Cruise diary

Wednesday 23rd July 2003

Bramley writes...

"Time for an update on the science! The science operations so far have involved making multibeam echosounder maps of the seafloor and some sampling of the rocks and water column at 2500-3000m. The first sites we dredged were about 120 nautical miles north of the the Seychelles. Here, several ridges rise above the 4000m deep abyssal seafloor. It is thought these ridges are the remains of the margins of the Seychelles as they were rifted away from India some 60million years ago. To test the theory we dredged rocks from these ridges - which turned out to be a type of basalt (trachite) typical of continental rifting. The basalts were very weathered and covered by a thick manganese coating. Both the weathering and coating indicates these rocks are of considerable age - something we will verify when we return to our labs.

We took 3 days to reach the Carlsberg Ridge from Seychelles. It is almost 800 nautical miles, and we were limited for speed by the exceptionally warm seawater which is used to cool the engines. Too hot and we have to slow down - but then the water is nearly 30ºC!

We started work on the Carlsberg Ridge on Tuesday 22nd July at 10pm. First we mapped the ridge axis to make sure we know where it is. It is strange but true that because no-one has been to this part of the ridge before, we only think it should be here because we predict it from our geological models for the Indian Ocean. As it turned out, our predictions were accurate, and the ridge appeared as a classic axial valley with 800m deep walls and a long thin volcanic ridge at the centre of the valley floor.

The Chief Scientist, Dr Bramley Murton, discusses operations with the ship's captain, Keith Avery.

Our first job was to navigate our dredge to the bottom - nearly 4000m down - and take rock samples. We did this, but failed to get any rocks, although the dredge was badly scratched and the inner nylon bag was shredded by some sharp rocks on the seafloor. This site is our most easterly and so is important. So we tried again, and this time managed to recover several kilogrammes of glassy basalt.

Ian Thomson, the A/B (ablebodied seaman) takes tea while waiting for the dredge to be deployed.

The Chief Scientist discusses log entries with Sophie (biologist). The log is kept up to date with entries on the ship's position and operations every half hour.

On the dredge wire we attached a plume sensor - to detect the dirty water that a hydrothermal vent might emit. Although only one hydrothermal vent has been found in the Indian Ocean, we have some previous experience of tracking down vent sites before from the Mid-Atlantic Ridge. Carla (water chemist) programmed the plume device (which is on loan from Ed Baker from NOAA, a US government research organisation in Seattle). Andy downloaded the data from the dredge station and we discovered a huge plume signal. Normally these signals are a few hundred metres thick and have a maximum particle density of up to 20% of that of the surface. Our plume signal is 600m thick and 75% of the surface water particle density! Such a huge plume signal makes this probably the most powerful hydrothermal plume in the world discovered to date.

Bram and Sophie looking at images of the seafloor processed from our SIMRAD multibeam sonar. The SIMRAD engineer (Geir Skogen, from Norway) shows new displays of volcanoes on the seafloor imaged a few minutes before.

The CTD being lowered into the ocean. This device records temperature, pressure and salinity.

Our next site we predict will be from a position closer to where the vents are located - at the shallowest part of the volcanic ridge in this area, and about 20 nautical miles northwest. Will we find the vents? If we do - will we recover the exotic vent animals that Sophie is studying? And if we do, what sort of creature will they be? The answer awaits us, 4000m below, in a region of the world that remains totally unexplored..."

Dave writes...

"It has been a week since I first left home on that fateful day. I feel as though I have learnt a lot, especially that a life at sea is fine for some people but it is not ideal for me.

Night manouvres: dredge going down

When dredging we aim for a target which is generally a topographic high (a lump sticking up out of the seabed!). We align the ship in front of it and then drift at half a knot toward the target. The aim is to get a hit and drag the dredge up the slope to get maximum yield. On the way to the target we let out more and more cable (a process called veering). As we veer the cable, out a piece of equipment that is attached to the cable called a pinger accurately measures the distance between it and the sea floor using sonar, so we know how much cable to let out. We know that we have made contact with the sea floor because the tension on the cable increases dramatically from ~3 tonnes to 6 tonnes as the dredge bites. The tension on the cable must not exceed 7.2 tonnes as that is the failure limit for the weakest link in the chain. If it is exceeded then the dredge is lost.

Once the dredge has been dragged a distance then the dredge can be hoisted and brought on deck for us to examine the fruits of our labour. I was in bed when the dredge was bought back on deck. Apparently there was some basaltic material and some volcanic glass. The volcanic glass is good evidence of recent submarine volcanic activity. When fresh magma is erupted onto the sea floor it does not have time to form minerals because it is quenched (cooled) too quickly for crystallisation to occur. The magma does not have time to form any ordered structure, so glass is the product. Glass will however rapidly break down (devitrify) and form clay minerals, so the discovery of fresh glass is good evidence of recent volcanism.

