R/V Atlantis - Voyage 7 Leg 4

Preliminary Data

 

 We have posted on this page our Mid-Cruise Report memo sent today to our program managers at the National Science Foundation, and some examples of preliminary maps and data plots produced on board. These works are the product of an extrodinary effort by everyone on board this cruise. From the ship's officers and crew, to the DSOG operations team, the HMRG sonar data processing team, the ABE group, and last, but not least, the scientific party. We thank them all for their dedication and expertise.

Keep checking back daily as we'll continue to add to the data products as they are produced. The plots appear after the Mid-Cruise Report.


Mid-Cruise Report

R/V Atlantis Voyage 7 Leg 4

To: Dave Epp ­ NSF ­MG&G
Dolly Dieter ­ NSF ­ Facilities

Cc: R. Pittenger, J. Luyten, R. Detrick - WHOI
B. Walden, J. Alberts, D. Yoerger, A. Bradley - WHOI

From: Hans Schouten, Maurice Tivey and Dan Fornari
On board R/V Atlantis ­ Voyage 7 Leg4

Date: Nov. 18, 2001

Dear Dave and Dolly:
We are mid-way through our field work here at the East Pacific Rise (EPR) 9deg 30-50'N on R/V Atlantis and thought it would be appropriate to provide a brief summary of what we have accomplished so far and what we plan to do during the time remaining on this cruise. Another reason for this summary is to provide the justification and request to re-schedule our 10 Alvin dives next Spring (2002).

We assume you have discussed with Barrie Walden and Dick Pittenger, the recent events regarding the Alvin view-port with WHOI ­ DSOG. We won't go into details on this, but will only point out that WHOI has gone to great efforts to try to get us a replacement view-port. This included air-dropping one out here while we are on station. Unfortunately, as of yesterday, the view-port sent to us sustained damaged and cannot be repaired at sea.

We'll first summarize what we have done operationally and scientifically, then discuss what we plan to do during the remaining time on this cruise. Finally, we lay out a justification that we hope you will consider for re-scheduling the 10 lost Alvin dives next Spring (CY2002) when there is time in the Atlantis/Alvin schedule.

Summary of Operations to Date:
Atlantis departed Manzanillo, Mexico on Nov. 5 and we arrived on station at the EPR on Nov. 7. After setting and surveying some transponder nets for use with Alvin and ABE (near 9deg 50'N and 9deg 30'N), we commenced sonar surveys with the DSL-120A. So far we have surveyed a box that extends N-S from 9deg 57'N to 9deg 27'N (~55 km) centered on the axial trough and which covers ~2.5 km to either side of the axis. The correspondence between the sonar imagery collected with the new DSL-120A system and the old sonar is excellent. We have preliminary backscatter mosaics and we are continuing to refine navigation but we can confirm that we are accurate to within 100-200 meters of the established position of the axial trough (ASCT). We are using the ASCT which Dan mapped in 1989 [Fornari et al., 1998 JGR] using Argo 100 kHz sonar imagery that was controlled by extensive transponder navigation and which has been confirmed many times over the past decade by Alvin dives located using transponder navigation. There is a slight shift of ~100-200 m to the west that needs to be applied to the data collected on this cruise.

