The pressure ramps up in the deep ocean

Over the past 48 hours we have been working through an area of the ocean called the continental slope, where the shallow shelf seas give way to the deep abyssal ocean. The slope is steep in oceanic terms, with the depth of the water changing from 250m on the shelf to 3600m over a distance of 300 km. Our stations became progressively deeper as we headed down the slope away from the Canadian coast and into the central Labrador Sea. Each station took a little longer to complete than the last one, and we needed to collect more water samples to get a good picture of the range of chemical concentrations over the full depth.

The pressure at the seafloor increases as the water above it gets deeper. Water is heavy – a cubic metre of water weighs a ton, so imagine the weight you would feel if there was a column of water 3000m high above your head. You would be squashed flat! Our instruments are protected from this great pressure by heavy metal cylinders that can withstand the weight of the water. Some are rated to just 2000m, so need to be removed from the CTD package in water that is deeper than that. If they were left on the frame they would certainly leak and the seawater would ruin the electronics. At worst they might implode, the force of which would destroy all the instruments around it. But most of the instruments we use on a regular basis are designed and built to withstand the pressure of water over 6000m.

Oceanographers are fond of demonstrating the effects of pressure in the deep sea by drawing colourful designs on white polystyrene cups, then sending them to the bottom of the ocean tucked inside a sock tied to the CTD frame. The cups shrink under the pressure (and stay shrunk) and the colours become supremely intense. There is an art to stuffing the cups with paper or fabric so that they retain their cup shape, and it is a fun moment of anticipation when you cut the cable ties, release the sock and reveal your mini-cup.

The deeper the cups go, the smaller they get and the brighter the colours become. I took the photograph below on a cruise last spring on RRS James Cook and I think it demonstrates the pressure effect very nicely. The cups were designed and shrunk by Claire Smalley who spent a considerable amount of time (and pens) drawing on these cups and marking the depth they were destined for.

Penny Holliday

Image: Polystyrene cups tied to the CTD frame shrink under the pressure of seawater at the depths they are labelled with.

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