Science begins in earnest - CTD and NIOZ corer

Louis Byrne, British Oceanographic Data Centre, NOC

Early start on Tuesday with the first sampling of the cruise scheduled to start at 0600. A few winch jitters before we finally got to witness the CTD disappearing beneath the waves at 0718, and there was much rejoicing! Apart from three CTD casts for the majority of today we have been using a NIOZ Corer to dig up sediment from the sea-bed, and it seems like an appropriate time to introduce two instruments which will be mentioned a lot of in this blog.

A ‘CTD’ is a metal frame consisting of numerous marine sensors encircled by a ring of 24 water bottles. CTDs are the bread and butter of marine research and are very useful for getting a picture of how the water column changes with depth.  The CTD is attached to a wire and lowered through the water column until it reaches 5-10 metres above the sea-bed. During its transit to the sea bed the CTD produces live output to a computer screen for Temperature, Salinity, Pressure, Fluorescence (essentially the chlorophyll in the water),
Transmission (particles in the water) and other variables. Scientists on deck then use this information to decide at which depths to fire (close) the water bottles as the CTD is raised back through the water column, in order to catch water from specific depths for analysis back on deck.
 

A 'CTD' as pictured as it is being lowered into the Celtic Sea.

The term corer is a general term used to describe instruments designed to collect samples of sediment from the seabed. The samples are then brought up to the surface where they may be analysed, or used in experiments. This could happen in one of the several laboratories on the ship, or they may be stored for analysis back home.

A NIOZ corer being setup on deck.

One of the scientists using water collected by the CTD and sediment from the NIOZ corer is Briony Silburn (from Cefas), who is running an experiment to investigate the impact of different types of fishing trawls on the seabed. When trawling, the heavy weights attached to the trawl nets are dragged along the sea floor; disturbing the sediment causing it to be mixed into the water column above (this is called resuspension). When this happens the nutrients contained within the sediments also get released.

Briony Silburn and her sediment resuspension experiment

Briony is using mud from the core which is added to a large container containing water from the Celtic Sea, and spinning magnets are used to continuously mix the water so that the mud doesn’t fall to the bottom. This experiment is repeated three times, using mud from the first two, four and six centimetres of the seabed. Over a 24 hour period the nutrient concentrations of the water are measured, and by comparing the measurements from all three experiments it is possible to get an idea of how the concentration of nutrients resuspended varies as the trawl weights penetrate deeper into the seabed. These nutrients are then measured on board the ship (more about them later).

This research can then be used to influence the design of trawl weights so that they are less harmful to the marine environment.

November weather

Ocean research cruise blog of Jonathan Sharples

 

The remarkable weather continued yesterday as we continued a series of measurements and zooplankton nets next to the moorings. A couple of scientists were even spotted sunbathing between net hauls. The wind continued to drop, and the sea finally reached a glassy state by sunset. Pretty good for November in the Celtic Sea.

The winning picture of the salps in the process of releasing faecal material into the water is below: look at the streaks of back trailing from the curl of colonial salps in the lower left of the picture. Some of these salp groups are reaching lengths close to 2 metres.

chain of salps

salps cought pooing

 This morning just as the sun came up we carried out one vertical profile with the CTD just next to the wirewalker mooring. That will provide Jo Hopkins with vital data for her to calibrate the instruments on the mooring. We are now pulling up the wirewalker, and will then head off to collect the 2 gliders that neeed to come back with us. The glider “pilot” back at the Oceanography Centre has sent instructions to the gliders to meet as at a specific location, so the gliders will have dutifully reached that position this morning and will now be bobbing about on the surface waiting for us.

ctd at down

The weather is due to close in tomorrow, with 25-30 knots of wind expected from mid afternoon through to mid afternoon on Friday. But the longer term forecast is suggesting a return to these calm, sunny conditions. Feels strange for this time of year, but none of us are complaining.

