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1. Pressure effects

2. CTD frame

From: Ben Ward
Date: Sat, 1 Dec 2007 22:45
Subject: Over the edge

So finally we are at sea. After all our fears of the Southern Ocean and the Drake Passage, so far the weather has been perfect. We are currently cruising towards are next station at about 13 knots (nautical miles per hour) and we are ready to deploy our 12th CTD cast. Over the last 12 hours, we have been passing over the edge of the continental shelf, with the sea floor dropping rapidly below us. The last station we sampled was the deepest we will visit on this crossing, at 4700 metres.

To mark this, several people on board decorated polystyrene cups. and before we lowered the probe, we put the cups into socks and tied them to the frame, making sure they were not too near the instruments (very wise with my smelly feet). You can see in the picture how the enormous pressures at the bottom of the ocean crushed the cups down to a fraction of their original size. This is a bit of a tradition for oceanographers on their first cruise, and I feel a bit more like a real one now.

While the CTD was at the bottom on our cup crushing mission, we thought we might do a bit of science as well. The CTD frame carries 12 strong plastic tubes that can be closed at any depth. They are called Niskin bottles, and we use them to collect water samples from different depths through the water column. At 4700 metres in the Southern Ocean we found some very cold and salty water that had sunk far south in the Southern Ocean, near Antarctica. This dense water is known as Antarctic Bottom Water - or AABW in the jargon. It is important to the global climate system because when it sinks warmer surface waters moves south to replace it. This is part of the Global Overturning Circulation, which warms the poles and cools the tropics.

While I have been typing we have started station number 12, on the northern side of the Drake Passage. We have our way, so things could get a bit rough.

Ben