Cruise JC030
Tuesday 2nd January 2009
Day 8 (I think) at Sea:
CTDs and a normal day!
JDAY 002
Lunch: Chicken
Dinner: Friday = Fish
Weather: Bumpy - some big rolls - Force 6
Distance Travelled Tpday: 103 Miles
Total Distance Travelled: 1506 Miles
Activity: CTDing
Sea Temperature:0.8•C
Air Temperature:-2•C
Ok, I realised that i haven't actually explained about CTD's in any great depth. My Bad! So the first part of today's blog is a little bit about CTD's:
Above: Us
CTD - Conductivity Temperature Depth
What is it and why do we use it?
A CTD — an acronym for Conductivity, Temperature, and Depth — is the primary tool for determining essential physical properties of sea water. It gives scientists a precise and comprehensive charting of the distribution and variation of water temperature, salinity, and density that helps to understand how the oceans affect life. Due to the array of equipment mounted on our CTD you wouldn't get much change out of £250,000!!.
Above: Our CTD for cruise JC030. The grey plastic cylinders are 20LNiskin bottles used to hold water samples. The metal frame is used to protect "the package" from damage. The bottom part of the frame contains the sensors for measuring Conductivity, Temperature and Depth. Our CTD also contains ADCP's (The yellow device at the bottom of the CTD - more about these again) that are used to measure the speed of currents. We also have a Transmissometer, Fluorometer, Altimeter, 10Khz pinger, Homer Beacon....Surround sound system (only joking)
Also visible in the above image are the white "taps" at the bottom of each bottle through which the scientists are able to extract the samples for analysis
Above: A conductivity sensor (top) and a temperature sensor (bottom) located on the CTD's "vain"
Above: Another conductivity and temperature sensor just visible inside the CTD Frame
Some information about Conductivity, Temperature and Depth
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Conductivity
Uh-oh, is it too early for science? Conductivity is a measure of the ability of water to pass an electrical current. Conductivity in water is affected by the presence of inorganic dissolved solids such as chloride, nitrate, sulfate, and phosphate anions (ions that carry a negative charge) or sodium, magnesium, calcium, iron, and aluminum cations (ions that carry a positive charge). Organic compounds like oil, phenol, alcohol, and sugar do not conduct electrical current very well and therefore have a low conductivity when in water. Conductivity is also affected by temperature: the warmer the water, the higher the conductivity. |
Temperature
Well temperature is not constant throughout the depths of the sea - or water column as we call it. Most of the solar radiation (light and heat fromm the sun) that hits the ocean is absorbed in the first few tens of meters of water. Waves and turbulence mix this heat downward quickly.
The polar seas (high latitude) can be as cold as -2 degrees Celsius (28.4 degrees Fahrenheit) while the Persian Gulf (low latitude) can be as warm as 36 degrees Celsius (96.8 degrees Fahrenheit). The average temperature of the ocean surface waters is about 17 degrees Celsius (62.6 degrees Fahrenheit).
There is a boundary between surface waters of the ocean and deeper layers that are not mixed. The boundary usually begins around 100-400 meters and extends several hundred of meters downward from there.
This boundary region, where there is a rapid decrease of temperature, is called the thermocline. 90 % of the total volume of ocean is found below the thermocline in the deep ocean. Here, temperatures approach 0 degrees Celsius. So even though surface waters can be a comfortable 20 degrees Celsius (good for swimming in!), the majority of our ocean water has a temperature between 0-3 degrees Celsius (32-37.5 degrees Fahrenheit).
The density of ocean water continuously increases with decreasing temperature until the water freezes.
Ocean water, with an average salinity of 35 psu, freezes at -1.94 degrees Celsius (28.5 degrees Fahrenheit).At very high latitudes, ocean water can reach these low temperatures and freeze. Dissolved salts in the water tend to be rejected by the forming ice so that sea ice is only about 1 % salt. An interesting tidbit for those of you interested in survival tactics - because of the lessened amount of salt, melted sea ice would be fit to drink even if sea water is not! Sea ice formation at high latitudes ultimately drives circulation of the deep waters of the ocean.
At the surface of the ocean, temperature measurements are often taken by thermometers placed on buoys. Recently, the Argo program has taken on the task of monitoring the state of ocean surface waters across the globe. The Argo program involves over 3000 - floats that measure salinity and temperature throughout the surface layer of the ocean.
You can visit the argo website:
http://www.argo.ucsd.edu/
Each float is programmed to sink 2,000 meters down, drifting at that depth for about 10 days. The float then makes its way to the surface measuring temperature and salinity the whole time. Data is transmitted to a satellite once the float reaches the surface, so that scientists have access to the state of the ocean within hours of the data collection. Each float lasts 4-5 years.
However, for greater depths in situ measurements are often made with CTD's like ours!
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Depth
We also measure depth using a pressure sensor. There's a lot more that goes into it..... But I wan't you to carry on reading the blog not fall asleep!! |
So how does our CTD work?
Our shipboard CTD is made up of a set of small probes attached to a large metal rosette wheel or carousel - on our CTD we have a whole suite of scientific sensors in addition to those mentioned for measuring conductivity, temperature and depth above we also have the following sensors:
Transmissometer
How to explain this :-s Ok, Our Transmissometer shines a thin beam of light through the water and a receiver measures how much light arrives at the other end of a set distance. This can be used to study how much suspended sediment is in the water
Above: The Transmissometer is just visible in this image below the grey plastic bottles.