Sonar Data ­ Preliminary Interpretations
The key preliminary observations from the sonar data are:
1) The trace of the ASCT over the 55 km area mapped shows excellent correlation to detailed features mapped in 1989 using Argo II [Haymon et al., 1991, EPSL; Fornari et al., 1998, JGR], and in 2000 using the old DSL-120 sonar [AHA-Nemo2 cruise]. As mentioned above, navigation on this cruise has largely been by layback but precision is within 100-200 meters, and the shift is nearly all one where the current data should be shifted slightly west. One area where the trace and character of the ASCT in the sonar data collected over the past two weeks do not match well is near 9deg 29'-26'N where we observe extensive sheet flow features issuing from the west margin of the ASCT over a 5 km long region.
2) The EPR crest over the mapped region shows extensive re-paving by successive volcanic flows as evidenced by scalloped flow-front margins consistent with eruptions sourced from the axis and flowing out onto the upper rise flank out to ~1-2 km. There are very few faults within ~2km of the ASCT. Most of those are low relief (<~10m) based on sonar shadow geometry and all but one trend nearly parallel to the N10W trend of the EPR axis in this area.
3) There are numerous indications of dendritic flow channels, likely floored by smooth surfaced sheet flows, which originate from the ASCT over areas as great as ~ 5km (along strike), and coalesce away from the trough at distances of ~500m to <1000m. Spacing between these areas of inferred sheet flows can be from a few hundred meters to several kilometers. These features are key areas where ground truth observations and sampling are required and where we hoped to focus our Alvin dives.
4) Fields of ~10-30m high lava mounds or ridges, several hundred meters wide and up to ~1 km long, occur at distances of >1.5 km from the ASCT and in some areas are concentrated at ~2-2.5 km in the region between ~9deg 53'-57'N, 9deg 43'-47'N and 9deg 31'-34'N on the east side of the axis, and at 9deg 43'N, 9deg 35'-36'N west of the axis. At times the pillow mounds appear cut by faults and fissures, while in some areas their construction appears to be younger than the tectonic features they overprint.
5) The region of the ASCT between 9deg26'-29'N appears very different between the DSL-120 sonar data collected last year on the Melville cruise (AHA-Nemo2) and this year. Specifically, over this ~5 km region, the ASCT looks much more subdued in this year's data; relief on the splayed ASCT walls appears much less and the trace of the ASCT seems to be gone from ~9deg 26'N to 9deg 27'N. The spatial coincidence of this anomaly in the ASCT trace and the extensive area of apparent sheet-flows cascading to the west, down the ridge crest is striking. Our working hypothesis is that there has been an eruption in this area during the past 18 months. We had tentatively planned several Alvin dives to investigate the nature of these flows, how they terminate to the west, and the character of the walls and floor of the ASCT in this region to further study these relationships.

ABE ­ Autonomous Underwater Vehicle Surveys
We have also conducted two ABE test dives to calibrate the transponder net to be used for vehicle navigation and to test the various survey sensors and vehicle systems. We just completed the first long (~18 hour) ABE dive (#55) which begins our detailed near-bottom mapping of the Central Axial Magnetic High (CAMH) and micro-bathymetry in the 9deg 50'N area. We estimate it will take another 3 ABE dives to complete our mapping in this area. We had expected to begin diving with Alvin in the next few days in the 9deg 30'N area so that we could carry out the stated objectives of our research in comparing the variations in the CAMH and the implications for building the upper ocean crust at these two sites on the EPR crest. Our initial field and navigation logistics constrained us to be operating ABE and Alvin simultaneously for this second part of the field program, and operating the vehicles in geographically different navigation nets (ie. 9deg 50'N and 9deg 30'N areas) due to overlap in transponder frequencies and ping cycles.

Other Survey Operations
We began rock coring specific volcanic features in the sonar data during turn-around times between ABE Dives and sonar surveys. Three rock cores have been carried out to date and more are planned. We are also working towards jury-rigging together parts of Dan's camera equipment to make a simple towed camera that could help ground-truth some of the sonar contacts. We hope to have this operational in a few days.

Planned Survey Work for the Remainder of This Cruise
Our funded research consists of a 3-part survey strategy ­ detailed sonar imagery using the DSL-120, ABE for near-bottom magnetics and Alvin for visual observations and sampling. One critical element of our plan is visual observations and sampling across seafloor volcanic and tectonic contacts, so that positive geological relationships can be established between the sonar data and ABE near-bottom magnetics and micro-bathymetry. Without these data our research concerning the processes which build the ocean crust at fast-spreading ridges will be severely compromised.

