Shelf Sea Biogeochemistry blog

Friday 14 November 2014

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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