Mud tension

by Peter Lamont

“Is this you off on another cruise then” our ferryman asked as I carried my small aluminium box – literally my sea chest – onto the slipway. “Where to this time?” he asked in response to my nod. “Oh just to Rockall and south west Iceland.” I replied.

In my time sampling the seabed I have been lucky enough to visit many remote areas of the oceans so Rockall and south west Iceland seems like the backyard to our laboratory, the Scottish Association for Marine Science in Oban on the west coast of Scotland. In the 1970s it did indeed become the backyard for a while as the laboratory had almost the exclusive use of a, then new, research vessel bearing the name of Challenger. The science programme of a series of ocean water sample positions out to Rockall planned by Dr. Dave Ellett allowed for plenty of time lost to bad weather and when the weather had been good the ship returned early, leaving time available for the biologists to explore the deeper water off the continental shelf.

I and my two colleagues are hoping to continue this tomorrow by re-sampling a patch of seabed near to a large undersea mountain called Anton Dohrn. This, by the way, is a big feature and if it were magically sliced off at the base and set down on the UK its near circular base would extend from Glasgow to Edinburgh, coast to coast, and would be almost two and a half times as high as Ben Nevis.

We have biological samples from a flat area near this seamount from 1975 to 1994 and managed to sample again in 2013 so, 2015 samples would be very valuable in confirming any changes. The water depth is 2200 m and we use a device called an epibenthic sled (epi means on the surface and benthic means the sea bottom). The sled has relatively wide ‘runners’ and is supposed to skim across the surface without sinking in. We are fairly sure it actually does this as the ship’s instruments measuring the tension on the trawl wire indicate little resistance. If it were to dig in, like an anchor, the tension would increase dramatically.

For us the tension is increasing as the time nears to try the sleds (we have two onboard). It is two years since the last time – have I remembered everything? How will the ship’s trawl winch behave? Will we get the time right when setting the door closure? Numerous little details that all seem equally important to the outcome.

Then, when (if!) we get some seabed mud will a new system I have put together work OK? The sled usually returns with something of the order of 200 litres of mud and that needs washed clear of the fine material. We were not so well prepared in 2013 but this time we have linked to the ship’s fire hose system which promises to deliver a copious flow of seawater strained through a filter to remove the surface plankton. Will it work? Will it work? I keep wondering.

Meanwhile my neighbours and the ferrymen hear the word “cruise” and probably think of a jolly with a bit of easy science such as dipping a little net in, while the 230 kg of sled and a quarter ton of mud are, we hope, the reality tomorrow.

Photos by Peter Lamont

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