As mentioned above, our original plan was to conduct nearly simultaneous ABE and Alvin dives to collect near-bottom data in the two study areas near 9deg 30'N and 9deg 50'N. ABE is being used to collect high-density (40 m line spacing) near-bottom magnetics and micro-bathymetry data (Imagenex 675kHz scanning sonar). These data will provide us with detailed maps of variations in the CAMH within the two areas and will allow us to correlate volcanic and tectonic features displayed in the sonar data with the location of the edge of the anomaly, and its width and character across the axis.

ABE surveying began in earnest with Dive #55; this dive was completed today. If all goes well, we hope to deploy ABE on a ~2 day cycle (24 hrs surveying, 24 hrs, recharging and downloading data). This scenario will provide us with an ability to acquire data on 6-7 more ABE dives before we have to head for port on the afternoon of Dec. 1. This plan is consistent with mapping the CAMH in two areas ~ 1km x 3km in size at 9deg 50'N and 9deg 30'N.

In the absence of Alvin dives, and while ABE is diving, the following work will also be carried out.
1) complete two more long DSL-120A sonar lines to extend the eastern and western margins of our sonar map out to ~3.5km on either side of the axis,
2) conduct a site-specific sonar survey of the area around the ASCT between 9deg 29'-26'N to investigate further the nature of the extensive sheet flows emanating from the west margin of the axial trough,
3) rock core specific targets displayed in the sonar data,
4) transponder navigated spot dredging of areas on either side of contacts displayed in the sonar data,
5) build a rudimentary deep sea camera out of parts on board to attempt to get some visual ground-truthing across volcanic and tectonic contacts in the sonar data.
Given the above information and plan, we confirm that we require the remainder of the cruise to finish the ABE diving and sonar surveying tasks .

Request for 10 Alvin Dives in Spring CY2002
Despite the significant efforts made by WHOI-DSOG to solve the view-port problem, it appears clear at this time that we will not do any diving on this cruise. As you can imagine, all of us are disappointed at this setback. But, we are continuing our field work and trying to maximize the amount of data we can collect using the tools and vehicles at hand.
Given these extraordinary circumstances, and everyone's best efforts, we would like to officially request that you consider putting us on the CY2002 schedule for a 10 dive program with Alvin. After reviewing the schedule it is clear that time for this short dive series could be inserted in the early-mid Spring.
While our research budget did not have funds for extra travel, we can commit to funding the personnel and travel costs associated with the rescheduled dives out of our existing grant and using some of Dan's recently awarded WHOI Senior Scientist Chair funds which will be in effect starting Jan. 2002. We realize that normally, CY2002 scheduling is complete, however, given the circumstances and our critical need for the Alvin dives to complete our research, we feel justified in making this request. We hope you will be favorably disposed towards granting our request.
If you have any questions concerning the status of our field work or our request for the Alvin dives next Spring please email us on board R/V Atlantis.

Best Regards,
Hans, Maurice, and Dan

hschouten@atlantis.whoi.edu
mtivey@atlantis.whoi.edu
dfornari@atlantis.whoi.edu


Preliminary Cruise Maps and Data Plots

(Check back often for new maps and plots. The latest data will be at the bottom of the page)

DSL-120A sonar imagery for the East Pacific Rise crest between 9° 53.5'N and 9° 51.5'N. White areas are strong acoustic returns, grays and blacks are lower returns and acoustic shadows, respectively. Red arrows point to pillow mounds. Magenta arrows point to narrow fissures along the EPR axis. Blue arrows point to scalloped lava flow fronts - the downslope ends of lava flows that erupted at the EPR axis and flowed generally east and west down the ridge crest to either side. Thin yellow lines represent the path of the sonar fish over the seafloor. The HMRG data processors - Margo Edwards, Paul Johnson, Akel Sterling and Tomoko Kurokawa have been terrific at processing the data in record time. The DSOG team and crew of R/V Atlantis have been great at maintaining the system, tweaking it so it makes the best records possible, and have been expertly launching and recovering it. Thanks to all of them!