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Fluorometer
Now this one is a bit more difficult. A fluorometer or fluorimeter is a device used to measure parameters of fluorescence: its intensity and wavelength distribution of emission spectrum after excitation by a certain spectrum of light.
These parameters are used to identify the presence and the amount of specific molecules in a medium. Modern fluorometers are capable of detecting fluorescent molecule concentrations as low as 1 part per trillion.
Above: Our Fluorometer |
Above: An image showing the sensors and electrical units located in the bottom section of the CTD package when some of the bottles are removed. The yellow device is the ADCP used for measuring currents. The silver device with the black "cap" is a homing beacon - this allows us to locate the CTD on the seabed with a ROV should (God forbid) the wire part. The large cylinder in the centre is a Seabird 911+ which contains the pressure sensor that is used for measuring depth.
The carousel also has a number of large plastic bottles attached to it - these are called Niskin bottles and there are 24 (20L) bottles on our CTD. These bottles are used to trap water samples that can then be analyzed to determine the properties of the water at different depths.
The carousel is lowered on a cable down to a position just above the seafloor, and scientists observe the water properties from the scientific sensors in real time via a conducting cable connecting the CTD to a computer on the ship.
However, the water in the bottles must be returned to the ship for analysis.
A remotely operated device allows the water bottles to be closed selectively as the instrument ascends. A standard CTD cast, depending on water depth, requires two to five hours to collect a complete set of data. Water sampling is often done at specific depths so scientists can learn the physical properties of the water column are at that particular place and time.
Above: Each bottle lid is connected via a strong line to a round mechanical "firing" device. This device is controlled by a computer on the ship via the CTD cable. When the CTD operator fires a bottle e.g. bottle 3 - the corresponding trigger for bottle three "lets go" of the wire. This release tension on the lids which are spring loaded - causing them to snap closed.
 
Above: These two images show the top and bottom lids/plugs used to seal the bottles. In the left image you can see the snug fit that each plug has. In the right image you can see the springs that cause the plugs to snap shut when tension is released from the "firing mechanism"
Above: A CTD ready for deployment. These bottles are cocked in the open position. You can see in this image that each bottle has a number printed on it. As a bottle is "Fired" from the control computer onboard the ship , the operator makes a note of the bottle number and the depth it was fired at. This allows the scientists to cross reference the sample with a positon in the water column - E.g. bottle 3 - 3200m.
Above: The CTD control computer in the main lab (RRS Discovery). The operator can "fire" the bottles from here and monitor the realtime data from the sensors,
Deploying the CTD
There a lots of ways to deploy CTD's. Most oceanographic institutes have different ways depending on the size of the CTD and the ship involved. Both RRS Discovery and RRS James Cook have a device called a
Parallelogram that makes deploying CTD's a lot easier. The RRS James Cook has a special
adaptation
to this system specifically for deploying CTD's called a Hydro Boom. The Hydro Boom allows the CTD to be moved from the CTD hangar to a position over the starboard side of the ship where it can be deployed. The system also acts in reverse - recovering the CTD and then placing it back inside the CTD lab where the scientists can extract samples from the Niskin bottles and data from the sensors can be downloaded.
Above: RRS James Cook's Parallelogram in the retracted position. The Hydro Boom is visible just in front of the shutter doors. The Hydro Boom serves as a common overboarding support structure for CTD cables. The Hydro Boom is located amidships on the starboard side, the area of least ship motion.
Above: The Hydro Boom being used to deploy the CTD
Above: The Hydro Boom on Maria S. Merian
Above: RRS Discovery deploying a CTD using the Parallelogram
TODAY
Ok, back to the blog. Right - what happened today? Hmmm, well we just carrying on CTDing - something we will be doing for the next 3-4 weeks? Everyone is continuing in their watches and falling into the routing of CTD deployment, recovery and sampling around the clock. I bet your getting bored of CTD's but that's why we are here!
The weather was bit bumpier today and the wind has got up quite a bit and even I had problems walking into the wind earlier! In the evening we had some big rolls which sent anything that was not safely stowed flying across the room. I was sat on my bed reading when I got attacked by the desk lamp. After a dozen or so "big rolls" I made my way up to the various rooms onboard to check our equipment. A few loose items had been thrown about but luckily the majority of stuff was secured or locked away.
At 11PM an iceberg passed within one mile of the ship. I was in my cabin at that point and people thought I might of gone to bed so missed the spectacle! The iceberg was litup with the ship's searchlight as it passed by.
Tiny spotted a solitary whale in the morning. The "blow" was quite large and I think it might of actually been a Blue Whale. Unfortunately we were some distance away and could not get any pictures of it.
The CTD recovery later in the evening was bit more exciting than the previous recoveries and the CTD moved about quite a bit when it came close to the surface because of the strong currents. Once it was out of the water the strong wind blew it about quite a bit. However, the skilled crew soon had the CTD inboard and the scientist swung into action!
Above: Engineers hard at work!
Above: Nick playing up for the camera
Above: Anna and Jon looking at the electronic charts
Above: Marie with a very very complex bit of kit. I have no idea what it does!
Above: Another grey day and cold day!
Above: Anna and Helen take a look at the charts
Above: Dave getting ready to launch the CTD
Above: CTD being manouvered for deployment
Above: CTD over the water
Above: Going......
Above: Getting ready to deploy
Above: Jon, John and Anna at the CTD Control computer
Above: Bringing the CTD home
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