I have just found out what my duty is when the dredge reaches the surface. I have to help Bob, the tatooed Bosun’s Mate, to pull the dredge in so I will have to wear a full body harness and attach myself to a wire to prevent me from falling into the sea.

Everyone’s body clocks are well and truly messed up - you can say to "morning" to someone at 6 in the evening and no one would bat an eyelid. It is now 6:50pm. We sent down a dredge just before tea. I got to control the big blue A frame (as seen in numerous photos at the stern of the ship) while we manoeuvred the dredge into position over board. The weather has turned on us a bit. There has been intermittent rain and the wind and waves have been up to force 6. So I’ve started to feel a bit groggy again. Rex still has not been sleeping well whereas I can’t seem to stop myself..."

Tina writes...

"This morning we sent down the dredge again, at the same site as the last shift (Waypoint 4). The first dredge did not bring anything up, so we have returned for another go. The MAPR readings were positive however (a very large plume), so Carla is quite interested in sending down a CTD at this site for further readings.

Second time round, we dredged the bottom for over an hour at the site (drifting south slightly in the prevailing current). The line then began to ‘bite’, to about 6 tonnes of tension on the wire, which indicates (hopefully) that it is banging into and then picking up lots of the freshly formed basalt that we suspect might be at the bottom of the ridge. Then, after we saw the bites, we began hauling the dredge very slowly home (slow so we don’t tip it up and lose the sample). This is a crucial point, since we haven’t had a success at this site yet. I nipped off for bacon and eggs and coffee and then came back to watch the next shift (Bram, Carla and Sophie) bring up the haul. In all, the process of dredging took about 4 hours.

Carla and Jeff will also be sending down CTD. Jeff wasn’t very happy to be woken at 4:45am today to tell us how to read the ‘pinger’ echo sounder that tracks the dredge about 50m above the ocean floor. The hours and shifts are a little anti-social, and 4am seems like a perfectly normal time for me now. In fact life on board seems totally natural and almost too easy, compared with my usual details and strifes on land. Everything runs to a schedule, from meals to work, to laundry, and there’s always someone to talk to, and something happening.

Above: Morning manoeuvres...dragging the dredge on the bottom
Left: Some of the technology used to control equipment
Right: The team suffering from emotional dredge drain after 3 hours.

It was very exciting when the dredge finally surfaced. Not only did we find some real samples, including some glassy fragments on pillow rock, and basalts, but also the MAPR results repeated the same huge plume signature. This then lead to deployment of the CTD, which further reinforced the hydrothermal plume findings. This is a significant result for Carla’s research (supervised by Chris German at the SOC) and also for the cruise as a whole, as it agrees with the general hypothesis that the Ridge is actually here! Remember no-one was completely sure before we came that we would find what we hoped to find. It’s amazing to be involved in genuine exploration.

Rex and Bram consult about breakfast

Dredge up, with interesting rocks.

This morning, Dave needed a boilersuit and tried my tech services one on - needless to say it’s about four sizes too small, but it did cheer us up at 4:40 in the morning!

I slept through lunch and now we are back on shift, steaming towards Waypoint 5. We are keeping a steady 7 knots (7 nautical miles an hour, where a nautical mile is 1852m) to arrive at the dredging site by about 5:30pm local time. The weather has become more difficult, with a Force 5 blowing outside and we are rolling around much more than before (this will make dredging more of a challenge). The few technical inaccuracies in our diaries are kindly being corrected by the Captain (known on board as The Master or ‘Old Man’ to the rest of us!). So we may start looking more knowledgeable than before about the ship’s workings.

On the afternoon shift, we sent down the next dredge (winds now blowing Force 5-6, choppy seas). I radioed our progress to the bridge (who need to know what is going on at the stern of the boat while they hold position) and Rex and Dave aided Bob the Bosun’s Mate with control and deployment of the dredge. While positioning the dredge, it’s held in place by ropes tying it to the A-frame, these are released as it goes over the railings. The railings are kept in place throughout, and we all have to wear hydrostatic lifejackets whilst on the back deck (the kind that are slim to work easily in, but would inflate if they hit water). This is along with the usual safety precautions of hard hats and safety boots. When you are out at the stern of the ship, with the waves bumping you up and down, reeling out a heavy metal chain bag and bucket, suddenly you realise why these precautions are so important.

The deployment went smoothly, and now we are reeling out the dredge to its full 4000m depth or so. The length of cable paid out is longer, since the dredge is dragging behind the ship in the current as we move it across the site we want to sample. There is in total about 8000m of cable available, so we definitely won’t be running out.

The ping echo sounder was attached 100m above the dredge, and then the MAPR. We had a bit of trouble configuring the MAPR software, and Carla helped us out with a masterclass..."

Collecting the haul

Dave's new look

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