DSL-120A sonar imagery for the East Pacific Rise crest between 9° 48.5'N and 9° 51.5'N. White areas are strong acoustic returns, grays and blacks are lower returns and acoustic shadows, respectively. Red arrows in this image point to areas of sheet lava flows. We believe these flows help transport lava rapidly away from the ridge axis. Magenta arrows to narrow fissures along the EPR axis. Blue arrows point to scalloped lava flow fronts - the downslope ends of lava flows that erupted at the EPR axis and flowed generally east and west down the ridge crest to either side. Han's calls these "cow flaps". Thin yellow lines represent the path of the sonar fish over the seafloor.


DSL-120A sonar imagery for the East Pacific Rise crest between 9° 47.0'N and 9° 48.5'N. White areas are strong acoustic returns, grays and blacks are lower returns and acoustic shadows, respectively. Red arrows point to pillow mounds. Magenta arrows point to narrow fissures that comprise the axial summit collapse trough (ASCT). Blue arrows point to scalloped lava flow fronts - the downslope ends of lava flows that erupted at the EPR axis and flowed generally east and west down the ridge crest to either side. Thin yellow lines represent the path of the sonar fish over the seafloor.


Track map of ABE Dives #54 AND #55 on the crest of the East Pacific Rise in the 9° 50'N area made by Maurice Tivey. Red lines show where ABE surveyed the seafloor from an altitude of 20 meters collecting near-bottom magnetic data and Imagenex 675kHz scanning altimeter data. The bathymetry in this map is from multibeam sonar and the pixel size is 80 m on a side. This is about 80 times coarser than the resolution of the Imagenex bathymetry shown below. Depths are in meters.


Micro-bathymetry produced and imaged by Dana Yoerger and Maurice Tivey based on ABE Dive #55 Imagenex data. The color shading is every 2 meters and the contours are every 1 meter. Color bar at the bottom shows the range of depths.


ABE Dive #55 near-bottom magnetic data showing the edge of the Central Axial Magnetic High (CAMH) [blue linear feature in the middle of the map] made by Maurice Tivey. We are very excited by the detailed spatial character of the magnetic anomaly and the very intriguing correlation with the micro-bathymetry. ABE Dive #56 will extend the coverage of the micro-bathymetry and magnetics data west to the EPR axis.


Added November 21, 2001

Track map of the 3 ABE dives carried out so far on this cruise. The location of the deep sea camera lowering #1 was near the northern boundary between the ABE Dive #55 and #56 tracks.


Selected digital deep sea photos taken during Camera Tow #1. Altitude for most images was ~5 meters. Area covered by most of the photos is estimated to be ~3m x2m.


Added November 23, 2001

Imagenex micro-bathymetry map of the East Pacific Rise axis and eastern flank near the 9° 50'N area produced from ABE data. Dana Yoerger processed the data and Maurice Tivey made the image. The next map shows a blow-up of the western portion of this map. Colorbar shows depth contours in meters. Contour interval is 2 meters, grid interval is 5 meters.


Imagenex micro-bathymetry data from ABE of the axial summit collapse trough. Dana Yoerger processed the data and Maurice Tivey made the image. It shows what those of us working in this area for the past 10 years have wanted. A detailed micro-bathymetry map that would allow us to relate all the volcanic, hydrothermal and biological features we've been studying all this time. The contour interval is 0.5 meters. The axial trough floor is green colored and runs down the axis which is colored in reds and whites (white areas are shallowest). Color bar on right shows depth range in meters.


Measured magnetic field map over the axis and east flank of the East Pacific Rise. Age range of the crust covered by this map is 0 to ~20,000years. Data collected by ABE and processed by Maurice Tivey.


