Tuesday, 17 November 2015

Biogeochemical sensors added to the RAPID array (part 2)

Biogeochemical sensors and samplers are being deployed at four locations across the subtropical North Atlantic: two are at the western boundary (one at 1500m depth, one at 50m depth), one is on the western flank of the Mid-Atlantic Ridge, and the last is towards the eastern boundary.  At all four locations, oxygen sensors will also be installed at multiple depths throughout the full water column.

Locations of RAPID moorings and where biogeochemical observations will be made.
The largest component of this new instrumentation is the McLane Remote Access Sampler (RAS), which has space to collect 48 unique water samples.

The mechanism by which this happens is quite ingenious. Empty plastic bags (that are slightly more bespoke than a humble 5p carrier) are enclosed within sealed acrylic cylinders filled with water. Over the next 15 months or so, every 11 days an external valve will turn to select a new, unfilled bag port, and start to pump the water out of the bottom of the acrylic cylinder. This will create a pressure gradient within the cylinder that will lead to local seawater being drawn through the sample inlet, through a separate part of the multi-position valve and into the bag. A sample preservative placed in the tubing between the bag and the sample inlet will ensure that the chemistry of the water collected will not change between the time it is sampled and the time at which it is retrieved and analysed. When full, the valve turns back to its home position, and the bag and the water sample it contains become, like a desert island, isolated.

RAS and sensors being deployed at mooring EB1
It sounds so simple but these are quite intricate beasts. Each bag is pressure and leak tested before being evacuated of air; sample lines and cylinders are carefully filled with sample preservative and freshwater respectively before finally over 450 individual fittings are checked, tightened, checked again and probably checked again as we've likely forgotten where we got to. (A few are ok, but with 48 sample bottles it always takes longer to prepare than you think!) Only then will the system be allowed into the water. As can be imagined, this preparation can be a lengthy undertaking but many helpful hands have made light work.

Flower Power: birds-eye view of the multiple tubes leading from the central multi-position valve to individual sample bags / cylinders on the RAS
A number of alterations and additions have been made to the setup that will hopefully bolster its performance. From the extra retaining plates and sensor frames expertly installed by Tom the mooring tech, to the very particular type of plastic chosen for the plastic sampling bags, and the copper-nickel banjo bolts (essentially a bolt with a hole through it) installed at the sample inlet to ward off biofouling and inlet blockage caused by sinking particulates.

Hillbilly Sprinter: banjo bolt water inlet
In the additional frame attached to the bottom of the RAS are located the biogeochemical and temperature/salinity/pressure sensors. Towards the top is located a Seabird Satlantic Deep SeapHOx, a combined temperature, salinity, pressure, oxygen and pH sensor. These are the first of their deep-rated type deployed in anger, and use the same ISFET pH technology as has been successfully used in SOCCOM profiling floats in the Southern Ocean.

Part of the ABC sensor suite: pH sensor (black cylinder, foreground), temperature, salinity, pressure and oxygen sensor (silver cylinder with circular-holed guard, centre-ground), temperature/salinity/pressure sensor (below).
At the bottom of the frame is located the Contros HydroC pCO2 sensor. This uses a small pump to force local seawater past a membrane that is permeable to CO2. Over time, the CO2 in the seawater equilibrates with the airspace that is behind the membrane before being pumped into an enclosed detector and analysed by infrared spectroscopy. In order to stop water vapour condensing within the detector, the internal mechanisms need to be kept at a much higher temperature than the local environment. In the warm and balmy subtropics and for such a long deployment (up to 18 months) this becomes quite a power-intensive process, hence the weighty power pack. Using all the D-cell batteries crammed inside we could have had a remote-controlled car race involving everyone on board ship, and still have a few spare. Note for future cruise: bring remote-controlled cars.

Other part of the ABC sensor suite: Baby Bear (pump), Mummy Bear (pCO2 sensor), Daddy Bear (battery pack)
The final part of the ABC sensor suite is not attached to the RAS frame and is instead installed on the mooring wire, spaced at regular intervals down to the ocean bottom. In total, 24 combined temperature, salinity, pressure and oxygen sensors (Seabird SBE63 ODO mounted on microCAT CTD) will be installed along the four mooring wires where the RAS and other sensors will be located, with a fifth at the western boundary. Together they will create a high frequency time-series of the full-depth transatlantic distribution of oxygen that, as mentioned in part 1, will contribute greatly to an improved ability to estimate the transports of total, natural and anthropogenic carbon, and inorganic nutrients across the subtropical North Atlantic.

Not a flying fish on a zip wire but final part of ABC sensor suite (sort of): a temperature, salinity, pressure  sensor being deployed at MAR1. These are actually part of the RAPID sensors, but additional very similar instruments that have oxygen sensors too have been added for ABC Fluxes (we just don't have a nice photo of them on the wire going into the water!)

Together, the autonomous samplers and biogeochemical sensors provide a substantial chemical upgrade to the successful RAPID mooring array. In 15-18 months time, the data collected should hopefully shed new light on the drivers and processes controlling short and longer-term variability in chemical fluxes, both within the ocean and at the upper-ocean lower-atmosphere interface. But that’s for the future. As yet only two RAS/pH/pCO2/oxygen sensor sets are in the water at EB1 and MAR1, with more still to come at WB4, WBH2 and WB1. Better get to checking some fittings.

Splash: RAS and sensors deployment at EB1

Written by Pete.

1 comment:

  1. Great to see the biogeochemical sensors being added to the RAPID array across the Atlantic. We should have some interesting data to help us understand how the biology contributes to ocean uptake and storage of carbon.