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End of the iron line

We have nearly finished our transect sampling iron from the deep ocean back to the shelf. The iron group is fairly excited, because in all of the profiles we have done gradually working along and up a seabed canyon there has been evidence in the CTD data of lots of suspended particles near the seabed. There must be some flow of water down there that is pulling sediments, along with trace metals such as iron, up off the seabed which is exactly what the scientists are looking for.

iron nerve centre

Other lab work continues also, as the iron chemists need to know what else is happening in the water to help understand what they are seeing. Chata, a PhD student from the University of East Anglia, has spent the past 3 days trying to fix a machine she uses to measure argon, oxygen and nitrogen gas dissolved in seawater. The machine is refusing to work properly, so she is having to store samples for analysis later back at University. Oxygen and argon behaviour similarly in seawater, and in the rates they can be transferred from the atmosphere to the ocean. However, oxygen also has a biological component to how it changes – if the ocean’s microbial plants are growing, then (like all plants) they produce oxygen. Chata can compare what she sees the argon and the oxygen doing in the water, and any differences between them will tell her about how the biology in the ocean is working. She is also helping us by doing chemical analyses of water samples to measure the oxygen concentration, which will allow us to calibrate the oxygen sensor that we have on our CTD.

chata titrating oxygen samples

Due to finish this transect at about 0100 tomorrow. We then plan to start th second of our main study stations, this time sat at the edge of the continental shelf.

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Into the deep water

We’re in deep water now. Not the deepest in the ocean, but enough to make a normally shelf-focused oceanographer a little nervous. The forecast suggests things should quieten a little over the next day or two, so we headed out over the shelf edge and into the deep ocean aiming for a depth of 2,500 metres. Crossing the shelf edge always looks like we are going over a cliff when you look at the echosounder. On the shelf the depth had increased from 150 to 200 metres in about 100 km, but then over the shelf edge the depth suddenly increases from 200 to 2000 metres in about 30 km. So a change of 1800 metres over 30 km: if you cycled a slope like that you might get a bit out of breath, but it’s not the cliff edge that the echosounder makes it look.

cup creations

 Scientists can be easily amused. The one thing we really like to do when we work in deep water is decorate polystyrene cups and then send them down with the CTD. Amber Annett from Edinburgh University remembered to bring a supply of cups, pens, and a pair of old tights to hold the cups on the CTD frame. The lab is a hive of creative activity. Why do we do this strange ritual? The cups compress under the pressure of the water; the greater the pressure the smaller the cups become. The decorations also compress, so that you end up with miniature, highly-detailed cups when the CTD returns to the deck. We are due to work gradually back up the shelf slope to the shelf edge, lowering the CTD into 2,000, 1,500, 1,000 and 500 metres, so we could produce a series of cups scaled by the depth of the water. It’s fun, and also a great way of demonstrating the concept of water pressure to school kids.

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deep ctd

At the shelf edge, and rolling….

Ocean research cruise blog of Jonathan Sharples

 

November 14th. We are out at the edge of the continental shelf. Work had to stop early this morning as the waves reached 7 – 8 metres, making it too difficult to get our instruments over the side safely. We are now sat here trying to get a weather forecast, so that we can decide whether to stay out here and wait for the waves to settle down, or turn back onto the shelf and work in the shallow water.

Yesterday’s work started off very well. We managed to do 5 out of 6 sets of measurements as we headed southwest from the central Celtic Sea. We have collected a great set of information on the distribution of the autumnal nutrients out towards the shelf edge. Unfortunately we couldn’t collect any information on iron in the sea, as the instruments used to do that use a wire that has a lower breaking strain – we are fairly sure it wouldn’t survive the sudden snatches the wire gets when getting gear back onto the ship in these waves.

We’re rolling heavily now! The ship has turned direction slightly to try to get a signal to our back-up internet connection – then we can get a weather forecast and start to plan the next few days.

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The first data

Right on schedule, at 0500 our main set of instruments hit the water to collect the first data. This package of instruments (the "CTD") is made up of several sensors that measure water temperature, saltiness, oxygen concentration, and also how many of the microscopic ocean plants there are. The long grey tubes around the outside are used to trap water (20 litres at a time) from depths where the scientists want samples for their experiments.

CTD into sea

The first data is exactly what I was hoping for. Throughout the summer this part of the sea would have had a warm, sunlit layer above deeper, colder water. As autumn and winter approach this surface layer cools and starts to get thicker, until eventually the whole of the water from the surface to the seabed (140 metres deep here) reaches the same temperature. The red lines in the left panel of the computer screen show the temperature. It’s about 14 deg C in the upper 40 metres, then drops to 12 deg C in the deeper water. In summer it would have been about 18 deg C in the upper 30 metres, and 11 deg C below. So, we’ve got here just in time to see the change in conditions towards winter.
These early morning "CTD casts" are sampled a lot by the scientists. There was a big meeting last night to discuss who required how much water from what depths. Also the order in which the samples are taken is really important (samples for dissolved gases need to be taken first, while samples for salt can wait until the end).

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orderly queue