Preliminary DSL-120A sonar backscatter image of the East Pacific Rise axis and upper flanks between 9° 25'N and 9° 33'N. The yellow arrow points to the low-reflectivity sheet flows that we confirmed with the digital photographs taken during Camera Tow#2. The red dots on the record are hourly marks to help us refine the navigation for the various lines. White areas are strong reflectivity, dark areas are low reflectivity and acoustic shadows.


Selected digital photographs from Camera Tow#2. Most of the photos were taken from an altitude of ~5 meters. The photos are in sequence from start of tow to end of tow. The tow traversed a ~1.2 kilometer area covered with what looked like sheet flows in the DSL-120A sonar records. The location of the photo traverse is ~200 meters from the west rim of the axial trough near 9° 28'N.

Lightly sedimented ropy lava. The white ball in the center right of the image is the rock core head.

More ropy lava, note how streamlined the folds in the flow surface are. This suggests very rapid effusion rates for this flow.

The contact between the ropy flow and a hackely flow. This is probably the same lava flow but the morphology likey changed because of the underlying terrain the flow moved over.

Ropy/hackely lava. White dot in the center right of image is the rock corer head - this appears in all the rest of the photos in about the same place.

Ropy sheet flow.

Ropy sheet flows and what we think may be the margin or levee of the flow to the left.

Across the contact from the ropy/sheet flow is the lobate flow at upper left.

Collapse pits in the lobate flow. White dot is the rock core head.

More collapse in the lobate flow surface. Note the stalkless crinoid on the edge of the collapse at upper center.

Sedimented sheet flow.

Close-up of a collapse pit in a lobate lava flow. Note the anemone at left edge.

More collapse pits in a lobate flow.

Hackely lava flow.

The lava flow sampled by the camera rock corer. It is a sparsely sedimented sheet flow.

Calibration of the Towed Camera THING field of view at 5 meters altitude at 50 meters depth. Image area is 6.4 meters across by 4.8 meters top-to-bottom. Thanks to Andy Billings for coming up with the rig to do this test.


Added November 23, 2001

Summary map of ABE Dive tracks in the 9° 50'N study area. Dive #54 is magenta, Dive #55 is blue, Dive #56 is red, and Dive #57 is white. During these four dives ABE collected data along 138 kilometers of trackline in 61 hours and 25 minutes of traversing the seafloor. In addition to the micro-bathymetry and magnetics data ABE also collected over 1300 photographs of the seafloor and CTD (conductivity, temperature, depth) data which show the temperature anomalies associated with locations of hydrothermal vents in the axial trough in this region (we'll post a map of this in the next few days). Dana Yoerger processed the ABE navigation data and Maurice Tivey generated this image. The base map is shaded multibeam bathymetry. Color bar shows depth range in meters.


Added November 27, 2001

This image shows the preliminary DSL-120A sidescan sonar imagery overlaid onto the multibeam bathymetry for the East Pacific Rise from about 9° 25'N to 9° 57'N. Paul Johnson of HMRG created this image. It provides a stunning perspective of seafloor data that can be used to help us understand the spatial relationships of different volcanic and tectonic features on the ridge. North is towards the top of the image. The total swath of sidescan data we collected is 31.4 kilometers long (north to south) by 6.8 kilometers wide. The area covered amounts to just over 390 square kilometers. The resolution of the backscatter data allow us to 'see' seafloor features that are as small as 2 meters across. This contrasts with the underlying multibeam data that provide resolution of ~10 meters over an area the size of a football field at these depths (~2500 meters).


Preliminary shipboard micro-bathymetry map made from ABE Imagenex 675kHz scanning pencil beam altimeter plus pressure depth for the East Pacific Rise crest near 9° 50'N. Colorbar shows depth range. Contour interval is 1 meter, data were gridded at 5 meters. Data processed by Dana Yoerger and map image produced by Maurice Tivey.


 


Preliminary shipboard observed magnetic anomaly map made from data collected during ABE surveys on the East Pacific Rise axis near 9° 50'N. Data processed and imaged by Maurice Tivey.

