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Special Report: Sediment Traps

Hi folks!

Thus far it has been a quiet morning, so thought I’d put paw to keys as they say in this modern technological age.

Let me tell you about the sediment traps mentioned in other reports.
These are part of scientific data collecting and aimed at gathering information about particulate matter which falls through the ocean layers - whether man made, eg carbon particles from industrial activity, metals, sea creature faeces, volcanic dust or anything else which can fall through the ocean layers. The technology is relatively simple to understand, so read on.

In July 1999 people on an earlier expedition in this area deployed two instrumentation packages designed to remain in their positions and gather sediments over a period of time (some 15 months now). One package is at position 46 degrees 47.00’S; 142 degrees 05.92’E and the second at 53 degrees 44.93’S; 141 degrees 45.25’E. We are presently at the site of the first one.
The packages are similar so I will only describe the first one.

The device starts with a very heavy concrete block 4226metres below the surface of the ocean. Rising from that on rope and wire is a long string of gizmos - four sets of 12-15 glass floats, three current meters (measuring speed and direction of current at different depths) and three parflux gathering containers for the sediments. Near the bottom is an acoustic release. The whole string is about 3500metres long, finishing some 700metres below the surface. The sediment containers themselves are a series of glass tubes which can be electronically rotated to come under the collecting funnel - a bit like filling jars in a jam factory. There are I think 15-20 tubes in each set and these are moved I think about once a month to a new tube - electronically activated from Hobart by the scientists. The tubes are sealed when filled. Sediment falling into the funnel is what is trapped in the glass tubes.

When we arrived at the scene to retrieve the instruments, the first thing was to send a radio signal to wake the instruments up (they are put into hibernation to conserve battery power). The idea then was to trigger a special release mechanism - the acoustic release. Theoretically the whole string then disconnects from the heavy mooring concrete block and the assembly then rises to the surface because of the buoyancy of the glass floats. While these floats are 45-50 centimetres in diameter and bright yellow and, as you can see there are a lot of them, the task of locating them when they reached the surface was very difficult. In fact when the gadgets reach the surface, a VHF beacon is activated to give the ship something to focus on with direction finding equipment. The signals we were picking up were very faint and confused, so we had a lot of people up on the decks just looking.

We released the mechanisms about 1615 on Thursday but despite crossing and crisscrossing the area (our ships track looks like a plateful of demented spaghetti) we did not spot the floats until about 1730 on Friday.

It then took us until well after 2200 on Friday to actually bring all the wire rope, floats, current meters and stuff on board. We have been in some pretty heavy seas and manoeuvring the ship around to grapple and drag the stuff on board was a bit harrowing. Still all done safely, and the scientists are happy. We are still in the same area and the scientists involved with the project have been busy reassembling the equipment to be deployed again to the ocean floor. We expect that to be this afternoon after lunch - it will take an hour or two to carefully put it into the water via the ramp on the trawl deck at the stern of the ship. The last item to go overboard will be the new concrete block and that will start dragging the whole string back into the ocean depths for another 12 months or so.
Once that is out we will set sail for our next set of sediment traps 400 or so miles away, but on route Gordon will be deploying the Continuous Plankton Recorder - but more about that in another report.

That’s enough science and technology for now.

Until next time.



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