientist. The PI's will share responsibility for data
acquisition and analysis. ARGO II watches require 5 people. DSOG provides 2
people per watch and the science party must supply 3 per watch. One of the
watchstanders is designated as a datalogger. This person watches the real-time
video and logs observations digitally in real time. In this way we can manage
the huge visual dataset. The datalogger files are subsequently edited by going
back to the tapes to review and verify the logged observations. By this means
the classification of features is standardized and erroneous data are deleted
from the files. The end product is a set of digitized and categorized
GIS/ArcInfo files that can be plotted in any combination (for example, black
smokers and fissures; vent communities and Age 1 lavas; etc.). This is a very
powerful approach to data management that has worked beautifully for the EPR
9-10N ARGO I dataset.
Dan Scheirer has been separately funded by NSF to carry out an ancillary study
of magnetics measured with a magnetometer mounted on the Argo, and to analyze
high resolution bathymetry collected with a Mesotech sonar that will also be
incorporated on to the Argo sled.
We will also collect CTD and transmissometer data using instruments mounted on
the Argo sled and on the towing cable.