Trackline showing ABE Dive #58 track on the East Pacific Rise axis near 9° 28'N, our southern survey area. the fine black line running down the axis is the Axial Summit Collapse Trough (ASCT) as mapped using Argo I sonar data in 1989. Contours are in meters based on multibeam bathymetry. White is shallowest area.


Added November 29, 2001

ABE Imagenex micro-bathymetry map showing the well-defined terraces formed on the east side of the East Pacific Rise axis in this area. The axial summit trough is clearly imaged by the detailed bathymetry as the narrow trough running just inside the left edge of the image. The dotted lines show the locations of the DSL-120A sonar tracks (see the next map). Paul Johnson of HMRG made this image.


DSL-120A sonar image of the same area covered by the ABE dives near 9° 50'N showing the prominent flow fronts on either side of the axial trough which runs along left side of the record. The trough is close to or under the nadir in this image but is visible at the bottom just above the letters "k and i" in kilometer. Dotted lines show the path of the sonar fish.


Older, partly sediment-covered lobate lava flows near the easter edge of the ABE survey box in the 9° 50'N area.

The rock corer about to hit on the lobate lava flows seen at the start of Camera Tow #10 which started about in the middle of the ABE survey area near 9° 50'N. The corer is the white ball at the end of the monofilament line visible coming from mid-right in the photo.

The core head hitting the lava flow. You can see the coil of line at the mid-right edge of the photo. In the next photo, it will be lifted off as we started our traverse during Tow#10.

The small cloud of sediment is being stirred up as the corer is lifted off the lava flow surface.

Near the end of Tow#10 we crossed the collapsed lobate lava flows near the axial summit collapse trough (ASCT). A large anemone is attached to the edge of the collapse near the middle of the photo.


Added December 2, 2001

Bottom water temperature derived from the CTD (conductivity, temperature, depth) sensor on ABE, plotted over ABE micro-bathymetry contours for the 9° 50'N area. The red areas show anomalously high bottom water temperatures caused by the numerous high temperature and diffuse flow hydrothermal vents in the axial trough in this area. Next map shows a detailed view of the axial trough CTD data.


CTD derived bottom water temperature from ABE dives in the 9° 50'N area, showing a blow-up of the axial trough and the principal hydrothermal vents labeled.


Track plot of Towed Camera THING surveys carried out during this cruise in the 9° 50'N study area.


Track plot of Towed Camera THING surveys carried out during this cruise in the 9° 28'-29'N study area.


ABE tracks for Dives #58 to #62 plotted over multibeam bathymetry for the southern survey site on the East Pacific Rise crest near 9° 28'N. Depths in meters.


Micro-bathymetry map derived from ABE Dives #58 to #62 for the 9° 28'-29'N area on the East Pacific Rise crest. There are several key volcanic and tectonic features (labeled) that are resolved by the ABE bathymetry and the DSL-120A sidescan sonar imagery in this area. Data were processed by Dana Yoerger and Maurice Tivey made the contour map. Contour interval is 1 meter, grid size is 5 meters.


DSL-120A sidescan sonar map of our southern survey area on the East Pacific Rise crest centered on 9° 28'N. Labels point to features shown on the ABE micro-bathymetry map above. White is strong acoustic return, gray to black areas are weaker returns or acoustic shadows, respectively. Trace of axial trough is shown in yellow.


Observed magnetic field derived from ABE surveys in the 9° 28'-29'N area on the East Pacific Rise crest over shaded ABE micro-bathymetry. Note the Central Axial Magnetic High (CAMH) anomaly edge is well-correlated with the prominent East Graben feature labeled in the micro-bathymetry map above. Note also the distinct difference between the character of the CAMH on either side of the axial trough.

Observed magnetic field derived from ABE surveys in the 9° 28'-29'N area on the East Pacific Rise crest with micro-bathymetry contours underneath .

 

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