Thursday, 4 August 2011

INFOMAR marine geologist describes unique discoveries after returning from the Mid-Atlantic Ridge

Marine geologist Maria Judge based at the Geological Survey of Ireland, reviews data onboard.

The VENTuRE Cruise: Researching and Recovering information from the deep.
As we traversed closer to our destination, the vast grey sea turned a deep sky blue, reflecting mellow sun on a calm day. Onboard the scientists and crew, glistening with excitement, have remained calm and focused on the task at hand: that delicate task of locating hydrothermal vents up to 3000m below this vast ocean expanse.

(Top) The ships crew recover the CTD rosette to the deck. (Below) CTD data received in real-time indicates target locations for the ROV dives.

With the weather on our side, we began the science of locating a sulphide 'plume', as plumes of 'smokey' water are consistently expelled from hydrothermal Vents. For this we use a CTD rosette, guided through the water column at a quarter of a knot in a yoyo motion, during a procedure called Tow-Yo CTDing. Heads were scratched as directions and methodologies were teased out. Finally, as one small pink line deviated across a graph followed closely by the blue line, any tension and risk of not uncovering a signal was quenched! The pink line represents the Eh signal, the blue, temperature. A negative deviation in Eh is indicative of intercepting a layer of seawater containing traces of hydrothermal vent fluid. Our graph is essentially the measure of redox potential, found where fluid rich in hydrogen sulphide mixes with seawater. We look for large anomalies that indicate a strong signal, the strength of the signals represent our proximity to a vent site. When a deflection in Eh is coincident with a temperature increase we can be sure we have located a 'hot spot' close to the vent site. Considering pressure at 3000m, the fluid emitted directly from the vents are roughly ~350°C, this instantly diffusing to ~5°C a few meters from the vent as it mixes with cold sea water, beyond this the background temperature decreases to ~3°C.

After forty hours of CTD sampling, hunting Eh and concurrent temperature signals it appeared as though we had a possible plume signal boxed in. The ROV was deployed in reconnaissance mode, for this we chose a traverse to image. Rigged with a full suite of cameras recording continuously, the ROV has an aft facing camera, a downward facing camera, a forward facing camera and a High Definition camera also facing forward. Most scientists on board are accustomed to vent exploration and as such have enormous regard for the exceptionally unique and incredibly beautiful environment, so once the ROV was close to the bottom, grand excitement had engulfed the scientists on watch. Data management and logging tasks became increasingly difficult under the whirlwind of nervous suspense.
Feeling sick with anticipation, the day watch team had their gaze fixed on a monitor positioned in the now silent science lab. Hanging on to the edge of our chairs, we first spotted the trace of smoke in the water, as powerful lights and lasers on the ROV caught glittering sulphide particles and black soot in the water column. After some foggy navigation through smoky water, elegant columnar chimneys emerged into view. News travelled throughout the ship like electricity; soon the science lab was giddy with animated chatter.
Pilots steer the ROV along the vent target as scientists observe the real-time video in the ROV control cabin.

With the diligent and ever so patient ROV crew carefully navigating through the challenging terrain of the vent field, and a full compliment of scientist fully concentrating on the TV monitors, it was not long before we had a good handle on the main attributes of the vent field. Bellowing black smoke, these rust colored chimney structures have the capability of supporting a community of florescence microbial mats, shell fish, fish, crabs, shrimp and tube worms. Such fauna live solely on the sulphide-rich fluids emitted from the hydrothermal vents. These are one of very few environments in the world that do not rely on any photosynthesis to generate or sustain its community. Hence the significance to the biologists on board, who are keen to compare life around the vent to that already described at other sites.
The vents are also geologically significant for their precipitation of massive amounts of metals in the form of metal sulphide. Ancient ore deposits formed from past hydrothermal venting (called Volcanic Massive Sulphides), now exposed terrestrially, have been extensively mined e.g. the Troodos Ophiolite in Cyprus. Exploration for such deposits is still underway with old volcanic systems now uncovered on land targeted.
Samples are collected from the depths of the ocean by the ROV manipulator arms and stored in compartments until the ROV is recovered.

To understand the geological controls of the vent field, it is important for us to acquire information about the surrounding rock and its tectonic history. For this we first shot a high-resolution multibeam survey over the vents from the ROV. Multibeam uses sonar imaging to present a 3D view of the sea bed over a larger extent than we can image with our cameras. The vents here have an unusual and dramatic setting: perched on the face of a near vertical cliff scarp almost 200m high, they are unique amongst the 210 known vent fields in the world. Such a setting makes surveys and studies of this vent field very difficult and challenging. Plus the hot water surrounding the vents distorts the sonar beams and gives false reflections in mid-water above the seafloor. Our search for more information about this part of the Mid-Atlantic spreading ridge allowed us to utilize time during bad weather when the ROV can not operate. (Top) The rock dredge was deployed to collect rock samples from the seafloor. (Below) Scientist Maria Judge examines rock samples acquired from the Mid-Atlantic Ridge.

During a window in which our ship pitched and rolled in 3m swells, we took the opportunity to drop a dredge over two previously unsampled flat topped sea mount volcanoes. The dredge is a heavy iron net that is dragged across the seafloor. It cuts through layers of soft sediment until it hits the hard rocky substrate below and drags up whatever it can dislodge along the way. By dredging these hitherto unknown volcanoes, we have gained some samples which, under more detailed scrutiny back home, will cast more light on the volcanic and magmatic processes responsible for creating such an incredibly dynamic environment.
With all Geologists, Biologists and Chemists satisfied by the samples and information acquired from the new vent discovery, we turn on our heels and speed towards the next target, the Moira Mounds. Here we are currently gathering video footage of a protected area at the Porcupine Seabight, famous for its magnificent cold-water coral habitats.
High-octane spirits among the team propelled planning, preparation and procedures toward an exciting scientific discovery and an awe-inspiring reward for all who worked on board. I feel fortunate to have worked with such an adept team. We are looking forward to sharing more details of this wonderful discovery on our return and safe docking in Cork on Thursday the 4th of August
The captain, crew and scientific team gather for a group photo after a highly successful survey.

For more on the VENTuRE cruise, please check out the Science blog spot,
UCC's student website,
And the Marine Institute website.
There is also an article in the Irish Times

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